Calculations

[Grazer]

Hi, 
I use Calculations in Photoshop to create alpha masks.
Just wondering is there an equivalent somewhere in AP?

Thanks

Grazer

[James Ritson]

Hi Grazer, you can indeed do this in Photo. There are several approaches, one involving the Channels panel and blend modes, but here's something that is closer to your calculations approach. In this example we'll add the Red and Blue channels together, invert the blue channel and add an offset - seems this is used for alpha selections of hair?

1. Duplicate the image layer you want to create an alpha mask from. Uncheck/hide the original (so you can preview the alpha effect)
2. Filter > Apply Image
3. Click Use Current Layer As Source, then check Equations to enable channel equations
4. In DA (Destination Alpha), you can type this:
   1. SR+SB to add red and blue channels together
5. Now we need to invert the blue channel, so your equation becomes:
   1. SR+(1-SB)
6. Finally, to add an offset to the overall result, we can type:
   1. SR+(1-SB)-0.2
7. Click Apply and you'll now have a pixel layer with a modified alpha channel. At this point, you can either work on the pixel layer or go to Layer > Rasterise to Mask to convert it to a mask layer (which you can then drag inside other layers to mask them).

Hope that's what you're after and that you find it useful. In theory, you should be able to achieve most if not all of the Calculations behaviour using the channel equations in Photo's Apply Image dialog. Bear in mind that values in channel equations go from 0 to 1 and are in float. You can use expressions like lerp(SR, SB, 0.5) to linear interpolate between two channels, and as seen above use bracketed notation to isolate expressions.