[libGD] #88 [Task opened] Bug loading png images in grayscale + alpha

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User who did this - Daniel Diaz (daniel.diaz)

Attached to Project - libGD
Summary - Bug loading png images in grayscale + alpha
Task Type - Bug Report
Category - General
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Normal
Reported Version - 2.0.34
Due in Version - Undecided
Due Date - Undecided
Details - Hi,

There is a bug in gd_png.c when dealing with a png in grayscale + alpha (color_type == PNG_COLOR_TYPE_GRAY_ALPHA). Tested version is last CVS (gd_png.c,1.23).

The existing does not make a difference between PNG_COLOR_TYPE_GRAY and PNG_COLOR_TYPE_GRAY_ALPHA. In the PNG_COLOR_TYPE_GRAY_ALPHA mode, row_pointers contain 2 bytes par pixel, (rowbytes = 2 *width) but existing code simply uses the first width bytes (forgetting the width next others). The result is a wrong image : only the left half is loaded in memory (but with wrong gray colors).

I attach an image.png file in this format. If you use:
  pngtogd image.png foo.gd
  gdtopng foo.gd new-image.png
you will see that new-mage1.png is wrong (see attached file).

A preliminar solution consists in adding the following code to gd_png.c:415 (just before the default:)

  case PNG_COLOR_TYPE_GRAY_ALPHA:
    for (h = 0; h < height; ++h) {
      int boffset = 0;
      for (w = 0; w < width; ++w) {
        register png_byte idx = row_pointers[h][boffset++];
        im->pixels[h][w] = idx;
        open[idx] = 0;
        // add code to deal with alpha at row_pointer[h][boffset]
        // for now we simply ignore it !
        boffset++;
      }
    }
    break;

Obviously I here simply ignore alpha bytes (I don't know how alpha channel is handled in libgd, sorry).


More information can be found at the following URL:
http://bugs.libgd.org/?do=details&task_id=88

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