Merging RGB channels back into a single image with ImageMagick (php)

Question

So I have a situtation with ImageMagick and php where I need to processes each of the RGB channels separately and then merge them back together into the final image. So in the code I have below, $red, $green and $blue are the channels (as gray scale images). The following code is what I have tried (and a couple variations) but each time I end up with an image that only has Cyan, Magenta or Yellow showing through. In this case, the resulting image is Cyan.

$im->removeImage();
$im->addImage($red);
$im->addImage($green);
$im->addImage($blue);
$img = $im->combineImages(self::CHANNEL_ALL);

$im->removeImage();
$im->removeImage();
$im->removeImage();
$im->addImage($img);

I think part of my problem is that the PHP documentation doesn't say much about how to use combineImages and there are no samples so far as I can find. So it's very likely that I'm using that particular method incorrectly, and I suspect it has to do with how I am combining the images in the single Imagick object to begin with.

EDIT

This question ultimately boils down to this: How do I recreate the following script using only php?

convert tmp_r.png tmp_g.png tmp_b.png -combine tmp_rgb.png

Answer 1

[EDIT] I have to admit, looking further into the documentation, Im not sure what the constant CHANNEL_ALL does. They do state that you can concatenate channels by logically ORing them together. You might try:

$im->combineImages(imagick::CHANNEL_RED | imagick::CHANNEL_GREEN | imagick::CHANNEL_BLUE);

[ORIGINAL] I've been looking into this API, and honestly what I think you are looking for is the convert function, NOT the combine function.

Look At the below provided link and click specifically on "Combining RGB Channel Images" http://www.imagemagick.org/Usage/color_basics/

Try that, leave a comment if you need further help :-)

Answer 2

So I think I've figured out how to get this to work. The missing piece was a call to flattenImages(). I'm not exactly sure why this worked, but it seems to be what I was looking for. Here's the code (keep in mind that $this is in the context of a member method of a class that extends Imagick):

$this->removeImage(); // gets rid of the old, unprocessed image
$rgb = clone $this;

$rgb->addImage($red);
$rgb->addImage($green);
$rgb->addImage($blue);
$rgb->flattenImages(); // this is what was missing above

$rgb = $rgb->combineImages(self::CHANNEL_ALL);

$this->addImage($rgb);

Can anyone comment on why this might be? I expected flattenImages() to merge the three images into one and destroy some of the information, but it appears that it actually tells ImageMagick to process all of the contained images together whereas it was processing them independently previously.

Answer 3

Try this:

$im->addImage($red);
$im->addImage($green);
$im->addImage($blue);
$im->combineImages(imagick::CHANNEL_RED | imagick::CHANNEL_GREEN | imagick::CHANNEL_BLUE);

btw combineImages doesn't return imagick object, but true/false indicating success or failure, so $im will contain your combined image.

Edit: Apparently combineImages sucks big time, so here's an alternative: imagick::compositeImage

$im->compositeImage($red, imagick::COMPOSITE_COPY, 0, 0, imagick::CHANNEL_RED);
$im->compositeImage($green, imagick::COMPOSITE_COPY, 0, 0, imagick::CHANNEL_GREEN);
$im->compositeImage($blue, imagick::COMPOSITE_COPY, 0, 0, imagick::CHANNEL_BLUE);