`imagecropauto()` not working with transparent background

[ThomasLandauer]

Description

The following code:

<?php
$gdImage = imagecreatetruecolor(100,100);
imagesavealpha($gdImage, true);
imagefill($gdImage, 0, 0, imagecolorallocatealpha($gdImage, 0, 0, 0, 127));
// imagepng($gdImage, '/tmp/image_transparent.png');
$result = imagecropauto($gdImage, IMG_CROP_TRANSPARENT);
var_dump($result); // object(GdImage)#2 (0) { }
// imagepng($result, '/tmp/image_black.png');

Resulted in this output:

object(GdImage)#2 (0) { }

But I expected this output instead:

bool(false)

Docs at https://www.php.net/manual/en/function.imagecropauto.php are saying:

If the complete image was cropped, imagecrop() returns false

And this should be the case here IMO.

When doing the same with white background, it works as expected:

<?php
$gdImage = imagecreatetruecolor(100,100);
imagesavealpha($gdImage, true);
imagefill($gdImage, 0, 0, imagecolorallocate($gdImage, 255, 255, 255));
// imagepng($gdImage, '/tmp/image_white.png');
$result = imagecropauto($gdImage, IMG_CROP_WHITE);
var_dump($result); // bool(false)

PHP Version

PHP 8.3.1

Operating System

Kubuntu 22.04

[nielsdos]

imagecolorallocatealpha($gdImage, 0, 0, 0, 127) creates the color 0x7f'00'00'00, but the transparent color from gdImageGetTransparent that gdImageCropAuto compares against is 0xff'ff'ff'ff, but even after changing the arguments of imagecolorallocatealpha to imagecolorallocatealpha($gdImage, 255, 255, 255, 127) it doesn't work because then the color is 0x7f'ff'ff'ff, where the alpha is 0x7f instead of 0xff. (0x7f, aka 127, is strangely the highest allowed number, but that's gd-specific).
So that hints that the alpha channel value needs to be remapped.

I'm not entirely sure whether "full transparent cropping" with different rgb component should be considered the same, as I know that image editors can still display that information and the code as-is seems to be on purpose. However, the fact that the alpha channel seems to be compared incorrectly due to the 255 vs 127 mismatch seems like a bug to me.

The fix for that would be quite simple:

diff --git a/ext/gd/libgd/gd_crop.c b/ext/gd/libgd/gd_crop.c
index 676545c4db..769464f4dd 100644
--- a/ext/gd/libgd/gd_crop.c
+++ b/ext/gd/libgd/gd_crop.c
@@ -91,10 +91,6 @@ gdImagePtr gdImageCropAuto(gdImagePtr im, const unsigned int mode)
 	crop.height = 0;
 
 	switch (mode) {
-		case GD_CROP_TRANSPARENT:
-			color = gdImageGetTransparent(im);
-			break;
-
 		case GD_CROP_BLACK:
 			color = gdImageColorClosestAlpha(im, 0, 0, 0, 0);
 			break;
@@ -108,8 +104,9 @@ gdImagePtr gdImageCropAuto(gdImagePtr im, const unsigned int mode)
 			break;
 
 		case GD_CROP_DEFAULT:
+		case GD_CROP_TRANSPARENT:
 		default:
-			color = gdImageGetTransparent(im);
+			color = gdImageGetTransparent(im) & 0x7fffffff;
 			break;
 	}
 

although I'm not entirely sure if this is the right fix.
This would also need to be fixed in libgd upstream somehow, and the changes synced. But the annoying thing is that as far as I know there are fixes in PHP that are not in upstream libgd and vice versa, so I'm not sure how to go about this.

cc @devnexen You seem to be more familiar with libgd etc, maybe you can chime in quickly if you'd like 🙂

[devnexen]

Looking good at first glance, as far as upstreaming it, changing just the GD_CROP_TRANSPARENT case might fly better but that s just a guess, I never contributed to libgd.

[ranvis]

The following code works, so I believe that IMG_CROP_TRANSPARENT refers to the color previously specified using imagecolortransparent().
For the OP's intention, IMG_CROP_THRESHOLD would be better fit:

<?php
$gdImage = imagecreatetruecolor(100,100);
imagesavealpha($gdImage, true);
imagefill($gdImage, 0, 0, $c = imagecolorallocatealpha($gdImage, 0, 0, 0, 127));

$result = imagecropauto($gdImage, IMG_CROP_THRESHOLD, PHP_FLOAT_EPSILON, $c);
var_dump($result === false);

imagecolortransparent($gdImage, $c);
$result = imagecropauto($gdImage, IMG_CROP_TRANSPARENT);
var_dump($result === false);

Edit: changed arbitrary threshold value.

[ranvis]

My understanding is that the color for imagecolortransparent() refers to the color you explicitly specify to make that color appear transparent. At the time of image creation, the 'transparent color' is unspecified.
When using imagecropauto() and referring to this color, no cropping will occur if unspecified.

[nielsdos]

My understanding is that the color for imagecolortransparent() refers to the color you explicitly specify to make that color appear transparent. At the time of image creation, the 'transparent color' is unspecified. When using imagecropauto() and referring to this color, no cropping will occur if unspecified.

Yeah, that makes sense. The interaction with truecolor images is a bit weird though, but perhaps just a documentation problem as the expected behaviour is unclear.
See also #13183 (comment)

[cmb69]

I would argue that IMG_CROP_TRANSPARENT cannot refer to pixels with full alpha channel, since in that case the algorithm won't find all transparent pixels (because they may have different RGB values). So in my opinion, this is a documentation issue, although reading the description of IMG_CROP_DEFAULT:

Same as IMG_CROP_TRANSPARENT. Before PHP 7.4.0, the bundled libgd fell back to IMG_CROP_SIDES, if the image had no transparent color.

makes it clear that IMG_CROP_TRANSPARENT is actually about the transparent color (which is unrelated to the alpha channel).

Unfortunately, libgd never came around to stop offering direct manipulation of palette images, which causes this kind of confusion.