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Composite Modes

Up until this point if you wanted to blend a video or graphic layer with the layers below it the only way to do so was by changing it's opacity. This produces a linear blend of the pixel values from the top and bottom images based on the opacity percentage you assign. However, this also usually results in a muddy image which doesn't show any distinct details from either of the two images.

Composite modes also allow you to blend two layers together as well. However, instead of using a linear blend, each pixel in the top image is combined with the pixel below it based on a mathematical formula applied to their brightness and/or color values. The result is a composite image which contains portions of each image - and the resulting blend will change as the content of the videos change as well.

Understanding Pixel Values

Video is simply a series of still images played sequentially. Each of those still images is composed of three channels - red, green and blue (RGB). Each channel is simply a grayscale image which represents the intensity of that particular color at each pixel position. Every pixel in each channel of a layer has a numerical value which corresponds to it's color value. In 24 bit video (3 channels X 8 bits per channel) each pixel has a possible range of 256 values, where is O is completely black, and 255 is pure white. Applied to 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 & brightness from the top layer's pixels will be combined with half the color & brightness 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.

For example, instead of just adding half of the value of one pixel to half of the other, we could add the full values together, making 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 available in FCP's composite modes.

These modes may be familiar to you if you have done much work in Photoshop - it's layer modes work exactly the same way. Photoshop does have a wider variety of modes, but Final Cut's modes cover all the basics.

Applying Composite Modes

In order for composite modes to have any effect, you must have two or more layers containing video or graphics in your sequence. The layers are always blended from the top down, so with only two layers you will always apply the mode to the topmost layer. If you have more than two layers the second layer will be blended with the first using it's composite mode, and then each additionally layer will be combined with the result. You can apply as many different layers with different modes as you like, however each layer can only have a single composite mode applied to it, and you cannot change the mode used over time.

Assuming you have two layers of video in your sequence, select the top layer (V2) and then from the menu bar choose Modify>Composite Mode>Add ('normal' is the default mode and does not blend images at all). Your video should now blend with the layer below it, and everything should become brighter overall. Go ahead and repeat this process with some of the other modes to see how the resulting composite changes.

You can also access the composite modes using a contextual menu - simply right-click (or control-click with a single button mouse) on the clip that you wish to change, and then select composite mode from the contextual menu which pops up - a sub menu will drop down which allows you to select any of the modes.

To remove the effects of a composite mode simply repeat the above process but set the mode back to 'normal' and your video will no longer blend with the clips below it.

Modes can be used in conjunction with opacity - after applying a mode to a layer, try reducing the opacity of that layer to see how the composite changes.

Available Modes

A full description of each mode with illustrations is available in the FCP user manual, vol. III pages 363-366. This is available from the help menu in Final Cut Pro. I'll provide only a basic description of each here.
The default mode - no blending is performed.
Pixel values in the top image are added to values in the bottom, and the overall image becomes lighter. There is no value brighter than pure white, so white areas tend to wash out and lose detail. Black areas in the top layer become fully transparent.
The values of the topmost pixels are subtracted from the pixels below, resulting in a darker image overall. Areas that are black stay black.
Difference is like subtract, except that negative values (which would just show up as black in subtract) are translated to color values, which can result in unusual colors in dark parts of the image.
Multiplies the values of the top and bottom pixels together. Dark areas of the top clip stay visible, while light areas tend to become transparent, and the overall image becomes slightly darker.
Screen multiplies the inverse values of the top and bottom images together. The result is similar to add, but without the extreme brightening that add usually results in.
Overlay combines Multiply and Screen, using Screen for bright areas of the top image and Multiply for dark areas.
Hard Light
This applies the colors of the topmost clip to the ones below it without changing the overall brightness of the video.
Soft Light
Similar to hard light but with less intensity, so that the video below is less effected.
The top layers pixels are visible when they are darker than the pixels below, and they are transparent when they are lighter than the pixels below.
The opposite of Darken - the top pixels are visible when they are lighter than the pixels below, and the pixels that are darker than the layer below become transparent.
Travel Matte - Alpha or Luma
These use the layer below to create transparency in the selected layer, either based on the alpha channel of the lower layer or the brightness values. This usually involves having a third layer below - the selected layer becomes transparent based on the alpha or brightness values of the layer below it, and the layer below that one shows through the transparency.

I recommend reading through the FCP manual if you'd like a more detailed explanation of what each mode does, but the best way to really get a feel for them is to experiment by combining a variety of different footage with different modes.

There are a lot of ways in which these modes can be used - you can blend different layers together, blend a layer into itself, even blend a copy of a layer with effects applied to the original layer in order to control the amount or way the effect becomes visible. Think of them as a glue for sticking different components of your composites together - and be sure to experiment with them.