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Ok, lay those fine pictures aside just one moment, we must now find an efficient method to work with those big files. Fortunately, Photoshop has some useful functions that makes painting a joy instead of a pain! I assume that you have some knowledge of Photoshop. I won’t give all details for basic actions. For example, you’ll find out that it’s often necessary to deselect before doing another selection. Here we go!

### 3.1 Introduction When dealing with large surfaces and selections, I found channels to be very useful. They can be modified and painted like a grayscale layer, and can be loaded instantly as a selection. By default, the selected areas are drawn black, and shades of grey show some partial selection. Some useful operations can be done between channels, and a selection can be “saved” as a channel, even to another document (it must be the same size). Thus, to make thinks clearer, we will save/load all the selections to/from channels of a work document, and make changes to the shapes and create shadows inside this document. The first time you save a selection, go to SELECT > SAVE SELECTION…, select “document > new”, name the channel. This will automatically create a new grayscale document. For further saves, choose the document name (usually Untitled-1 by default) and a new channel (give it a name). You can stack the channels and delete them as soon as you don’t need them. #### Calculations with channels We’ll make extensive use of two similar Photoshop’s functions : IMAGE > CALCULATIONS and IMAGE > APPLY IMAGE. “CALCULATIONS” is when you want to save the result in a new channel; “APPLY IMAGE” is to modify the active channel. The operation is defined by the blending mode. We’ll use three of them. The results may seem paradoxical, but remember that it’s much like mathematic operations on each pixel, black’s value is 0 and white’s ~1 (255/256). Thus to “add” doesn’t mean that you put together all black surfaces. The addition is between the same pixel of each channel, and for instance, white added to black results in white. When you are not sure, refer to the examples given here :
Channels: | Alpha 1 | Alpha 2
Calculations : Effect on selection : | Multiply Alpha 1 with Alpha 2 Gathering of areas | Add Alpha 1 and Alpha 2 Intersection of areas
| Add reverse of Alpha 2 to Alpha 1 Clip Alpha 2 from Alpha 1 area |