Resizing images with `imagecopyresampled` from PHP GD

Question

Using the PHP GD library, I generate image resource that is opaque white, with some "holes" here and there, which are fully transparent disk shapes. I've attached it bellow, although you'll need to first save it and open it (in Windows) with the Image Preview application in order to see the holes in it, due to the blue-ish background. Otherwise you'll only see white.

The original image:

From that image resource, which is 2550 x 3000px, I need to create a smaller version. I do so using imagecopyresampled() . All is fine with the resulting image, with one exception: here and there, it contains grey pixels (RGB: 254,254,254):

The resized image:

Part of the code I use is bellow:

$previewPxWidth = $this->viewportWidth_;
$previewPxHeight = round($this->viewportWidth_ / $schematic['paper.aspect.ratio']);

$preview = imagecreatetruecolor($previewPxWidth, $previewPxHeight);
$noColor = imagecolorallocatealpha($preview, 255, 255, 255, 127);
imagesavealpha($preview,true);
imagefill($preview, 0, 0, $noColor);

imagecopyresampled($preview, $sheet, 0, 0, 0, 0, $previewPxWidth, $previewPxHeight, $sheetPxWidth, $sheetPxHeight);
imagedestroy($sheet);

header("Content-type: image/png");
header("Content-disposition: inline; filename=image.png");
imagepng($preview);

Where are those very light and (apparently) randomly positioned grey pixels coming from and how can I get rid of them?

Answer 1

尝试imageAlphaBlending（），但是ImageMagic是最好的方法。