ydb/contrib/libs/libpng/patches/apng.patch

Index: LICENSE
===================================================================
--- a/LICENSE
+++ b/LICENSE
@@ -1,6 +1,12 @@
 COPYRIGHT NOTICE, DISCLAIMER, and LICENSE
 =========================================
 
+This modified version of libpng code adds animated PNG support and is
+released under the libpng license described below. The modifications are
+Copyright (c) 2006-2007 Andrew Smith, Copyright (c) 2008-2025 Max Stepin,
+and are delimited by "#ifdef PNG_APNG_SUPPORTED / #endif" directives
+surrounding them in the modified libpng source files.
+
 PNG Reference Library License version 2
 ---------------------------------------
 
Index: pngread.c
===================================================================
--- a/pngread.c
+++ b/pngread.c
@@ -160,6 +160,9 @@
 
       else if (chunk_name == png_IDAT)
       {
+#ifdef PNG_READ_APNG_SUPPORTED
+         png_have_info(png_ptr, info_ptr);
+#endif
          png_ptr->idat_size = length;
          break;
       }
@@ -259,6 +262,17 @@
          png_handle_iTXt(png_ptr, info_ptr, length);
 #endif
 
+#ifdef PNG_READ_APNG_SUPPORTED
+      else if (chunk_name == png_acTL)
+         png_handle_acTL(png_ptr, info_ptr, length);
+
+      else if (chunk_name == png_fcTL)
+         png_handle_fcTL(png_ptr, info_ptr, length);
+
+      else if (chunk_name == png_fdAT)
+         png_handle_fdAT(png_ptr, info_ptr, length);
+#endif
+
       else
          png_handle_unknown(png_ptr, info_ptr, length,
              PNG_HANDLE_CHUNK_AS_DEFAULT);
@@ -266,6 +280,72 @@
 }
 #endif /* SEQUENTIAL_READ */
 
+#ifdef PNG_READ_APNG_SUPPORTED
+void PNGAPI
+png_read_frame_head(png_structp png_ptr, png_infop info_ptr)
+{
+    png_byte have_chunk_after_DAT; /* after IDAT or after fdAT */
+
+    png_debug(0, "Reading frame head");
+
+    if ((png_ptr->mode & PNG_HAVE_acTL) == 0)
+        png_error(png_ptr, "attempt to png_read_frame_head() but "
+                           "no acTL present");
+
+    /* do nothing for the main IDAT */
+    if (png_ptr->num_frames_read == 0)
+        return;
+
+    png_read_reset(png_ptr);
+    png_ptr->flags &= ~PNG_FLAG_ROW_INIT;
+    png_ptr->mode &= ~PNG_HAVE_fcTL;
+
+    have_chunk_after_DAT = 0;
+    for (;;)
+    {
+        png_uint_32 length = png_read_chunk_header(png_ptr);
+
+        if (png_ptr->chunk_name == png_IDAT)
+        {
+            /* discard trailing IDATs for the first frame */
+            if (have_chunk_after_DAT != 0 || png_ptr->num_frames_read > 1)
+                png_error(png_ptr, "png_read_frame_head(): out of place IDAT");
+            png_crc_finish(png_ptr, length);
+        }
+
+        else if (png_ptr->chunk_name == png_fcTL)
+        {
+            png_handle_fcTL(png_ptr, info_ptr, length);
+            have_chunk_after_DAT = 1;
+        }
+
+        else if (png_ptr->chunk_name == png_fdAT)
+        {
+            png_ensure_sequence_number(png_ptr, length);
+
+            /* discard trailing fdATs for frames other than the first */
+            if (have_chunk_after_DAT == 0 && png_ptr->num_frames_read > 1)
+                png_crc_finish(png_ptr, length - 4);
+            else if (png_ptr->mode & PNG_HAVE_fcTL)
+            {
+                png_ptr->idat_size = length - 4;
+                png_ptr->mode |= PNG_HAVE_IDAT;
+
+                break;
+            }
+            else
+                png_error(png_ptr, "png_read_frame_head(): out of place fdAT");
+        }
+        else
+        {
+            png_warning(png_ptr, "Skipped (ignored) a chunk "
+                                 "between APNG chunks");
+            png_crc_finish(png_ptr, length);
+        }
+    }
+}
+#endif /* READ_APNG */
+
 /* Optional call to update the users info_ptr structure */
 void PNGAPI
 png_read_update_info(png_structrp png_ptr, png_inforp info_ptr)
Index: pngget.c
===================================================================
--- a/pngget.c
+++ b/pngget.c
@@ -1288,4 +1288,166 @@
 #  endif
 #endif
 
+#ifdef PNG_APNG_SUPPORTED
+png_uint_32 PNGAPI
+png_get_acTL(png_structp png_ptr, png_infop info_ptr,
+             png_uint_32 *num_frames, png_uint_32 *num_plays)
+{
+    png_debug1(1, "in %s retrieval function", "acTL");
+
+    if (png_ptr != NULL && info_ptr != NULL &&
+        (info_ptr->valid & PNG_INFO_acTL) != 0 &&
+        num_frames != NULL && num_plays != NULL)
+    {
+        *num_frames = info_ptr->num_frames;
+        *num_plays = info_ptr->num_plays;
+        return (1);
+    }
+
+    return (0);
+}
+
+png_uint_32 PNGAPI
+png_get_num_frames(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_num_frames()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->num_frames);
+    return (0);
+}
+
+png_uint_32 PNGAPI
+png_get_num_plays(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_num_plays()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->num_plays);
+    return (0);
+}
+
+png_uint_32 PNGAPI
+png_get_next_frame_fcTL(png_structp png_ptr, png_infop info_ptr,
+             png_uint_32 *width, png_uint_32 *height,
+             png_uint_32 *x_offset, png_uint_32 *y_offset,
+             png_uint_16 *delay_num, png_uint_16 *delay_den,
+             png_byte *dispose_op, png_byte *blend_op)
+{
+    png_debug1(1, "in %s retrieval function", "fcTL");
+
+    if (png_ptr != NULL && info_ptr != NULL &&
+        (info_ptr->valid & PNG_INFO_fcTL) != 0 &&
+        width != NULL && height != NULL &&
+        x_offset != NULL && y_offset != NULL &&
+        delay_num != NULL && delay_den != NULL &&
+        dispose_op != NULL && blend_op != NULL)
+    {
+        *width = info_ptr->next_frame_width;
+        *height = info_ptr->next_frame_height;
+        *x_offset = info_ptr->next_frame_x_offset;
+        *y_offset = info_ptr->next_frame_y_offset;
+        *delay_num = info_ptr->next_frame_delay_num;
+        *delay_den = info_ptr->next_frame_delay_den;
+        *dispose_op = info_ptr->next_frame_dispose_op;
+        *blend_op = info_ptr->next_frame_blend_op;
+        return (1);
+    }
+
+    return (0);
+}
+
+png_uint_32 PNGAPI
+png_get_next_frame_width(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_width()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_width);
+    return (0);
+}
+
+png_uint_32 PNGAPI
+png_get_next_frame_height(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_height()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_height);
+    return (0);
+}
+
+png_uint_32 PNGAPI
+png_get_next_frame_x_offset(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_x_offset()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_x_offset);
+    return (0);
+}
+
+png_uint_32 PNGAPI
+png_get_next_frame_y_offset(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_y_offset()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_y_offset);
+    return (0);
+}
+
+png_uint_16 PNGAPI
+png_get_next_frame_delay_num(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_delay_num()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_delay_num);
+    return (0);
+}
+
+png_uint_16 PNGAPI
+png_get_next_frame_delay_den(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_delay_den()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_delay_den);
+    return (0);
+}
+
+png_byte PNGAPI
+png_get_next_frame_dispose_op(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_dispose_op()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_dispose_op);
+    return (0);
+}
+
+png_byte PNGAPI
+png_get_next_frame_blend_op(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_get_next_frame_blend_op()");
+
+    if (png_ptr != NULL && info_ptr != NULL)
+        return (info_ptr->next_frame_blend_op);
+    return (0);
+}
+
+png_byte PNGAPI
+png_get_first_frame_is_hidden(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_first_frame_is_hidden()");
+
+    if (png_ptr != NULL)
+       return (png_byte)(png_ptr->apng_flags & PNG_FIRST_FRAME_HIDDEN);
+
+    PNG_UNUSED(info_ptr)
+
+    return 0;
+}
+#endif /* APNG */
 #endif /* READ || WRITE */
Index: png.h
===================================================================
--- a/png.h
+++ b/png.h
@@ -23,6 +23,12 @@
  * COPYRIGHT NOTICE, DISCLAIMER, and LICENSE
  * =========================================
  *
+ * This modified version of libpng code adds animated PNG support and is
+ * released under the libpng license described below. The modifications are
+ * Copyright (c) 2006-2007 Andrew Smith, Copyright (c) 2008-2025 Max Stepin,
+ * and are delimited by "#ifdef PNG_APNG_SUPPORTED / #endif" directives
+ * surrounding them in the modified libpng source files.
+ *
  * PNG Reference Library License version 2
  * ---------------------------------------
  *
@@ -328,6 +335,10 @@
 #   include "pnglibconf.h"
 #endif
 
