PHP Imagemagick convert greyscale to RGB

Question

I made a tool where people can upload photos and modify them, including desaturation, resulting in a greyscale image. I generate the final image using PHP's GD library.

When printing these images the colors appear wrong so using Image Magick I add a color profile.

This works great except for images that have been greyscaled. The color profile gets added, but when I open the image in Photoshop, it says "The embedded ICC profile cannot be used because the ICC profile is invalid. Ignoring the profile". In Photoshop the image is set to Greyscale rather than RGB, hence the attached RGB profile is wrong. I need it to be RGB.

I'm using the following code to add all the possible information in an attempt to make the image RGB:

<?php
$i = new Imagick();
$i->readimage('image.jpg');
$i->setimagetype(Imagick::IMGTYPE_TRUECOLOR);
$i->setimagecolorspace(Imagick::COLORSPACE_RGB);
$i->profileimage('icc', file_get_contents('AdobeRGB1998.icc'));
$i->writeimage($d);
$i->destroy();
?>

Does anyone know how to successfully set the image to RGB and attach the profile?

I did try the different methods and combinations for 'setImageProfile' and 'profileImage', also for colorspace and imagetype, but the result is always the same.

Answer 1

This worked for me to have it recognized as a truecolor image. Assuming $img is the Imagick object containing the greyscaled image, I check if it is indeed greyscale and then edit 1 random pixel and modify its red value by adding or substracting 5 values, depending on red being greater than 5 or not.

<?php
if ($img->getImageType() == Imagick::IMGTYPE_GRAYSCALE)
{
    // Get the image dimensions
    $dim = $img->getimagegeometry();

    // Pick a random pixel
    $x = rand(0, $dim['width']-1);
    $y = rand(0, $dim['height']-1);

    // Define our marge
    $marge = 5;
    //$x = 0;
    //$y = 0;

    // Debug info
    echo "\r\nTransform greyscale to true color\r\n";
    echo "Pixel [$x,$y]\n";

    // Get the pixel from the image and get its color value
    $pixel = $img->getimagepixelcolor($x, $x);
    $color = $pixel->getcolor();
    array_pop($color); // remove alpha value

    // Determine old color for debug
    $oldColor   = 'rgb(' . implode(',',$color) . ')';
    // Set new red value
    $color['r'] = $color['r'] >= $marge ? $color['r']-$marge : $color['r'] + $marge;
    // Build new color string
    $newColor   = 'rgb(' . implode(',',$color) . ')';

    // Set the pixel's new color value
    $pixel->setcolor($newColor);

    echo "$oldColor -> $newColor\r\n\r\n";

    // Draw the pixel on the image using an ImagickDraw object on the given coordinates
    $draw = new ImagickDraw();
    $draw->setfillcolor($pixel);
    $draw->point($x, $y);
    $img->drawimage($draw);

    // Done, 
    unset($draw, $pixel);
}
// Do other stuff with $img here
?>

Hope this helps anyone in the future.

Answer 2

@a34z says in a comment:

"Somehow I must let PS know it is an RGB image with only grey pixels in it or something like that."

It is a fundamental error to assume that an RGB image could even contain 'gray' pixels as such!

RGB images do have pixels that are always composed of a mix of 3 colors: R ed + G reen + B lue. These are the 3 channels which are available, no more. There is no such thing as a gray channel in RGB.

What makes an RGB image look gray to our eyes is the fact that each of the 3 numerical channel values are equal or less strictly speaking, at least 'similar enough'. Of course, there is also software that can analyze the color values of the 3 channels and tell you which pixels are 'gray'. ImageMagick's histogram output would happily tell you which shades of gray you would say and use different names for those Grays. But don't be fooled by that color name: the pixel will still be composed from 3 colors with the same (or very similar) intensities, and ImageMagick will also report these values.

If you really need a pure grayscale image (that uses only one channel for the level of gray, not three), then you have to convert it to such an image type.

The two images may still look the same (if the conversion was done correctly, and if your monitor is calibrated, and if your not red-green-blind) -- but their internal file structure is different.

RGB images need ICC profiles that deal with RGB (if any), such as sRGB. For grayscale you cannot use sRGB, there you may want to use DeviceGray or something...