#62 Optimization leads to PNG corruption

[Computer Guru]

When optimizing the attached alpha transparency testcase.png (for verification, the sha1 should be 51c5d42e98cce3f04b451bfe1439aff2dcdd1f43, in case SF.net resizes), the result is a corrupted PNG (corrupted.png).

The compression log is below:

[I] mqudsi@QBook ~/Downloads> optipng -fix ./testcase.png
Processing: ./testcase.png
2168x1472 pixels, PNG format
4x8 bits/pixel, RGB+alpha
IDAT size = 209550 bytes
file size = 213359 bytes
Reducing:
3x8 bits/pixel, RGB
Trying: 8 combinations
zc = 9 zm = 9 zs = 0 f = 0 IDAT size = 87172
Encoding:
zc = 9 zm = 9 zs = 0 f = 0 IDAT size = 87172
Output:
IDAT size = 87172 bytes (122378 bytes decrease)
file size = 90825 bytes (122534 bytes = 57.43% decrease)

[I] mqudsi@QBook ~/Downloads> open testcase.png
[I] mqudsi@QBook ~/Downloads> optipng --version
OptiPNG version "Hg"
Copyright (C) 2001-2012 Cosmin Truta and the Contributing Authors.

Using opnglib version @OPNGLIB_VERSION@
Using opngreduc
Using libpng version 1.6.8
Using zlib version 1.2.8
Using pnmio version 0.3
Using minitiff version 0.1
Using cexcept version 2.0.1-optipng

This program is open-source software, distributed under the zlib license.
See LICENSE.txt for more details.

[hgysin]

This works fine with the current version (7.9.1)
does not create currupted output anymore.

---

$ optipng testcase.png
** Processing: testcase.png
2168x1472 pixels, 4x8 bits/pixel, RGB+alpha
Reducing image to 3x8 bits/pixel, RGB
Input IDAT size = 209550 bytes
Input file size = 213359 bytes

Trying:
  zc = 9  zm = 8  zs = 0  f = 0         IDAT size = 85874

Selecting parameters:
  zc = 9  zm = 8  zs = 0  f = 0         IDAT size = 85874

Output IDAT size = 85874 bytes (123676 bytes decrease)
Output file size = 89527 bytes (123832 bytes = 58.04% decrease)

---

$ optipng --version
OptiPNG version 7.9.1
Copyright (C) 2001-2025 Cosmin Truta and the Contributing Authors.

This program is open-source software. See LICENSE for more details.

Portions of this software are based in part on the work of:
  Jean-loup Gailly and Mark Adler (zlib)
  Glenn Randers-Pehrson and the PNG Development Group (libpng)
  Miyasaka Masaru (BMP support)
  David Koblas (GIF support)

Using libpng version 1.6.50 and zlib version 1.3.1