white is duplicated in palette

Bug #67325

imagetruecolortopalette: white is duplicated in palette

Submitted:

2014-05-22 11:42 UTC

Modified:

2016-09-03 14:34 UTC

Votes:	2
Avg. Score:	4.5 ± 0.5
Reproduced:	2 of 2 (100.0%)
Same Version:	1 (50.0%)
Same OS:	1 (50.0%)

From:

icp at freigre dot com

Assigned:

cmb (profile)

Status:

Closed

Package:

GD related

PHP Version:

5.5.12

OS:

Private report:

CVE-ID:

None

[2014-05-22 11:42 UTC] icp at freigre dot com

Description:
------------
This bug has been already posted 2004 here:
https://bugs.php.net/bug.php?id=28949

But the bug is still alive in PHP 5.5!

I convert a truecolor image with 1000s of different colors into a palette image using

imagetruecolortopalette ($image, no, 256)

But the palette of the resulting image doesn't contain the 256 most appearing individual colors of the original truecolor image. Many of the palette colors are just white (RGB value: #FF FF FF).
For example just 120 colors are individual, the rest of 136 palette entries is white.
This results in a worse color reproduction of the palette image than it should be using 256 individual colors.

Please fix this bug, since imagetruecolortopalette is very important for creating small-filesized GIF and PNG images. Their filesize is dramatically lower than its truecolor counterpart while retaining satisfying output quality.

Patches

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[2014-11-08 07:42 UTC] remi@php.net

I can reproduce, with php 5.4.34 (bundled libgd) and 5.6.3RC1 (system libgd)

Result (with or without dither) have 187 different colors, 70 are white.

[2014-11-10 14:02 UTC] pajoye@php.net

Given that the default algo works with similar colors to create palettes, it sounds like we may not be able to fix it.



@remi have you tried using the new algo? Afair it is not exposed in php but it provides a way better quality in that regard.

[2016-09-03 14:34 UTC] cmb@php.net

-Status: Open +Status: Analyzed -Assigned To: +Assigned To: cmb

[2016-09-03 14:34 UTC] cmb@php.net

I can confirm the issue. The culprit has already been hinted at in
bug #28949, namely the change to cater to `total==0`[1]. Just from
looking at the code, the condition in the following line is
obviously wrong; we don't want to test `count` but rather `total`
to avoid the division by zero. After fixing this, some quick tests
confirm that there are no more duplicate palette colors.

As this issue also affects libgd, I'm going to fix it there first,
and will port the fix to PHP's bundled libgd.

> have you tried using the new algo? Afair it is not exposed in
> php but it provides a way better quality in that regard.

Actually, gdImageTrueColorToPaletteSetMethod() and
gdImageTrueColorToPaletteSetQuality() are not yet available in
PHP's bundled libgd. Seems it doesn't get boring. :-)

[1] <https://github.com/php/php-src/blob/PHP-7.0.11/ext/gd/libgd/gd_topal.c#L762-L774>
    <https://github.com/libgd/libgd/blob/gd-2.2.3/src/gd_topal.c#L661-L670>

[2016-09-03 14:34 UTC] cmb@php.net

-Summary: imagetruecolortopalette: many colors in palette are missing +Summary: imagetruecolortopalette: white is duplicated in palette

[2016-09-03 18:21 UTC] cmb@php.net

-Status: Analyzed +Status: Closed