+#define PNG_APNG_SUPPORTED
+#define PNG_READ_APNG_SUPPORTED
+#define PNG_WRITE_APNG_SUPPORTED
+
 #ifndef PNG_VERSION_INFO_ONLY
 /* Machine specific configuration. */
 #  include "pngconf.h"
@@ -423,6 +434,17 @@
  * See pngconf.h for base types that vary by machine/system
  */
 
+#ifdef PNG_APNG_SUPPORTED
+/* dispose_op flags from inside fcTL */
+#define PNG_DISPOSE_OP_NONE        0x00
+#define PNG_DISPOSE_OP_BACKGROUND  0x01
+#define PNG_DISPOSE_OP_PREVIOUS    0x02
+
+/* blend_op flags from inside fcTL */
+#define PNG_BLEND_OP_SOURCE        0x00
+#define PNG_BLEND_OP_OVER          0x01
+#endif /* APNG */
+
 /* This triggers a compiler error in png.c, if png.c and png.h
  * do not agree upon the version number.
  */
@@ -745,6 +767,10 @@
 #define PNG_INFO_IDAT 0x8000U  /* ESR, 1.0.6 */
 #define PNG_INFO_eXIf 0x10000U /* GR-P, 1.6.31 */
 #define PNG_INFO_cICP 0x20000U
+#ifdef PNG_APNG_SUPPORTED
+#define PNG_INFO_acTL 0x40000U
+#define PNG_INFO_fcTL 0x80000U
+#endif
 
 /* This is used for the transformation routines, as some of them
  * change these values for the row.  It also should enable using
@@ -782,6 +808,10 @@
 #ifdef PNG_PROGRESSIVE_READ_SUPPORTED
 typedef PNG_CALLBACK(void, *png_progressive_info_ptr, (png_structp, png_infop));
 typedef PNG_CALLBACK(void, *png_progressive_end_ptr, (png_structp, png_infop));
+#ifdef PNG_APNG_SUPPORTED
+typedef PNG_CALLBACK(void, *png_progressive_frame_ptr, (png_structp,
+    png_uint_32));
+#endif
 
 /* The following callback receives png_uint_32 row_number, int pass for the
  * png_bytep data of the row.  When transforming an interlaced image the
@@ -3241,6 +3271,75 @@
  *  END OF HARDWARE AND SOFTWARE OPTIONS
  ******************************************************************************/
 
+#ifdef PNG_APNG_SUPPORTED
+PNG_EXPORT(252, png_uint_32, png_get_acTL, (png_structp png_ptr,
+   png_infop info_ptr, png_uint_32 *num_frames, png_uint_32 *num_plays));
+
+PNG_EXPORT(253, png_uint_32, png_set_acTL, (png_structp png_ptr,
+   png_infop info_ptr, png_uint_32 num_frames, png_uint_32 num_plays));
+
+PNG_EXPORT(254, png_uint_32, png_get_num_frames, (png_structp png_ptr,
+   png_infop info_ptr));
+
+PNG_EXPORT(255, png_uint_32, png_get_num_plays, (png_structp png_ptr,
+   png_infop info_ptr));
+
+PNG_EXPORT(256, png_uint_32, png_get_next_frame_fcTL,
+   (png_structp png_ptr, png_infop info_ptr, png_uint_32 *width,
+   png_uint_32 *height, png_uint_32 *x_offset, png_uint_32 *y_offset,
+   png_uint_16 *delay_num, png_uint_16 *delay_den, png_byte *dispose_op,
+   png_byte *blend_op));
+
+PNG_EXPORT(257, png_uint_32, png_set_next_frame_fcTL,
+   (png_structp png_ptr, png_infop info_ptr, png_uint_32 width,
+   png_uint_32 height, png_uint_32 x_offset, png_uint_32 y_offset,
+   png_uint_16 delay_num, png_uint_16 delay_den, png_byte dispose_op,
+   png_byte blend_op));
+
+PNG_EXPORT(258, png_uint_32, png_get_next_frame_width,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(259, png_uint_32, png_get_next_frame_height,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(260, png_uint_32, png_get_next_frame_x_offset,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(261, png_uint_32, png_get_next_frame_y_offset,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(262, png_uint_16, png_get_next_frame_delay_num,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(263, png_uint_16, png_get_next_frame_delay_den,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(264, png_byte, png_get_next_frame_dispose_op,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(265, png_byte, png_get_next_frame_blend_op,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(266, png_byte, png_get_first_frame_is_hidden,
+   (png_structp png_ptr, png_infop info_ptr));
+PNG_EXPORT(267, png_uint_32, png_set_first_frame_is_hidden,
+   (png_structp png_ptr, png_infop info_ptr, png_byte is_hidden));
+
+#ifdef PNG_READ_APNG_SUPPORTED
+PNG_EXPORT(268, void, png_read_frame_head, (png_structp png_ptr,
+   png_infop info_ptr));
+#ifdef PNG_PROGRESSIVE_READ_SUPPORTED
+PNG_EXPORT(269, void, png_set_progressive_frame_fn, (png_structp png_ptr,
+   png_progressive_frame_ptr frame_info_fn,
+   png_progressive_frame_ptr frame_end_fn));
+#endif /* PROGRESSIVE_READ */
+#endif /* READ_APNG */
+
+#ifdef PNG_WRITE_APNG_SUPPORTED
+PNG_EXPORT(270, void, png_write_frame_head, (png_structp png_ptr,
+   png_infop info_ptr, png_bytepp row_pointers,
+   png_uint_32 width, png_uint_32 height,
+   png_uint_32 x_offset, png_uint_32 y_offset,
+   png_uint_16 delay_num, png_uint_16 delay_den, png_byte dispose_op,
+   png_byte blend_op));
+
+PNG_EXPORT(271, void, png_write_frame_tail, (png_structp png_ptr,
+   png_infop info_ptr));
+#endif /* WRITE_APNG */
+#endif /* APNG */
+
 /* Maintainer: Put new public prototypes here ^, in libpng.3, in project
  * defs, and in scripts/symbols.def.
  */
@@ -3249,7 +3348,11 @@
  * one to use is one more than this.)
  */
 #ifdef PNG_EXPORT_LAST_ORDINAL
+#ifdef PNG_APNG_SUPPORTED
+  PNG_EXPORT_LAST_ORDINAL(271);
+#else
   PNG_EXPORT_LAST_ORDINAL(251);
+#endif /* APNG */
 #endif
 
