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Build a Keyer
Build a Keyer
Deke Kincaid, updated 2005-08-11 00:43:49 UTC 81,938 views  Rating:
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Page 2 of 3

The Keyer

Still can�t believe how simple this technique is. We are going to be using the inverse of the blue channel of our image combined with red channel of our image to make a matte. Basically the inversed blue will do most of the work and the red channel will pick up some of the stubborn areas.

1. FileIn and load the clip

NB I jumped to frame 15 of the sequence. A wise compositor once told me that it was always bets to pull your key at the hardest or least desirable frame in the sequence. Here the woman on shot left has some nasty motion blur on her hand so I am guessing this is a good place to start. Remember that motion blurred areas should be partially transparent. As you work try not to loose all the detail in this area.

After you complete this tutorial you should probably go back cut a rough matte for these problem areas and re-key them separately, you can then recombine them with the larger key before the switchMatte node

2. Now make a color=> reorder node. Set its channels to �rrrn� ( red, red, red, none). Rename this node rrr_reorder for future reference.

3. Branch another reorder node from the fam19_bs node and set this ones channels to bbbn (blue, blue,blue,none ). Rename this node bbbn_reorder.

4. Now select rrr_reorder and create a color=>expand node. Name this redChannel_expand.

5. Select bbbn_reorder, create a color=>invert and then a color=> expand node. Name this inv_blueChannel_expand

We are going to use these expand nodes to clamp the luminance of their respective inputs. You can just do this by eye.

6. Open the �+� next to �Low Colour� and �High Colour� and select the �l� button for luminance, you can now use the slider. Conversely you can hold down the �l� key while dragging over the colour pot for luminance or the �o� key for overall

Basically you are just going to increase the low colour and decrease the high colour to act as a clamp on these values.

For the blue reorder you want to clamp the values such that the screen in the back ground goes black and our talent in the fg becomes white. Try to retain detail on the edges of the talent while getting rid of black regions on the talent.

Here are some values I prepared earlier :

inv_blueChannel_reorder :

You might notice some stubbornness around the woman�s shoulder :

Don�t panic too much about this now we will fix this up using the red channel.

7. Using the same technique as in the previous step, clamp the red channel up so that only the areas which were stubborn or previously full of black patches are now white.

Here are my values (feel free to tweak till you�re happy with the results )

8. Next create a layer=> screen node and put redChannel expand in the foreground input and inv_blueChannel_expand in the background input

9. Finally place another color=>expand node under this screen and rename this overall_expand. This node can be used as a bit of a fine tuner for semi-transparent areas and the like

The result will be your matte.

10. Create a layer=> switchMatte node and put fam19_bs into the foreground input and overall_expand into the background input. Set the matte channel to either r,g or b and set matteMult to 1 / on.

If we look at the result we can see we have a basic key, except there is a lot of blue spill and some edge problems: