simplify formula to make it more numerically stable

plugins/ImageReader/ImageReader.py

@@ -130,7 +130,7 @@ class ImageReader(MeshReader):
 
         if use_transparency_model:
             p = 1.0 / math.log(transmittance_1mm / 100.0, 2) # base doesn't matter here. use base 2 for fast computation
-            min_luminance = 2.0 ** ((peak_height - base_height) / p)
+            min_luminance = (transmittance_1mm / 100.0) ** (peak_height - base_height)
             for (y, x) in numpy.ndindex(height_data.shape):
                 mapped_luminance = min_luminance + (1.0 - min_luminance) * height_data[y, x]
                 height_data[y, x] = base_height + p * math.log(mapped_luminance, 2) # use same base as a couple lines above this