fix reading of palette PNG images (#937)

examples/decoder_png.cc

@@ -143,12 +143,16 @@ InputImage loadPNG(const char* filename, int output_bit_depth)
 
 
  /* Expand paletted colors into true RGB triplets */
- if (color_type == PNG_COLOR_TYPE_PALETTE)
- png_set_expand(png_ptr);
+ if (color_type == PNG_COLOR_TYPE_PALETTE) {
+ png_set_palette_to_rgb(png_ptr);
+ bit_depth = 8;
+ }
 
  /* Expand grayscale images to the full 8 bits from 1, 2, or 4 bits/pixel */
- if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8)
- png_set_expand(png_ptr);
+ if (color_type == PNG_COLOR_TYPE_GRAY && bit_depth < 8) {
+ png_set_expand_gray_1_2_4_to_8(png_ptr);
+ bit_depth = 8;
+ }
 
  /* Set the background color to draw transparent and alpha images over.
  * It is possible to set the red, green, and blue components directly
@@ -228,34 +232,15 @@ InputImage loadPNG(const char* filename, int output_bit_depth)
  }
  }
 
+ int band = png_get_channels(png_ptr, info_ptr);
+
  /* clean up after the read, and free any memory allocated - REQUIRED */
  png_destroy_read_struct(&png_ptr, &info_ptr, (png_infopp) NULL);
 
 
- // OK, now we should have the png image in some way in
- // row_pointers, have fun with it
-
- int band = 0;
- switch (color_type) {
- case PNG_COLOR_TYPE_GRAY:
- case PNG_COLOR_TYPE_GRAY_ALPHA:
- band = 1;
- break;
- case PNG_COLOR_TYPE_PALETTE:
- case PNG_COLOR_TYPE_RGB:
- case PNG_COLOR_TYPE_RGB_ALPHA:
- band = 3;
- break;
- default:
- assert(false); // , "unknown color type in png image.");
- } // switch
-
-
-
-
  struct heif_error err;
 
- bool has_alpha = (color_type & PNG_COLOR_MASK_ALPHA);
+ bool has_alpha = (band == 2 || band == 4);
 
  if (band == 1 && bit_depth == 8) {
  err = heif_image_create((int) width, (int) height,