Noise on transparent area after resizing via ngx_http_image_filter_module

Question

Nginx

Hello.
There is a png picture, a significant part of it is completely transparent (when filling a transparent area in paint, the entire area is filled evenly). After resizing via ngx_http_image_filter_module, a subtle noise appears on the transparent area (seen again when trying to fill this area in paint). I didn’t find anything in the documentation about the quality of resizing png images, maybe someone faced a problem?

Answer 1

Old versions of GD had this problem with gdImageCopyResampled used in ngx_http_image_filter_module: grokbase.com/t/php/php-bugs/047pm8gqye/29315-new-bug-in-imagecopyresampled
Not sure if it was fixed, so they think it is such a feature of the algorithm. If it does not help, then I can advise you to play with the size of the resulting image. Powers of two should work just fine.