Merge Blend Modes
Probably the most commonly used Node in compositing is the Merge node. The way that it lays the background over the foreground is determined by a set of mathematics called blend modes, which are available via the 'operations' drop down menu, at the top of the Merge node properties panel. The default blend mode is 'over' in which the 'A' feed is laid over the 'B' feed, according to the values in its alpha. There are, however, a whole bunch of other blend modes. Some of these are very useful, some are less so.
Uses of the Blend Modes
Broadly: blend modes serve two purposes:
To combine mattes
It is often the case that in the process of pulling mattes from an image a single pull is not sufficient and several mattes needs to be combined into one. For example, a soft matte that preserves hair detail but has a noisy centre needs to be combined with a hard-edge matte that has a solid centre. Many of the blend modes serve this need very well ('max' and 'min' I have found most useful in this regard). See Combining Alphas for details of how this is done.
To augment picture values
If an image needs 'improving', then most of the time color nodes such a Multiply, Grade and ColorLookup will be used. However, sometimes blend modes are more effective, particularly in the passing of RGB information from one image to another. An example is when a random, color-noisy image is used to 'sweeten-up' a background using the overlay mode. This way of using blend modes is more often used in Photoshop compositing, but in perfectly viable in motion compositing. However, in Nuke the 'Video colorspace' parameter option will sometimes need selecting.
The Merge Blend Modes Parameters
Nuke respects color values that extend below black (i.e. less than zero) and above white (i.e. more than one). However, in Photoshop these values are ignored (or 'clipped'). Sometimes a Nuke blend will not behave as you expect it to unless you tick this value, which converts the operation to Photoshop style.
Some blend modes respect alphas (e.g. the 'over' mode), some don't. Sometimes this parameter value needs changing, sometimes it doesn't. Helpful much?
The following is a selection of blend modes. I have only listed the modes for which I have found uses and have tried to give an indication of how useful they are.
|Blend name||Short description||Real-world use||Use rating|
|atop||'Cookie-cuts' the BG by the FG.||Sometimes useful for combining mattes.||*|
|color-dodge||Lightens the BG according to FG colors. Does not affect BG black.||A soft, light-colored radial placed over the background will add a vivid highlight. Beautiful for landscape effects.||***|
|color-burn||Darkens the BG according to FG colors. Does not affect BG white. Kinda the opposite of color-dodge||Good for making things appear burned.||*|
|conjoin-over||A bit like over, but ignores the transparency of the FG.||Good for 'thickening up' fire.||*|
|difference||Shows the difference between the BG and FG.||Good for comparing frames and also can be used as a difference matte.||**|
|disjoin-over||If both the FG and BG have mattes and the two mattes are 'oposite' of each other (like jigsaw puzzle pieces) then the edges of the mattes are not fringed.||Good for compositing CG output where each element has a different hold-out matte.||*|
|hypot||Lightens the BG according to the values of the FG.||Good for adding reflections.||*|
|in||'Cookie-cuts' FG by BG.||Good for combining mattes.||*|
|mask||The oposite of in.||Good for combining mattes.||*|
|max||Compares FG with BG and shows the brightest.||Very good for combining mattes.||***|
|min||Compares FG with BG and shows the darkest.||Very good for combining mattes.||***|
|multiply||Multiplies FG and BG together.||The clasic shadow blend mode. Good for combining the CG shadow pass with a background.||***|
|over||The default merge operation.||Lays FG over BG according to matte of FG. Cant live without it.||****|
|overlay||Darkens and lightens the BG according to the values of the FG.||Good for adding texture and general ambient contrast.||**|
|plus||Lightens BG according to the values of the FG.||Good for adding light-sabre lights.||**|
|screen||Lightens BG according to the values of the FG.||Good for adding light-sabre lights.||**|
|under||Places the FG under the FG.||Good for straightforward compositing in instances where the primacy of the B feed needs preserving.||***|