Use an even smaller epsilon. Allow the user to avoid the epsilon business if...

Use an even smaller epsilon. Allow the user to avoid the epsilon business if they want to preserve their alpha values.

stb_image_resize.h

@@ -241,6 +241,15 @@ STBIRDEF int stbir_resize_region(  const void *input_pixels , int input_w , int
 #define STBIR_PROGRESS_REPORT(float_0_to_1)
 #endif
 
+// This value is added to alpha just before premultiplication to avoid
+// zeroing out color values. It is equivalent to 2^-80. If you don't want
+// that behavior (it may interfere if you have floating point images with
+// very small alpha values) then you can define STBIR_NO_ALPHA_EPSILON to
+// disable it.
+#ifndef STBIR_EPSILON
+#define STBIR_EPSILON ((float)1 / (1 << 20) / (1 << 20) / (1 << 20) / (1 << 20))
+#endif
+
 //
 //
 ////   end header file   /////////////////////////////////////////////////////
@@ -1035,6 +1044,7 @@ static void stbir__decode_scanline(stbir__info* stbir_info, int n)
 		break;
 	}
 
+#ifndef STBIR_NO_ALPHA_EPSILON
 	if (!(stbir_info->flags & STBIR_FLAG_PREMULTIPLIED_ALPHA))
 	{
 		for (x = -stbir__get_filter_pixel_margin_horizontal(stbir_info); x < max_x; x++)
@@ -1042,7 +1052,7 @@ static void stbir__decode_scanline(stbir__info* stbir_info, int n)
 			int decode_pixel_index = x * channels;
 
 			// If the alpha value is 0 it will clobber the color values. Make sure it's not.
-			float alpha = (decode_buffer[decode_pixel_index + alpha_channel] += (float)1 / 17179869184); // 1/2^34 should be small enough that it won't affect anything.
+			float alpha = (decode_buffer[decode_pixel_index + alpha_channel] += STBIR_EPSILON);
 
 			for (c = 0; c < channels; c++)
 			{
@@ -1053,6 +1063,7 @@ static void stbir__decode_scanline(stbir__info* stbir_info, int n)
 			}
 		}
 	}
+#endif
 
 	if (edge_horizontal == STBIR_EDGE_ZERO)
 	{
@@ -1220,7 +1231,8 @@ static void stbir__encode_scanline(stbir__info* stbir_info, int num_pixels, void
 	int x;
 	int n;
 
-	if (!(stbir_info->flags&STBIR_FLAG_PREMULTIPLIED_ALPHA)) 
+#ifndef STBIR_NO_ALPHA_EPSILON
+	if (!(stbir_info->flags&STBIR_FLAG_PREMULTIPLIED_ALPHA))
 	{
 		for (x=0; x < num_pixels; ++x)
 		{
@@ -1234,9 +1246,10 @@ static void stbir__encode_scanline(stbir__info* stbir_info, int num_pixels, void
 					encode_buffer[pixel_index + n] *= reciprocal_alpha;
 
 			// We added in a small epsilon to prevent the color channel from being deleted with zero alpha. Remove it now.
-			encode_buffer[pixel_index + alpha_channel] -= (float)1 / 17179869184; // 1/2^34 should be small enough that it won't affect anything.
+			encode_buffer[pixel_index + alpha_channel] -= STBIR_EPSILON;
 		}
 	}
+#endif
 
 	switch (decode)
 	{