Until now, when we want to see through one layer to the layer below, we've been limited to directly manipulating transparency - either by adjusting opacity or using an alpha channel. Sometimes we'll want to go beyond just seeing through the layer and instead actually blend two or more layers together. To achieve this we use Layer Modes. Layer Modes in AE work the same as they do in Photoshop, so you may already be familiar with them.
Each pixel in each channel of a layer has a numerical value which corresponds to it's degree of color and/or brightness. O is completely black, and 255 is pure white. Used in the color channels this value determines how much of each particular color is present in that pixel.
When we do normal transparency we simply combine the pixels from two layers linearly. If we set opacity to 50%, then half the color from the top layer's pixels will be combined with half the color from the bottom layer's pixels. However, since the values we are combining are just numbers, we can combine them using many different mathematical formulas...layer modes are just different ways of blending pixels between two or more layers using math to combine the values of each pixel.
for example, istead of just adding half of the value of one pixel to half of the other, we could add both values together. This will make the overall image brighter. If instead you subtracted the top layers values' from the bottom layer then everything will look darker overall. You can add the values only if the top one is brighter than the bottom one, or subtract them only if the top one is darker, or vice versa. The values could be multiplied, you could add only the color values without changing the brightness, or you could multiply the color values only. There's many, many different possible ways to combine the pixels, and many of them are provided in AE's modes.
Applying Layer Modes
Each layer can have a different layer mode. Layer modes only affect the visibility of layers below the layer we are setting the mode for. The mode will affect the visibility of all layers below the current layer though, not just the one immediately below.
To set the mode for a layer:
- On your timeline, check the third column from the left. By default it will display a bunch of different checkboxes for each layer that allow you to turn on motion blur, change the preview quality, etc. At the bottom of the column you will see a button that says "Switches/Modes". Click on this and the column should change to show drop-down menus for each layer under the heading "mode". These should all say "normal" at this point.
- Choose the layer who's mode you want to change. Click on the drop-down mode menu for that layer. A menu will appear with 33 different mode options. Select the mode you want. The Comp window will update to show the effect of the new mode.
That's it...you can repeat the same process if you want to change the mode at any time.
The modes are loosly grouped in the menu by how they affect the image.
With 33 different modes I'm not going to try to explain them all here. Honestly, I only know exactly what about half of them are actually doing, mathematically speaking. Usually this gives me a good idea where to start depending on the effect I'm trying to achieve, but I then will often try out different modes until I find the one that's perfect for what I want.
Realistically, the best way to get familiar with them is to just start trying them and watching how the image changes.
If you want to learn more about how each mode actually combines the pixels, take a look at chapter 12 in your book - it has a list of all modes with an explanation of how each works.