'Image Calculations' demystified

Started Feb 8, 2006 | Discussions thread
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'Image Calculations' demystified
Feb 8, 2006

Photoshop's "Image Calculations" is kind of a daunting and mysterious thing at first, but once learned it's quite an effective way to convert to black & white. Here I'll attempt to explain the important parts of it.

The only thing Calculations really does is to simulate the combination of two layers. The dialog is very complicated at first sight, but there are only 3 or 4 essential settings. There are three groups of controls: the first group controls the top layer, the second group controls the bottom layer, and the third group controls the blending mode and layer mask that are to be used for the top layer.

Here's the photo I'll be working on.

What I've decided to do for this image is use the Blue channel as a base layer, with the Red channel as the top layer with soft light blending and a layer mask derived from the Green channel. This was decided by trial-and-error in the Calculations dialog--indeed, the power of Calculations is the ability to experiment with different channels and blending modes very quickly. Here are the corresponding settings for the Calculations dialog, with the important ones circled.

And now the result. Notice that Calculations created a new alpha channel to store the result. If I want to process it further, I can copy and paste it as a new layer, or select it and convert to grayscale mode.

Since all Calculations really does is simulates two independent layers, I can get the same result "manually" by creating two layers from the color channels (Blue and Red in this case), setting the appropriate blending mode and layer mask. To prove my point, I've done it here. Take a look at the layers palette to see the details.

That's really all there is to it. After some final contrast adjustment and sloppy dodging and burning, it looks like this:

That's it! Have fun...


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