Remove grey outline on transparent animations

fixes #87

src/render.cpp

@@ -40,6 +40,19 @@ void write_png(
  PNG_FILTER_TYPE_DEFAULT
  );
 
+ // remove outline on tranparent images. references:
+ // https://github.com/smohantty/ImageTest/blob/6427edb586fd9a2d92087e25a9ff0b361fc85870/imagetest.cpp#L83-L105
+ // https://github.com/Samsung/rlottie/issues/466
+ size_t total_bytes = width * height * lp_COLOR_BYTES;
+ for (size_t i = 0; i < total_bytes; i += lp_COLOR_BYTES) {
+ unsigned char a = buffer[i + 3];
+ if (a != 0 && a != 255) {
+ buffer[i] = (buffer[i] * 255) / a;
+ buffer[i + 1] = (buffer[i + 1] * 255) / a;
+ buffer[i + 2] = (buffer[i + 2] * 255) / a;
+ }
+ }
+
  unsigned char** row_pointers = (unsigned char**)png_malloc(png_ptr, height * sizeof(png_byte*));
  if (row_pointers == nullptr) {
  png_destroy_write_struct(&png_ptr, &info_ptr);