Question

I’m attempting to convert some effects created in Photoshop into code for use with php/imagemagick. Right now I’m specifically interested in how to recreate Photoshop’s RGB levels feature. I’m not really familiar with the Photoshop interface, but this is the info that I am given:

RGB Level Adjust
  Input levels: Shadow 0, Midtone 0.92, Highlight 255
  Output levels: Shadow 0, Highlight 255

What exaclty are the input levels vs. the output levels? How would I translate this into ImageMagick? Below you can see what I have tried, but it does not correctly render the desired effect (converting Photoshop’s 0-255 scale to 0-65535):

$im->levelImage(0, 0.92, 65535);
$im->levelImage(0, 1, 65535);

This was mostly a stab in the dark since the parameter names don’t line up and for output levels the number of parameters don’t even match. Basically I don’t understand exactly what is going on when photoshop applies the adjustment. I think that’s my biggest hurdle right now. Once I get that, I’ll need to find corresponding effects in ImageMagick.

Can anyone shed some light on what’s going on in Photoshop and how to replicate that with ImageMagick?

Answer 1

So I stumbled across this website: http://www.fmwconcepts.com/imagemagick/levels/index.php

Based on the information there, I was able to come up with the following php which seems pretty effective at simulating what Photoshop does with input and output and all that.

function levels($im, $inshadow, $midtone, $inhighlight, $outshadow, $outhighlight, $channel = self::CHANNEL_ALL) {
    $im->levelImage($inshadow, $midtone, $inhighlight, $channel);
    $im->levelImage(-$outshadow, 1.0, 255 + (255 - $outhighlight), $channel);
}

This assumes that the parameters to levelImage for blackpoint and whitepoint are on a scale of 0-255. They might actually be 0-65535 on your system. If that’s they can it’s easy enough to adjust it. You can also check what value your setup uses with $im->getQuantumRange(). It will return an array with a string version and a long version of the quantum. From there it should be easy enough to normalize the values or just use the new range.