 #ifdef __cplusplus
Index: pngpriv.h
===================================================================
--- a/pngpriv.h
+++ b/pngpriv.h
@@ -620,6 +620,10 @@
 #define PNG_HAVE_CHUNK_AFTER_IDAT 0x2000U /* Have another chunk after IDAT */
 #define PNG_WROTE_eXIf            0x4000U
 #define PNG_IS_READ_STRUCT        0x8000U /* Else is a write struct */
+#ifdef PNG_APNG_SUPPORTED
+#define PNG_HAVE_acTL            0x10000U
+#define PNG_HAVE_fcTL            0x20000U
+#endif
 
 /* Flags for the transformations the PNG library does on the image data */
 #define PNG_BGR                 0x0001U
@@ -857,6 +861,16 @@
 #define png_tRNS PNG_U32(116,  82,  78,  83)
 #define png_zTXt PNG_U32(122,  84,  88, 116)
 
+#ifdef PNG_APNG_SUPPORTED
+#define png_acTL PNG_U32( 97,  99,  84,  76)
+#define png_fcTL PNG_U32(102,  99,  84,  76)
+#define png_fdAT PNG_U32(102, 100,  65,  84)
+
+/* For png_struct.apng_flags: */
+#define PNG_FIRST_FRAME_HIDDEN       0x0001U
+#define PNG_APNG_APP                 0x0002U
+#endif
+
 /* The following will work on (signed char*) strings, whereas the get_uint_32
  * macro will fail on top-bit-set values because of the sign extension.
  */
@@ -1653,6 +1667,49 @@
 
 #endif /* PROGRESSIVE_READ */
 
+#ifdef PNG_APNG_SUPPORTED
+PNG_INTERNAL_FUNCTION(void,png_ensure_fcTL_is_valid,(png_structp png_ptr,
+   png_uint_32 width, png_uint_32 height,
+   png_uint_32 x_offset, png_uint_32 y_offset,
+   png_uint_16 delay_num, png_uint_16 delay_den,
+   png_byte dispose_op, png_byte blend_op),PNG_EMPTY);
+
+#ifdef PNG_READ_APNG_SUPPORTED
+PNG_INTERNAL_FUNCTION(void,png_handle_acTL,(png_structp png_ptr,
+   png_infop info_ptr, png_uint_32 length),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_handle_fcTL,(png_structp png_ptr,
+   png_infop info_ptr, png_uint_32 length),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_handle_fdAT,(png_structp png_ptr,
+   png_infop info_ptr, png_uint_32 length),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_have_info,(png_structp png_ptr,
+   png_infop info_ptr),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_ensure_sequence_number,(png_structp png_ptr,
+   png_uint_32 length),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_read_reset,(png_structp png_ptr),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_read_reinit,(png_structp png_ptr,
+   png_infop info_ptr),PNG_EMPTY);
+#ifdef PNG_PROGRESSIVE_READ_SUPPORTED
+PNG_INTERNAL_FUNCTION(void,png_progressive_read_reset,(png_structp png_ptr),
+   PNG_EMPTY);
+#endif /* PROGRESSIVE_READ */
+#endif /* READ_APNG */
+
+#ifdef PNG_WRITE_APNG_SUPPORTED
+PNG_INTERNAL_FUNCTION(void,png_write_acTL,(png_structp png_ptr,
+   png_uint_32 num_frames, png_uint_32 num_plays),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_write_fcTL,(png_structp png_ptr,
+   png_uint_32 width, png_uint_32 height,
+   png_uint_32 x_offset, png_uint_32 y_offset,
+   png_uint_16 delay_num, png_uint_16 delay_den,
+   png_byte dispose_op, png_byte blend_op),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_write_fdAT,(png_structp png_ptr,
+   png_const_bytep data, size_t length),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_write_reset,(png_structp png_ptr),PNG_EMPTY);
+PNG_INTERNAL_FUNCTION(void,png_write_reinit,(png_structp png_ptr,
+   png_infop info_ptr, png_uint_32 width, png_uint_32 height),PNG_EMPTY);
+#endif /* WRITE_APNG */
+#endif /* APNG */
+
 /* Added at libpng version 1.6.0 */
 #ifdef PNG_GAMMA_SUPPORTED
 PNG_INTERNAL_FUNCTION(void,png_colorspace_set_gamma,(png_const_structrp png_ptr,
Index: pnginfo.h
===================================================================
--- a/pnginfo.h
+++ b/pnginfo.h
@@ -270,5 +270,18 @@
    png_bytepp row_pointers;        /* the image bits */
 #endif
 
+#ifdef PNG_APNG_SUPPORTED
+   png_uint_32 num_frames; /* including default image */
+   png_uint_32 num_plays;
+   png_uint_32 next_frame_width;
+   png_uint_32 next_frame_height;
+   png_uint_32 next_frame_x_offset;
+   png_uint_32 next_frame_y_offset;
+   png_uint_16 next_frame_delay_num;
+   png_uint_16 next_frame_delay_den;
+   png_byte next_frame_dispose_op;
+   png_byte next_frame_blend_op;
+#endif
+
 };
 #endif /* PNGINFO_H */
Index: pngstruct.h
===================================================================
--- a/pngstruct.h
+++ b/pngstruct.h
@@ -398,6 +398,27 @@
    png_byte filter_type;
 #endif
 
+#ifdef PNG_APNG_SUPPORTED
+   png_uint_32 apng_flags;
+   png_uint_32 next_seq_num;         /* next fcTL/fdAT chunk sequence number */
+   png_uint_32 first_frame_width;
+   png_uint_32 first_frame_height;
+
+#ifdef PNG_READ_APNG_SUPPORTED
+   png_uint_32 num_frames_read;      /* incremented after all image data of */
+                                     /* a frame is read */
+#ifdef PNG_PROGRESSIVE_READ_SUPPORTED
+   png_progressive_frame_ptr frame_info_fn; /* frame info read callback */
+   png_progressive_frame_ptr frame_end_fn;  /* frame data read callback */
+#endif
+#endif
+
+#ifdef PNG_WRITE_APNG_SUPPORTED
+   png_uint_32 num_frames_to_write;
+   png_uint_32 num_frames_written;
+#endif
+#endif /* APNG */
+
 /* New members added in libpng-1.2.0 */
 
 /* New members added in libpng-1.0.2 but first enabled by default in 1.2.0 */
Index: pngwrite.c
===================================================================
--- a/pngwrite.c
+++ b/pngwrite.c
@@ -127,6 +127,10 @@
        * the application continues writing the PNG.  So check the 'invalid'
        * flag here too.
        */
+#ifdef PNG_WRITE_APNG_SUPPORTED
+   if ((info_ptr->valid & PNG_INFO_acTL) != 0)
+      png_write_acTL(png_ptr, info_ptr->num_frames, info_ptr->num_plays);
+#endif
 #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
          /* Write unknown chunks first; PNG v3 establishes a precedence order
           * for colourspace chunks.  It is certain therefore that new
@@ -399,6 +403,11 @@
    if ((png_ptr->mode & PNG_HAVE_IDAT) == 0)
       png_error(png_ptr, "No IDATs written into file");
 
