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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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Spill Supressor:

There are lots of ways to spill suppress. Some keyers have the function built in there are free macros and there is the spillSupress node (ugh!). To be honest my preferred method is to use the hueCurves node but I will cover in a later tutorial !

We are going to make our own spill suppressor with just a few reorders and a couple of transfer modes.

Because our footage is blue screen and because we will, in theory, be compositing it onto a medium luminance background, we need to construct a suppressor that will give medium edges without the tell-tale blue chrominance. To do this we will be using the brightness of the background of the green channel to replace the brightness values of the same area in our red and blue channels.

We will be turning our screen to medium grey. If we look at our green channel the screen is medium grey. So we must make the screen in all these channels the same medium grey. We do this by replacing the dark grey pixels of our red background and the bright blue pixels with the medium grey ones from our green channel; resulting in the same colour value for all the channels and hence �medium grey�

There is an easy way to do this. The min and max nodes, as the name suggest allow us to layer based on the maximum or minimum value of pixels in an image.

Confused? Good let�s proceed then.

11. Create a color=>reorder node. Rename it rrrn_reorder and set its channels to �rrrn�

12. Create a color=>reorder node. Rename it bbbn_reorder and set its channels to �bbbn�

13. Create a color=>reorder node. Rename it gggn_reorder and set its channels to �rrrn�

14. connect fam19_bs to each of these nodes.

15. select gggn_reorder and branch create a layer=>max node

16. repeat last step but now branch create a layer=>min node

17. place bbbn_reorder in the background input of min1 and rrrn_reorder in the background input of max1.

18. select fam19_bs and brach creat another reorder node set its channels to �ngnn� (none,green,none,none)

19. select max1 and create a reorder node and set its channels to �rnnn�

20. select min and make yet another reorder node set its channels to �nbnn�

The reason for this last step was to create three branches the one on the left has our new red channel, the one in the middle has our original green channel and the one on the right has a new blue channel. We will need to recombine these to get our final rgb image. Which we can now do will do this using the add function.

21. Select rnnn_reorder and creat a layer=> iAdd. Place ngnn_reorder into the background input.

22. Select the iAdd node and create another iAdd and put nbnn_reorder into the background input.

if we look at our resulting image we have a nicely de-saturated background and our talent has come out of the reordering and layering relatively unharmed.

Putting it together :

All that remains now is to multiply the colour suppressed image with the matte.

23. Select the final iAdd node, the one you just made in step 22 and place it into the input of the switchMatte node you made in step 10.

24. Create a colour node and set its r,g and b values to 0.5. Now select switchMatte and create a layer over node. Connect the colour node to the background input of this over node.


If we look at the results we see that there are still a few problems with the edges of the key. Also the woman�s hand at the left of shot is less than ideal. As previously stated you will probably have to go back and make a separate keyer for this portion of the image then recombine this with our larger key. I would encourage you to think of keying as more than just a one step process. You will generally have to make multiple keys and then combine them to produce the matte. In production this is almost always the case. For example in this shot I keyed the girl on the right separate to the other two actors and then went through and made separate keys for the motion blurred hands and some other fix-up�s to get more detail in the hair.

Making multiple keys work together and Edge treatments will be covered in a later tutorial.