Here is another video I made to help illustrators paint in Photoshop. What is a good way to stay inside the lines while you paint? One of the tools you can use is the quick mask tool. It’s prominently featured in the main palette, but not that many illustrators use this mode.
–excerpt from my recent article published in the Journal of Natural Science Illustration Volume 44, No. 3–
This is the fastest way to create an alpha channel. A brush or an eraser is used to create quick masks. Any type of brush can be chosen, in any grayscale value.
To create a hard-edged shape for an outline, use a brush with 100% hardness, 100% opacity, and 100% black. When the desired shape has been painted in, click off Quick Mask Mode (toggle the keyboard shortcut: Q) to see the selection activated.
To add or subtract from a selection shape, return to quick mask mode (Q), and simply paint in any grayscale value. The eraser tool can be used just as the brush tool except that the grayscale is reversed: lighter value is more visible and darker value is less visible. Again, to have a softer brush, use a softer edged brush. Any aspect of the brush can be applied to the quick mask, but there can only be one quick mask per file at any time.