Move GD binary transparency handling from Cloner to GifEncoder

[olivervogel]

No description provided.

[guillaume-sainthillier]

Found it: it isn't the GD version but how libgd is built. setup-php installs PHP 8.3–8.5 from the ondrej PPA, whose libgd3 (2.3.3-13+deb.sury.org) is built with libimagequant. PHP 8.6 runs against Ubuntu's own libgd3 (2.3.3-9ubuntu5), which isn't. With libimagequant, imagegif() turns truecolor images into a palette based on each pixel's alpha and ignores an opaque transparent color:

$gd = imagecreatetruecolor(2, 1);
imagesetpixel($gd, 0, 0, 0x000000FF);
imagecolortransparent($gd, 0x000000FF);
// after imagegif(): transparent index 1 with Ubuntu's libgd, -1 with the PPA build

A fully transparent color (alpha 127) survives with both, which is why the current Cloner approach (0x7FFFFFFF) passes. So the pixels that should become transparent need alpha 127 when imagegif() runs. This can be done natively, without iterating over pixels: flatten as in #1547, then restore alpha 127 where the source was fully transparent:

$mask = Cloner::clone($native); // copy incl. alpha
imagealphablending($mask, false); // the filters must not blend
imagefilter($mask, IMG_FILTER_BRIGHTNESS, 255); // rgb -> white, alpha kept
imagefilter($mask, IMG_FILTER_COLORIZE, 0, 0, 0, -126); // alpha 127 -> 1, others -> 0
imagefilter($mask, IMG_FILTER_COLORIZE, 0, 0, 0, 126); // -> 127 (transparent) or 126 (keep)
imagecolortransparent($mask, 0x7EFFFFFF); // imagecopy() skips the pixels to keep
imagealphablending($gd, false);
imagecopy($gd, $mask, 0, 0, 0, 0, imagesx($gd), imagesy($gd));
imagecolortransparent($gd, 0x7FFFFFFF);

I tested this with Ubuntu's libgd, the PPA build and PHP's bundled GD. Fully transparent pixels and sources with a transparent color (e.g. tile.png) stay transparent, and semi-transparent pixels blend with the background. Opaque pixels that equal the background color also stay opaque, which an opaque background key can't guarantee.

[olivervogel]

Thanks. This helps.