Imagick::paintOpaqueImage ignores translucent pixels - php

Question

I noticed Imagick::paintOpaqueImage only operates on pixels with an alpha value of 1. This leaves converted images with lots of leftover pixels of the colors I'm trying to replace. Take for example this test image
And this code to replace the blue pixels with red ones.
$img->paintOpaqueImage('rgb(12,0,245)', 'rgb(255,0,0)', 0);
The result is only the solid blue pixels are replaced.
Note all the pixels in that test image (aside from the fully transparent ones) are the same color blue. The only difference is the alpha value. Also note I've used a value of 0 for the $fuzz parameter. Originally I was bumping this way up before I discovered what the real problem was, and that was causing its own undesirable results.
I've hacked together a solution using ImagickPixelIterator; it clones the current pixel and sets the alpha value of both the current pixel and cloned pixel to 1 to coerce ImagickPixel::isSimilar to work agnostically with regard to alpha values.
$img = new Imagick('./test-paint-opaque-image.png');
$iterator = new ImagickPixelIterator($img);
$target = new ImagickPixel('rgb(12,0,245)'); // blue
$fill = new ImagickPixel('rgb(255,0,0)'); // red
$fuzz = 0;
foreach($iterator as $pixels) {
foreach($pixels as $curPixel) {
// Modify the alpha of the comparePixel so it won't throw off the isSimilar() check
$comparePixel = clone $curPixel;
$fOrigAlpha = $curPixel->getColorValue(Imagick::COLOR_ALPHA);
// Bail on fully transparent pixels
if($fOrigAlpha == 0)
continue;
// It seems the only way isSimilar will work is when the alpha is 1 for both pixels...
$comparePixel->setColorValue(Imagick::COLOR_ALPHA, 1);
$curPixel->setColorValue(Imagick::COLOR_ALPHA, 1);
if($comparePixel->isSimilar($target, $fuzz)) {
$curPixel->setColorValue(Imagick::COLOR_RED, $fill->getColorValue(Imagick::COLOR_RED));
$curPixel->setColorValue(Imagick::COLOR_GREEN, $fill->getColorValue(Imagick::COLOR_GREEN));
$curPixel->setColorValue(Imagick::COLOR_BLUE, $fill->getColorValue(Imagick::COLOR_BLUE));
// Set the modified alpha back to what it was after the color change
if($fOrigAlpha > 0) {
echo "Setting alpha to $fOrigAlpha\n";
$curPixel->setColorValue(Imagick::COLOR_ALPHA, $fOrigAlpha);
}
}
}
$iterator->syncIterator();
}
The result is a shiny red image, with translucent (and fully transparent) pixels preserved as I would like.
The main problem, and what finally brings me to my question is that this method is wicked slow. Is there a way to use PHP Imagick::paintOpaqueImage directly to make this sort of color transformation?

Answer 1

You can deactivate the alpha channel prior to replacing the color value then reactivate it again after with setImageAlphaChannel(), like so:
$img = new Imagick('./test-paint-opaque-image.png');
$img->setImageAlphaChannel(Imagick::ALPHACHANNEL_DEACTIVATE);
$img->opaquePaintImage('rgb(12,0,245)', 'rgb(255,0,0)', 0, false);
$img->setImageAlphaChannel(Imagick::ALPHACHANNEL_ACTIVATE);
$img->writeImage('./out.png');
Result: