- This extracts part of the image based upon the luminance or brightness of the image.
- It is very powerful and can create soft realistic edge detail easily.
- Excellent for shadow or highlight detail
- Limited particularly in the midrange where the exposure separation of the screen and the FG is negligible.
- Examine the separate RGB channels to see what colour separates your subject more effectively.
- Sometimes examining the different channels in a alternative colour space to RGB is effective in identifying seperation (HLS,YUV)
- Select a single RGB channel or combine channels together before preforming a luma key to get the best separation. Subtracting one channel from another can be effective
Building your own luma keyerThe luma keyer node is the same a scaling the monochrome RGB channels of an image and then copying them to the alpha channel.
Luminance EquationAs our eyes are more sensitive to different colours the luma matte must respond in a similar manner if all the RGB channels are used (so to get the approximate correct values for a colour monitor from "Digital Compositing for Film & Video" by Steve Wright 2002):
Highlight Edges Edges like backlit blonde hair are not defined enough. Luma key the higlights sharpen then do a additive merge back over the top
Keying in non-RGB colour spaces Doing a Luma key in the Luminance channel of HLS or YUV is good for DV footage as well as pulling good luma keys
Defocused & sharp edges that require to be keyed Create three (or more) keys based upon luminance levels in the plate. i.e. a Dark/mid/light (a combination of keys can be used.. say you used three difference keys they could be differenced from three colour slugs of the darkest/mid-tone/lightest areas of the plate.) The dark key goes on subtractively, the mids at 0.5 and the highlights as an additive key. The edges then blend naturally and keep the same as in the plate on shoft/hard edges that cross. It is important to note that you are keying in the image here in passes rather than building up a matte and then applying it. This makes it really important to keep checking the original plate so all the layers sit together and match it. Practically I have found that this technique is most advantageous in the highlight and shadows key passes, where the mid-tone pass is just more of a punch. The weakness of this is that it needs some adjusting for each individual shot, and colour correction on the FG of the plate is tricky as it is made up of several layers - although this can be overcome by creating a matte that is combined from the total of the different keys.