+#ifdef PNG_WRITE_APNG_SUPPORTED
+   if (png_ptr->num_frames_written != png_ptr->num_frames_to_write)
+      png_error(png_ptr, "Not enough frames written");
+#endif
+
 #ifdef PNG_WRITE_CHECK_FOR_INVALID_INDEX_SUPPORTED
    if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE &&
        png_ptr->num_palette_max >= png_ptr->num_palette)
@@ -2446,4 +2455,42 @@
 }
 #endif /* SIMPLIFIED_WRITE_STDIO */
 #endif /* SIMPLIFIED_WRITE */
+
+#ifdef PNG_WRITE_APNG_SUPPORTED
+void PNGAPI
+png_write_frame_head(png_structp png_ptr, png_infop info_ptr,
+    png_bytepp row_pointers, png_uint_32 width, png_uint_32 height,
+    png_uint_32 x_offset, png_uint_32 y_offset,
+    png_uint_16 delay_num, png_uint_16 delay_den, png_byte dispose_op,
+    png_byte blend_op)
+{
+    png_debug(1, "in png_write_frame_head");
+
+    /* there is a chance this has been set after png_write_info was called,
+    * so it would be set but not written. is there a way to be sure? */
+    if ((info_ptr->valid & PNG_INFO_acTL) == 0)
+        png_error(png_ptr, "png_write_frame_head(): acTL not set");
+
+    png_write_reset(png_ptr);
+
+    png_write_reinit(png_ptr, info_ptr, width, height);
+
+    if ((png_ptr->apng_flags & PNG_FIRST_FRAME_HIDDEN) == 0 ||
+        png_ptr->num_frames_written != 0)
+        png_write_fcTL(png_ptr, width, height, x_offset, y_offset,
+                       delay_num, delay_den, dispose_op, blend_op);
+
+    PNG_UNUSED(row_pointers)
+}
+
+void PNGAPI
+png_write_frame_tail(png_structp png_ptr, png_infop info_ptr)
+{
+    png_debug(1, "in png_write_frame_tail");
+
+    png_ptr->num_frames_written++;
+
+    PNG_UNUSED(info_ptr)
+}
+#endif /* WRITE_APNG */
 #endif /* WRITE */
Index: pngpread.c
===================================================================
--- a/pngpread.c
+++ b/pngpread.c
@@ -209,6 +209,89 @@
 
    chunk_name = png_ptr->chunk_name;
 
+#ifdef PNG_READ_APNG_SUPPORTED
+   if (png_ptr->num_frames_read > 0 &&
+       png_ptr->num_frames_read < info_ptr->num_frames)
+   {
+      if (chunk_name == png_IDAT)
+      {
+         /* Discard trailing IDATs for the first frame */
+         if ((png_ptr->mode & PNG_HAVE_fcTL) != 0 ||
+             png_ptr->num_frames_read > 1)
+            png_error(png_ptr, "out of place IDAT");
+
+         PNG_PUSH_SAVE_BUFFER_IF_FULL
+         png_crc_finish(png_ptr, png_ptr->push_length);
+         png_ptr->mode &= ~PNG_HAVE_CHUNK_HEADER;
+      }
+
+      else if (chunk_name == png_fdAT)
+      {
+         PNG_PUSH_SAVE_BUFFER_IF_LT(4)
+         png_ensure_sequence_number(png_ptr, 4);
+
+         if ((png_ptr->mode & PNG_HAVE_fcTL) == 0)
+         {
+            /* Discard trailing fdATs for frames other than the first */
+            if (png_ptr->num_frames_read < 2)
+               png_error(png_ptr, "out of place fdAT");
+
+            PNG_PUSH_SAVE_BUFFER_IF_FULL
+            png_crc_finish(png_ptr, png_ptr->push_length);
+            png_ptr->mode &= ~PNG_HAVE_CHUNK_HEADER;
+         }
+
+         else
+         {
+            /* frame data follows */
+            png_ptr->idat_size = png_ptr->push_length - 4;
+            png_ptr->mode |= PNG_HAVE_IDAT;
+            png_ptr->process_mode = PNG_READ_IDAT_MODE;
+         }
+      }
+
+      else if (chunk_name == png_fcTL)
+      {
+         PNG_PUSH_SAVE_BUFFER_IF_FULL
+         png_read_reset(png_ptr);
+         png_ptr->mode &= ~PNG_HAVE_fcTL;
+
+         png_handle_fcTL(png_ptr, info_ptr, png_ptr->push_length);
+
+         if ((png_ptr->mode & PNG_HAVE_fcTL) == 0)
+            png_error(png_ptr, "missing required fcTL chunk");
+
+         png_read_reinit(png_ptr, info_ptr);
+         png_progressive_read_reset(png_ptr);
+
+         if (png_ptr->frame_info_fn != NULL)
+            (*(png_ptr->frame_info_fn))(png_ptr, png_ptr->num_frames_read);
+
+         png_ptr->mode &= ~PNG_HAVE_CHUNK_HEADER;
+      }
+
+      else if (chunk_name == png_IEND)
+      {
+         PNG_PUSH_SAVE_BUFFER_IF_FULL
+         png_warning(png_ptr, "Number of actual frames fewer than expected");
+         png_crc_finish(png_ptr, png_ptr->push_length);
+         png_ptr->process_mode = PNG_READ_DONE_MODE;
+         png_push_have_end(png_ptr, info_ptr);
+      }
+
+      else
+      {
+         PNG_PUSH_SAVE_BUFFER_IF_FULL
+         png_warning(png_ptr, "Skipped (ignored) a chunk "
+                              "between APNG chunks");
+         png_crc_finish(png_ptr, png_ptr->push_length);
+         png_ptr->mode &= ~PNG_HAVE_CHUNK_HEADER;
+      }
+
+      return;
+   }
+#endif /* READ_APNG */
+
    if (chunk_name == png_IDAT)
    {
       if ((png_ptr->mode & PNG_AFTER_IDAT) != 0)
@@ -275,6 +358,9 @@
 
    else if (chunk_name == png_IDAT)
    {
+#ifdef PNG_READ_APNG_SUPPORTED
+      png_have_info(png_ptr, info_ptr);
+#endif
       png_ptr->idat_size = png_ptr->push_length;
       png_ptr->process_mode = PNG_READ_IDAT_MODE;
       png_push_have_info(png_ptr, info_ptr);
@@ -437,6 +523,20 @@
    }
 #endif
 
+#ifdef PNG_READ_APNG_SUPPORTED
+   else if (chunk_name == png_acTL)
+   {
+      PNG_PUSH_SAVE_BUFFER_IF_FULL
+      png_handle_acTL(png_ptr, info_ptr, png_ptr->push_length);
+   }
+
+   else if (chunk_name == png_fcTL)
+   {
+      PNG_PUSH_SAVE_BUFFER_IF_FULL
+      png_handle_fcTL(png_ptr, info_ptr, png_ptr->push_length);
+   }
+
+#endif /* READ_APNG */
    else
    {
       PNG_PUSH_SAVE_BUFFER_IF_FULL
@@ -569,7 +669,11 @@
       png_byte chunk_tag[4];
 
       /* TODO: this code can be commoned up with the same code in push_read */
+#ifdef PNG_READ_APNG_SUPPORTED
+      PNG_PUSH_SAVE_BUFFER_IF_LT(12)
+#else
       PNG_PUSH_SAVE_BUFFER_IF_LT(8)
+#endif
       png_push_fill_buffer(png_ptr, chunk_length, 4);
       png_ptr->push_length = png_get_uint_31(png_ptr, chunk_length);
       png_reset_crc(png_ptr);
@@ -577,17 +681,60 @@
       png_ptr->chunk_name = PNG_CHUNK_FROM_STRING(chunk_tag);
       png_ptr->mode |= PNG_HAVE_CHUNK_HEADER;
 
