Call coalesceImages() turns image white

[olivervogel]

I have noticed a possible error or behavior that I cannot understand.

If I read the following Jpeg and write it directly in the same format, I get an image with the same size and a completely white image when I call coalesceImages(). If I omit the call to coalesceImages(), the image is written as desired. With other Jpeg images I do not get this effect.

$imagick = new Imagick();
$imagick->readImage('st-john-xl.jpg');
$imagick = $imagick->coalesceImages();
$imagick->writeImage('out.jpg');

Image which leads to the described effect:
http://pictures.4ever.eu/data/download/buildings/bridges/st-johns-bridge,-forest,-hill-245439.jpg?no-logo

I also tried it directly with ImageMagick and could not reproduce the effect.

convert st-john-xl.jpg -coalesce out.jpg

Unfortunately, I cannot understand the effect of the white background. Is this intentional, could it be an bug or is the image corrupt?

System Information

System which leads to the described effect.

• PHP 8.2.14
• Host OS: Debian
• imagick module version 3.7.0
• ImageMagick 6.9.11-60 Q16 x86_64 2021-01-25

To make things even more complicated, I was not able to observe the effect in the following setup.

• PHP: 8.2.14
• Host OS: Mac OS 13
• imagick module version 3.7.0
• ImageMagick 7.1.1-24 Q16-HDRI aarch64 21856

Thank you very much and keep up the good work.

See also Intervention/image#1261