swscale/swscale: don't reject scaling when color parameters are not supported but conversion is not required

Values in csp, prim, trc, etc, are irrelevant if there's no conversion needed.

Reviewed-by: Niklas Haas <ffmpeg@haasn.xyz>
Signed-off-by: James Almer <jamrial@gmail.com>

libswscale/swscale.c

@@ -1452,7 +1452,7 @@ int sws_frame_setup(SwsContext *ctx, const AVFrame *dst, const AVFrame *src)
 
         src_ok = ff_test_fmt(&src_fmt, 0);
         dst_ok = ff_test_fmt(&dst_fmt, 1);
-        if ((!src_ok || !dst_ok)) {
+        if ((!src_ok || !dst_ok) && !ff_props_equal(&src_fmt, &dst_fmt)) {
             err_msg = src_ok ? "Unsupported output" : "Unsupported input";
             ret = AVERROR(ENOTSUP);
             goto fail;

libswscale/utils.h

@@ -98,12 +98,10 @@ static inline int ff_color_equal(const SwsColor *c1, const SwsColor *c2)
             ff_prim_equal(&c1->gamut, &c2->gamut);
 }
 
-/* Tests only the static components of a colorspace, ignoring per-frame data */
-static inline int ff_fmt_equal(const SwsFormat *fmt1, const SwsFormat *fmt2)
+/* Tests only the static components of a colorspace, ignoring dimensions and per-frame data */
+static inline int ff_props_equal(const SwsFormat *fmt1, const SwsFormat *fmt2)
 {
-    return fmt1->width      == fmt2->width      &&
-           fmt1->height     == fmt2->height     &&
-           fmt1->interlaced == fmt2->interlaced &&
+    return fmt1->interlaced == fmt2->interlaced &&
            fmt1->format     == fmt2->format     &&
            fmt1->range      == fmt2->range      &&
            fmt1->csp        == fmt2->csp        &&
@@ -111,6 +109,14 @@ static inline int ff_fmt_equal(const SwsFormat *fmt1, const SwsFormat *fmt2)
            ff_color_equal(&fmt1->color, &fmt2->color);
 }
 
+/* Tests only the static components of a colorspace, ignoring per-frame data */
+static inline int ff_fmt_equal(const SwsFormat *fmt1, const SwsFormat *fmt2)
+{
+    return fmt1->width      == fmt2->width      &&
+           fmt1->height     == fmt2->height     &&
+           ff_props_equal(fmt1, fmt2);
+}
+
 static inline int ff_fmt_align(enum AVPixelFormat fmt)
 {
     const AVPixFmtDescriptor *desc = av_pix_fmt_desc_get(fmt);