+#ifdef PNG_READ_APNG_SUPPORTED
+      if (png_ptr->chunk_name != png_fdAT && png_ptr->num_frames_read > 0)
+      {
+          if ((png_ptr->flags & PNG_FLAG_ZSTREAM_ENDED) != 0)
+          {
+              png_ptr->process_mode = PNG_READ_CHUNK_MODE;
+              if (png_ptr->frame_end_fn != NULL)
+                 (*(png_ptr->frame_end_fn))(png_ptr, png_ptr->num_frames_read);
+              png_ptr->num_frames_read++;
+              return;
+          }
+          else
+          {
+              if (png_ptr->chunk_name == png_IEND)
+                  png_error(png_ptr, "Not enough image data");
+              PNG_PUSH_SAVE_BUFFER_IF_FULL
+              png_warning(png_ptr, "Skipping (ignoring) a chunk between "
+                                   "APNG chunks");
+              png_crc_finish(png_ptr, png_ptr->push_length);
+              png_ptr->mode &= ~PNG_HAVE_CHUNK_HEADER;
+              return;
+          }
+      }
+      else
+#endif
+#ifdef PNG_READ_APNG_SUPPORTED
+      if (png_ptr->chunk_name != png_IDAT && png_ptr->num_frames_read == 0)
+#else
       if (png_ptr->chunk_name != png_IDAT)
+#endif
       {
          png_ptr->process_mode = PNG_READ_CHUNK_MODE;
 
          if ((png_ptr->flags & PNG_FLAG_ZSTREAM_ENDED) == 0)
             png_error(png_ptr, "Not enough compressed data");
 
+#ifdef PNG_READ_APNG_SUPPORTED
+         if (png_ptr->frame_end_fn != NULL)
+            (*(png_ptr->frame_end_fn))(png_ptr, png_ptr->num_frames_read);
+         png_ptr->num_frames_read++;
+#endif
+
          return;
       }
 
       png_ptr->idat_size = png_ptr->push_length;
+
+#ifdef PNG_READ_APNG_SUPPORTED
+      if (png_ptr->num_frames_read > 0)
+      {
+         png_ensure_sequence_number(png_ptr, 4);
+         png_ptr->idat_size -= 4;
+      }
+#endif
    }
 
    if (png_ptr->idat_size != 0 && png_ptr->save_buffer_size != 0)
@@ -661,6 +808,16 @@
    if (!(buffer_length > 0) || buffer == NULL)
       png_error(png_ptr, "No IDAT data (internal error)");
 
+#ifdef PNG_READ_APNG_SUPPORTED
+   /* If the app is not APNG-aware, decode only the first frame */
+   if ((png_ptr->apng_flags & PNG_APNG_APP) == 0 &&
+      png_ptr->num_frames_read > 0)
+   {
+      png_ptr->flags |= PNG_FLAG_ZSTREAM_ENDED;
+      return;
+   }
+#endif
+
    /* This routine must process all the data it has been given
     * before returning, calling the row callback as required to
     * handle the uncompressed results.
@@ -1094,6 +1251,18 @@
    png_set_read_fn(png_ptr, progressive_ptr, png_push_fill_buffer);
 }
 
+#ifdef PNG_READ_APNG_SUPPORTED
+void PNGAPI
+png_set_progressive_frame_fn(png_structp png_ptr,
+   png_progressive_frame_ptr frame_info_fn,
+   png_progressive_frame_ptr frame_end_fn)
+{
+   png_ptr->frame_info_fn = frame_info_fn;
+   png_ptr->frame_end_fn = frame_end_fn;
+   png_ptr->apng_flags |= PNG_APNG_APP;
+}
+#endif
+
 png_voidp PNGAPI
 png_get_progressive_ptr(png_const_structrp png_ptr)
 {
Index: pngset.c
===================================================================
--- a/pngset.c
+++ b/pngset.c
@@ -305,6 +305,11 @@
    info_ptr->pixel_depth = (png_byte)(info_ptr->channels * info_ptr->bit_depth);
 
    info_ptr->rowbytes = PNG_ROWBYTES(info_ptr->pixel_depth, width);
+
+#ifdef PNG_APNG_SUPPORTED
+   /* for non-animated png. this may be overwritten from an acTL chunk later */
+   info_ptr->num_frames = 1;
+#endif
 }
 
