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I need to convert an image (png) to (webp) file.

After uploading a png file, the webp image has been generated, but the webp file didn't copy the transparency of a png file, instead it creates a black background.

This is my php code:

$type = wp_check_filetype($file, null);
$ext = $type['ext'];
if ($ext === 'png') {
    $im = imagecreatefrompng($file);
    imagepalettetotruecolor($im);
    $webp = imagewebp($im, str_replace('png', 'webp', $file));
}
imagedestroy($im);

The version of PHP is 5.6

Answers

Tested on 7.3.0 -- works.

DISCLAIMER: May only work on later or some PHP versions.

Only tested on 5.6.15 (didn't work, black background) and 7.3.0 (worked, transparent background).

Here's the code:

// get png in question

$pngimg = imagecreatefrompng($file);

// get dimens of image

$w = imagesx($pngimg);
$h = imagesy($pngimg);;

// create a canvas

$im = imagecreatetruecolor ($w, $h);
imageAlphaBlending($im, false);
imageSaveAlpha($im, true);

// By default, the canvas is black, so make it transparent

$trans = imagecolorallocatealpha($im, 0, 0, 0, 127);
imagefilledrectangle($im, 0, 0, $w - 1, $h - 1, $trans);

// copy png to canvas

imagecopy($im, $pngimg, 0, 0, 0, 0, $w, $h);

// lastly, save canvas as a webp

imagewebp($im, str_replace('png', 'webp', $file));

// done

imagedestroy($im);  

Edit 1. *** PROOF

The PHP GD library relies on the libgd library.

Link:

https://github.com/libgd/libgd

Relevant code on saves (file: gd_webp.c), Excerpt showing respect of Alpha channel when present:

            c = im->tpixels[y][x];
            a = gdTrueColorGetAlpha(c);
            if (a == 127) {
                a = 0;
            } else {
                a = 255 - ((a << 1) + (a >> 6));
            }
            *(p++) = gdTrueColorGetRed(c);
            *(p++) = gdTrueColorGetGreen(c);
            *(p++) = gdTrueColorGetBlue(c);
            *(p++) = a;

In regards to static int _gdImageWebpCtx (gdImagePtr im, gdIOCtx * outfile, int quality)

The PHP code I presented relies on the fact that alpha is indeed respected in the GD library and as such works if tested in later PHP version than you are using, specifically I tested in 7.3.0 but may work in early releases after your version.