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ImageMagick / Imagick: Convert a PNG with Alpha channel into a 2 color image (Color, Transparent)

I would like to know and find out, how I can colorize/replace any pixel of an image, that is not (fully) transparent with an opaque pixel.

For example, having a multicolored Logo with transparent pixels, I would like to convert it in to a logo with only the color #ff0000, and not change the transparent background.

I want to achieve this with the PHP Imagick Library. I cannot find any good documentation.

I thought that Imagick::thresholdImage would be a helper, but there is no documentation about the threshold parameter.

Best results are achieved with this fragment of code. But still not working perfectly. Some pixels - i guess those with alpha > 0 and < 1 are not replaced.

$image = new \Imagick($source);
$image->setImageFormat('png');

$fill = new \ImagickPixel('#ff0000');
$image->thresholdImage(0);
$image->paintOpaqueImage('#ffffff', $fill, 1);

$image->writeImage($destination);
Solution

  • I would like to know and find out, how I can colorize/replace any pixel of an image, that is not (fully) transparent with an opaque pixel.

    You almost certainly don't.

    The code below does what you are asking (I think) and the output looks terrible. Perhaps you should give an example input image, and a hoped for example output image, that you have edited in Photoshop, to show what you were hoping for.

    Input image:

    enter image description here

    Output image:

    enter image description here

    $imagick = new Imagick("fnord.png");

// Get the alpha channel of the original image.
$imagick->separateImageChannel(\Imagick::CHANNEL_ALPHA);

// Make all the colors above this pure white.
$imagick->whiteThresholdImage("rgb(254, 254, 254)");

// Make all the colors below this pure black.
$imagick->blackThresholdImage("rgb(254, 254, 254)");

// We want the mask the other way round
$imagick->negateImage(false);

$imagickCanvas = new \Imagick();

$imagickCanvas->newPseudoImage(
    $imagick->getImageWidth(),
    $imagick->getImageHeight(),
    "xc:rgb(255, 0, 0)"
);

// Copy the mask back as the alpha channel.
$imagickCanvas->compositeImage($imagick, \Imagick::COMPOSITE_COPYOPACITY, 0, 0);

// Write out the image.
$imagickCanvas->setImageFormat('png');
$imagickCanvas->writeImage("./output.png");