 #ifdef PNG_oFFs_SUPPORTED
@@ -1176,6 +1181,146 @@
 }
 #endif /* sPLT */
 
+#ifdef PNG_APNG_SUPPORTED
+png_uint_32 PNGAPI
+png_set_acTL(png_structp png_ptr, png_infop info_ptr,
+    png_uint_32 num_frames, png_uint_32 num_plays)
+{
+    png_debug1(1, "in %s storage function", "acTL");
+
+    if (png_ptr == NULL || info_ptr == NULL)
+    {
+        png_warning(png_ptr,
+                    "Call to png_set_acTL() with NULL png_ptr "
+                    "or info_ptr ignored");
+        return (0);
+    }
+    if (num_frames == 0)
+    {
+        png_warning(png_ptr,
+                    "Ignoring attempt to set acTL with num_frames zero");
+        return (0);
+    }
+    if (num_frames > PNG_UINT_31_MAX)
+    {
+        png_warning(png_ptr,
+                    "Ignoring attempt to set acTL with num_frames > 2^31-1");
+        return (0);
+    }
+    if (num_plays > PNG_UINT_31_MAX)
+    {
+        png_warning(png_ptr,
+                    "Ignoring attempt to set acTL with num_plays > 2^31-1");
+        return (0);
+    }
+
+    info_ptr->num_frames = num_frames;
+    info_ptr->num_plays = num_plays;
+
+    info_ptr->valid |= PNG_INFO_acTL;
+
+    return (1);
+}
+
+/* delay_num and delay_den can hold any 16-bit values including zero */
+png_uint_32 PNGAPI
+png_set_next_frame_fcTL(png_structp png_ptr, png_infop info_ptr,
+    png_uint_32 width, png_uint_32 height,
+    png_uint_32 x_offset, png_uint_32 y_offset,
+    png_uint_16 delay_num, png_uint_16 delay_den,
+    png_byte dispose_op, png_byte blend_op)
+{
+    png_debug1(1, "in %s storage function", "fcTL");
+
+    if (png_ptr == NULL || info_ptr == NULL)
+    {
+        png_warning(png_ptr,
+                    "Call to png_set_fcTL() with NULL png_ptr or info_ptr "
+                    "ignored");
+        return (0);
+    }
+
+    png_ensure_fcTL_is_valid(png_ptr, width, height, x_offset, y_offset,
+                             delay_num, delay_den, dispose_op, blend_op);
+
+    if (blend_op == PNG_BLEND_OP_OVER)
+    {
+        if ((png_ptr->color_type & PNG_COLOR_MASK_ALPHA) == 0 &&
+            png_get_valid(png_ptr, info_ptr, PNG_INFO_tRNS) == 0)
+        {
+          png_warning(png_ptr, "PNG_BLEND_OP_OVER is meaningless "
+                               "and wasteful for opaque images, ignored");
+          blend_op = PNG_BLEND_OP_SOURCE;
+        }
+    }
+
+    info_ptr->next_frame_width = width;
+    info_ptr->next_frame_height = height;
+    info_ptr->next_frame_x_offset = x_offset;
+    info_ptr->next_frame_y_offset = y_offset;
+    info_ptr->next_frame_delay_num = delay_num;
+    info_ptr->next_frame_delay_den = delay_den;
+    info_ptr->next_frame_dispose_op = dispose_op;
+    info_ptr->next_frame_blend_op = blend_op;
+
+    info_ptr->valid |= PNG_INFO_fcTL;
+
+    return (1);
+}
+
+void /* PRIVATE */
+png_ensure_fcTL_is_valid(png_structp png_ptr,
+    png_uint_32 width, png_uint_32 height,
+    png_uint_32 x_offset, png_uint_32 y_offset,
+    png_uint_16 delay_num, png_uint_16 delay_den,
+    png_byte dispose_op, png_byte blend_op)
+{
+    if (width == 0 || width > PNG_UINT_31_MAX)
+        png_error(png_ptr, "invalid width in fcTL (0 or > 2^31-1)");
+    if (height == 0 || height > PNG_UINT_31_MAX)
+        png_error(png_ptr, "invalid height in fcTL (0 or > 2^31-1)");
+    if (x_offset > PNG_UINT_31_MAX)
+        png_error(png_ptr, "invalid x_offset in fcTL (> 2^31-1)");
+    if (y_offset > PNG_UINT_31_MAX)
+        png_error(png_ptr, "invalid y_offset in fcTL (> 2^31-1)");
+    if (width + x_offset > png_ptr->first_frame_width ||
+        height + y_offset > png_ptr->first_frame_height)
+        png_error(png_ptr, "dimensions of a frame are greater than "
+                           "the ones in IHDR");
+
+    if (dispose_op != PNG_DISPOSE_OP_NONE &&
+        dispose_op != PNG_DISPOSE_OP_BACKGROUND &&
+        dispose_op != PNG_DISPOSE_OP_PREVIOUS)
+        png_error(png_ptr, "invalid dispose_op in fcTL");
+
+    if (blend_op != PNG_BLEND_OP_SOURCE &&
+        blend_op != PNG_BLEND_OP_OVER)
+        png_error(png_ptr, "invalid blend_op in fcTL");
+
+    PNG_UNUSED(delay_num)
+    PNG_UNUSED(delay_den)
+}
+
+png_uint_32 PNGAPI
+png_set_first_frame_is_hidden(png_structp png_ptr, png_infop info_ptr,
+                              png_byte is_hidden)
+{
+    png_debug(1, "in png_first_frame_is_hidden()");
+
+    if (png_ptr == NULL)
+        return 0;
+
+    if (is_hidden != 0)
+        png_ptr->apng_flags |= PNG_FIRST_FRAME_HIDDEN;
+    else
+        png_ptr->apng_flags &= ~PNG_FIRST_FRAME_HIDDEN;
+
+    PNG_UNUSED(info_ptr)
+
+    return 1;
+}
+#endif /* APNG */
+
 #ifdef PNG_STORE_UNKNOWN_CHUNKS_SUPPORTED
 static png_byte
 check_location(png_const_structrp png_ptr, int location)
Index: pngrutil.c
===================================================================
--- a/pngrutil.c
+++ b/pngrutil.c
@@ -877,6 +877,11 @@
    filter_type = buf[11];
    interlace_type = buf[12];
 
+#ifdef PNG_READ_APNG_SUPPORTED
+   png_ptr->first_frame_width = width;
+   png_ptr->first_frame_height = height;
+#endif
+
    /* Set internal variables */
    png_ptr->width = width;
    png_ptr->height = height;
@@ -2912,6 +2917,180 @@
 }
 #endif
 
+#ifdef PNG_READ_APNG_SUPPORTED
+void /* PRIVATE */
+png_handle_acTL(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
+{
+    png_byte data[8];
+    png_uint_32 num_frames;
+    png_uint_32 num_plays;
+    png_uint_32 didSet;
+
+    png_debug(1, "in png_handle_acTL");
+
+    if ((png_ptr->mode & PNG_HAVE_IHDR) == 0)
+    {
+        png_error(png_ptr, "Missing IHDR before acTL");
+    }
+    else if ((png_ptr->mode & PNG_HAVE_IDAT) != 0)
+    {
+        png_warning(png_ptr, "Invalid acTL after IDAT skipped");
+        png_crc_finish(png_ptr, length);
+        return;
+    }
+    else if ((png_ptr->mode & PNG_HAVE_acTL) != 0)
+    {
+        png_warning(png_ptr, "Duplicate acTL skipped");
+        png_crc_finish(png_ptr, length);
+        return;
+    }
+    else if (length != 8)
+    {
+        png_warning(png_ptr, "acTL with invalid length skipped");
+        png_crc_finish(png_ptr, length);
+        return;
+    }
+
+    png_crc_read(png_ptr, data, 8);
+    png_crc_finish(png_ptr, 0);
+
+    num_frames = png_get_uint_31(png_ptr, data);
+    num_plays = png_get_uint_31(png_ptr, data + 4);
+
+    /* the set function will do error checking on num_frames */
+    didSet = png_set_acTL(png_ptr, info_ptr, num_frames, num_plays);
+    if (didSet != 0)
+        png_ptr->mode |= PNG_HAVE_acTL;
+}
+
+void /* PRIVATE */
+png_handle_fcTL(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
+{
+    png_byte data[22];
+    png_uint_32 width;
+    png_uint_32 height;
+    png_uint_32 x_offset;
+    png_uint_32 y_offset;
+    png_uint_16 delay_num;
+    png_uint_16 delay_den;
+    png_byte dispose_op;
+    png_byte blend_op;
+
+    png_debug(1, "in png_handle_fcTL");
+
+    png_ensure_sequence_number(png_ptr, length);
+
+    if ((png_ptr->mode & PNG_HAVE_IHDR) == 0)
+    {
+        png_error(png_ptr, "Missing IHDR before fcTL");
+    }
+    else if ((png_ptr->mode & PNG_HAVE_IDAT) != 0)
+    {
+        /* for any frames other then the first this message may be misleading,
+        * but correct. PNG_HAVE_IDAT is unset before the frame head is read
+        * i can't think of a better message */
+        png_warning(png_ptr, "Invalid fcTL after IDAT skipped");
+        png_crc_finish(png_ptr, length-4);
+        return;
+    }
+    else if ((png_ptr->mode & PNG_HAVE_fcTL) != 0)
+    {
+        png_warning(png_ptr, "Duplicate fcTL within one frame skipped");
+        png_crc_finish(png_ptr, length-4);
+        return;
+    }
+    else if (length != 26)
+    {
+        png_warning(png_ptr, "fcTL with invalid length skipped");
+        png_crc_finish(png_ptr, length-4);
+        return;
+    }
+
+    png_crc_read(png_ptr, data, 22);
+    png_crc_finish(png_ptr, 0);
+
+    width = png_get_uint_31(png_ptr, data);
+    height = png_get_uint_31(png_ptr, data + 4);
+    x_offset = png_get_uint_31(png_ptr, data + 8);
+    y_offset = png_get_uint_31(png_ptr, data + 12);
+    delay_num = png_get_uint_16(data + 16);
+    delay_den = png_get_uint_16(data + 18);
+    dispose_op = data[20];
+    blend_op = data[21];
+
+    if (png_ptr->num_frames_read == 0 && (x_offset != 0 || y_offset != 0))
+    {
+        png_warning(png_ptr, "fcTL for the first frame must have zero offset");
+        return;
+    }
+
+    if (info_ptr != NULL)
+    {
+        if (png_ptr->num_frames_read == 0 &&
+            (width != info_ptr->width || height != info_ptr->height))
+        {
+            png_warning(png_ptr, "size in first frame's fcTL must match "
+                               "the size in IHDR");
+            return;
+        }
+
+        /* The set function will do more error checking */
+        png_set_next_frame_fcTL(png_ptr, info_ptr, width, height,
+                                x_offset, y_offset, delay_num, delay_den,
+                                dispose_op, blend_op);
+
+        png_read_reinit(png_ptr, info_ptr);
+
+        png_ptr->mode |= PNG_HAVE_fcTL;
+    }
+}
+
+void /* PRIVATE */
+png_have_info(png_structp png_ptr, png_infop info_ptr)
+{
+    if ((info_ptr->valid & PNG_INFO_acTL) != 0 &&
+        (info_ptr->valid & PNG_INFO_fcTL) == 0)
+    {
+        png_ptr->apng_flags |= PNG_FIRST_FRAME_HIDDEN;
+        info_ptr->num_frames++;
+    }
+}
+
+void /* PRIVATE */
+png_handle_fdAT(png_structp png_ptr, png_infop info_ptr, png_uint_32 length)
+{
+    png_ensure_sequence_number(png_ptr, length);
+
+    /* This function is only called from png_read_end(), png_read_info(),
+    * and png_push_read_chunk() which means that:
+    * - the user doesn't want to read this frame
+    * - or this is an out-of-place fdAT
+    * in either case it is safe to ignore the chunk with a warning */
+    png_warning(png_ptr, "ignoring fdAT chunk");
+    png_crc_finish(png_ptr, length - 4);
+    PNG_UNUSED(info_ptr)
+}
+
+void /* PRIVATE */
+png_ensure_sequence_number(png_structp png_ptr, png_uint_32 length)
+{
+    png_byte data[4];
+    png_uint_32 sequence_number;
+
+    if (length < 4)
+        png_error(png_ptr, "invalid fcTL or fdAT chunk found");
+
+    png_crc_read(png_ptr, data, 4);
+    sequence_number = png_get_uint_31(png_ptr, data);
+
+    if (sequence_number != png_ptr->next_seq_num)
+        png_error(png_ptr, "fcTL or fdAT chunk with out-of-order sequence "
+                           "number found");
+
+    png_ptr->next_seq_num++;
+}
+#endif /* READ_APNG */
+
 #ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
 /* Utility function for png_handle_unknown; set up png_ptr::unknown_chunk */
 static int
@@ -3217,7 +3366,11 @@
    if (PNG_USER_CHUNK_MALLOC_MAX < limit)
       limit = PNG_USER_CHUNK_MALLOC_MAX;
 # endif
+#ifdef PNG_READ_APNG_SUPPORTED
+   if (png_ptr->chunk_name == png_IDAT || png_ptr->chunk_name == png_fdAT)
+#else
    if (png_ptr->chunk_name == png_IDAT)
+#endif
    {
       png_alloc_size_t idat_limit = PNG_UINT_31_MAX;
       size_t row_factor =
@@ -4217,6 +4400,38 @@
          uInt avail_in;
          png_bytep buffer;
 
