Season's Greetings and Happy Holidays to everyone!

Grain merge and Grain extract are my favorite layer modes. They're versatile and useful for many purposes - they're powerful, yet simple. They can be used for texturing, lighting, shading, beveling, embossing... many things.

Grain merge can basically be understood as converting the top layer's pixel values (0..255) into signed integers (-128..127), then adding them to the pixel values of the stack. Grain extract is the opposite of that: it converts the top layer to a signed int, but then substracts it from the top of the stack.

Basically, on grain merge, any value above medium (>128) makes the result lighter, while any value below medium (<128) makes the result darker. This is why it works so well for bevels and embosses: since grain merge/extract uses addition/substraction, the lightness of the base layer gets adjusted linearily, making it the most easily predictable, which makes it easy to figure out the correct values for your bevel/emboss/texture etc.

They also work per colour channel, so you can use grain merge also for linear colour adjustments. For example, making red 138, green 128 and blue 118 would modify the base layer so that red gets +10, green is unchanged and blue gets -10. There are loads and loads of effects and adjustments that you can make with only grain merge/extract and some math.

The downside is, that the effect may not be entirely "natural" looking for all applications, especially if you want to create realistic lighting schemes. For such applications, it's better to use soft light / hard light, or some combination of multiply/addition/screen.

Wow! Thanks for that explanation. Do you mind if I include a rough quote of that in my article?

Sure, go ahead.