+#ifdef PNG_READ_APNG_SUPPORTED
+         png_uint_32 bytes_to_skip = 0;
+
+         while (png_ptr->idat_size == 0 || bytes_to_skip != 0)
+         {
+            png_crc_finish(png_ptr, bytes_to_skip);
+            bytes_to_skip = 0;
+
+            png_ptr->idat_size = png_read_chunk_header(png_ptr);
+            if (png_ptr->num_frames_read == 0)
+            {
+               if (png_ptr->chunk_name != png_IDAT)
+                  png_error(png_ptr, "Not enough image data");
+            }
+            else
+            {
+               if (png_ptr->chunk_name == png_IEND)
+                  png_error(png_ptr, "Not enough image data");
+               if (png_ptr->chunk_name != png_fdAT)
+               {
+                  png_warning(png_ptr, "Skipped (ignored) a chunk "
+                                       "between APNG chunks");
+                  bytes_to_skip = png_ptr->idat_size;
+                  continue;
+               }
+
+               png_ensure_sequence_number(png_ptr, png_ptr->idat_size);
+
+               png_ptr->idat_size -= 4;
+            }
+         }
+#else
          while (png_ptr->idat_size == 0)
          {
             png_crc_finish(png_ptr, 0);
@@ -4228,6 +4443,7 @@
             if (png_ptr->chunk_name != png_IDAT)
                png_error(png_ptr, "Not enough image data");
          }
+#endif /* READ_APNG */
 
          avail_in = png_ptr->IDAT_read_size;
 
@@ -4291,6 +4507,9 @@
 
          png_ptr->mode |= PNG_AFTER_IDAT;
          png_ptr->flags |= PNG_FLAG_ZSTREAM_ENDED;
+#ifdef PNG_READ_APNG_SUPPORTED
+         png_ptr->num_frames_read++;
+#endif
 
          if (png_ptr->zstream.avail_in > 0 || png_ptr->idat_size > 0)
             png_chunk_benign_error(png_ptr, "Extra compressed data");
@@ -4700,4 +4919,80 @@
 
    png_ptr->flags |= PNG_FLAG_ROW_INIT;
 }
+
+#ifdef PNG_READ_APNG_SUPPORTED
+/* This function is to be called after the main IDAT set has been read and
+ * before a new IDAT is read. It resets some parts of png_ptr
+ * to make them usable by the read functions again */
+void /* PRIVATE */
+png_read_reset(png_structp png_ptr)
+{
+    png_ptr->mode &= ~PNG_HAVE_IDAT;
+    png_ptr->mode &= ~PNG_AFTER_IDAT;
+    png_ptr->row_number = 0;
+    png_ptr->pass = 0;
+}
+
+void /* PRIVATE */
+png_read_reinit(png_structp png_ptr, png_infop info_ptr)
+{
+    png_ptr->width = info_ptr->next_frame_width;
+    png_ptr->height = info_ptr->next_frame_height;
+    png_ptr->rowbytes = PNG_ROWBYTES(png_ptr->pixel_depth,png_ptr->width);
+    png_ptr->info_rowbytes = PNG_ROWBYTES(info_ptr->pixel_depth,
+        png_ptr->width);
+    if (png_ptr->prev_row != NULL)
+        memset(png_ptr->prev_row, 0, png_ptr->rowbytes + 1);
+}
+
+#ifdef PNG_PROGRESSIVE_READ_SUPPORTED
+/* same as png_read_reset() but for the progressive reader */
+void /* PRIVATE */
+png_progressive_read_reset(png_structp png_ptr)
+{
+#ifdef PNG_READ_INTERLACING_SUPPORTED
+    /* Arrays to facilitate easy interlacing - use pass (0 - 6) as index */
+
+    /* Start of interlace block */
+    static const png_byte png_pass_start[] = {0, 4, 0, 2, 0, 1, 0};
+
+    /* Offset to next interlace block */
+    static const png_byte png_pass_inc[] = {8, 8, 4, 4, 2, 2, 1};
+
+    /* Start of interlace block in the y direction */
+    static const png_byte png_pass_ystart[] = {0, 0, 4, 0, 2, 0, 1};
+
+    /* Offset to next interlace block in the y direction */
+    static const png_byte png_pass_yinc[] = {8, 8, 8, 4, 4, 2, 2};
+
+    if (png_ptr->interlaced != 0)
+    {
+        if ((png_ptr->transformations & PNG_INTERLACE) == 0)
+            png_ptr->num_rows = (png_ptr->height + png_pass_yinc[0] - 1 -
+                                png_pass_ystart[0]) / png_pass_yinc[0];
+        else
+            png_ptr->num_rows = png_ptr->height;
+
+        png_ptr->iwidth = (png_ptr->width +
+                           png_pass_inc[png_ptr->pass] - 1 -
+                           png_pass_start[png_ptr->pass]) /
+                           png_pass_inc[png_ptr->pass];
+    }
+    else
+#endif /* READ_INTERLACING */
+    {
+        png_ptr->num_rows = png_ptr->height;
+        png_ptr->iwidth = png_ptr->width;
+    }
+    png_ptr->flags &= ~PNG_FLAG_ZSTREAM_ENDED;
+    if (inflateReset(&(png_ptr->zstream)) != Z_OK)
+        png_error(png_ptr, "inflateReset failed");
+    png_ptr->zstream.avail_in = 0;
+    png_ptr->zstream.next_in = 0;
+    png_ptr->zstream.next_out = png_ptr->row_buf;
+    png_ptr->zstream.avail_out = (uInt)PNG_ROWBYTES(png_ptr->pixel_depth,
+        png_ptr->iwidth) + 1;
+}
+#endif /* PROGRESSIVE_READ */
+#endif /* READ_APNG */
 #endif /* READ */
Index: pngwutil.c
===================================================================
--- a/pngwutil.c
+++ b/pngwutil.c
@@ -838,6 +838,11 @@
    /* Write the chunk */
    png_write_complete_chunk(png_ptr, png_IHDR, buf, 13);
 
+#ifdef PNG_WRITE_APNG_SUPPORTED
+   png_ptr->first_frame_width = width;
+   png_ptr->first_frame_height = height;
+#endif
+
    if ((png_ptr->do_filter) == PNG_NO_FILTERS)
    {
       if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE ||
@@ -1020,7 +1025,17 @@
 #endif
 
          if (size > 0)
+#ifdef PNG_WRITE_APNG_SUPPORTED
+         {
+            if (png_ptr->num_frames_written == 0)
+#endif
             png_write_complete_chunk(png_ptr, png_IDAT, data, size);
+#ifdef PNG_WRITE_APNG_SUPPORTED
+            else
+               png_write_fdAT(png_ptr, data, size);
+         }
+#endif /* WRITE_APNG */
+
          png_ptr->mode |= PNG_HAVE_IDAT;
 
          png_ptr->zstream.next_out = data;
@@ -1067,7 +1082,17 @@
 #endif
 
          if (size > 0)
+#ifdef PNG_WRITE_APNG_SUPPORTED
+         {
+            if (png_ptr->num_frames_written == 0)
+#endif
             png_write_complete_chunk(png_ptr, png_IDAT, data, size);
+#ifdef PNG_WRITE_APNG_SUPPORTED
+            else
+               png_write_fdAT(png_ptr, data, size);
+         }
+#endif /* WRITE_APNG */
+
          png_ptr->zstream.avail_out = 0;
          png_ptr->zstream.next_out = NULL;
          png_ptr->mode |= PNG_HAVE_IDAT | PNG_AFTER_IDAT;
@@ -1925,6 +1950,82 @@
 }
 #endif
 
+#ifdef PNG_WRITE_APNG_SUPPORTED
+void /* PRIVATE */
+png_write_acTL(png_structp png_ptr,
+    png_uint_32 num_frames, png_uint_32 num_plays)
+{
+    png_byte buf[8];
+
+    png_debug(1, "in png_write_acTL");
+
+    png_ptr->num_frames_to_write = num_frames;
+
+    if ((png_ptr->apng_flags & PNG_FIRST_FRAME_HIDDEN) != 0)
+        num_frames--;
+
+    png_save_uint_32(buf, num_frames);
+    png_save_uint_32(buf + 4, num_plays);
+
+    png_write_complete_chunk(png_ptr, png_acTL, buf, 8);
+}
+
+void /* PRIVATE */
+png_write_fcTL(png_structp png_ptr, png_uint_32 width, png_uint_32 height,
+    png_uint_32 x_offset, png_uint_32 y_offset,
+    png_uint_16 delay_num, png_uint_16 delay_den, png_byte dispose_op,
+    png_byte blend_op)
+{
+    png_byte buf[26];
+
+    png_debug(1, "in png_write_fcTL");
+
+    if (png_ptr->num_frames_written == 0 && (x_offset != 0 || y_offset != 0))
+        png_error(png_ptr, "x and/or y offset for the first frame aren't 0");
+    if (png_ptr->num_frames_written == 0 &&
+        (width != png_ptr->first_frame_width ||
+         height != png_ptr->first_frame_height))
+        png_error(png_ptr, "width and/or height in the first frame's fcTL "
+                           "don't match the ones in IHDR");
+
+    /* more error checking */
+    png_ensure_fcTL_is_valid(png_ptr, width, height, x_offset, y_offset,
+                             delay_num, delay_den, dispose_op, blend_op);
+
+    png_save_uint_32(buf, png_ptr->next_seq_num);
+    png_save_uint_32(buf + 4, width);
+    png_save_uint_32(buf + 8, height);
+    png_save_uint_32(buf + 12, x_offset);
+    png_save_uint_32(buf + 16, y_offset);
+    png_save_uint_16(buf + 20, delay_num);
+    png_save_uint_16(buf + 22, delay_den);
+    buf[24] = dispose_op;
+    buf[25] = blend_op;
+
+    png_write_complete_chunk(png_ptr, png_fcTL, buf, 26);
+
+    png_ptr->next_seq_num++;
+}
+
+void /* PRIVATE */
+png_write_fdAT(png_structp png_ptr,
+    png_const_bytep data, size_t length)
+{
+    png_byte buf[4];
+
+    png_write_chunk_header(png_ptr, png_fdAT, (png_uint_32)(4 + length));
+
+    png_save_uint_32(buf, png_ptr->next_seq_num);
+    png_write_chunk_data(png_ptr, buf, 4);
+
+    png_write_chunk_data(png_ptr, data, length);
+
+    png_write_chunk_end(png_ptr);
+
+    png_ptr->next_seq_num++;
+}
+#endif /* WRITE_APNG */
+
 /* Initializes the row writing capability of libpng */
 void /* PRIVATE */
 png_write_start_row(png_structrp png_ptr)
@@ -2778,4 +2879,39 @@
    }
 #endif /* WRITE_FLUSH */
 }
+
+#ifdef PNG_WRITE_APNG_SUPPORTED
+void /* PRIVATE */
+png_write_reset(png_structp png_ptr)
+{
+    png_ptr->row_number = 0;
+    png_ptr->pass = 0;
+    png_ptr->mode &= ~PNG_HAVE_IDAT;
+}
+
+void /* PRIVATE */
+png_write_reinit(png_structp png_ptr, png_infop info_ptr,
+                 png_uint_32 width, png_uint_32 height)
+{
+    if (png_ptr->num_frames_written == 0 &&
+        (width != png_ptr->first_frame_width ||
+         height != png_ptr->first_frame_height))
+        png_error(png_ptr, "width and/or height in the first frame's fcTL "
+                           "don't match the ones in IHDR");
+    if (width > png_ptr->first_frame_width ||
+        height > png_ptr->first_frame_height)
+        png_error(png_ptr, "width and/or height for a frame greater than "
+                           "the ones in IHDR");
+
+    png_set_IHDR(png_ptr, info_ptr, width, height,
+                 info_ptr->bit_depth, info_ptr->color_type,
+                 info_ptr->interlace_type, info_ptr->compression_type,
+                 info_ptr->filter_type);
+
+    png_ptr->width = width;
+    png_ptr->height = height;
+    png_ptr->rowbytes = PNG_ROWBYTES(png_ptr->pixel_depth, width);
+    png_ptr->usr_width = png_ptr->width;
+}
+#endif /* WRITE_APNG */
 #endif /* WRITE */