Restore any plate's original grain | Edited

Written by Mohamed Selim on .

How to restore the plate’s original grain after pulling a key



This technique is meant for instances where you might want to use a keyer’s pre-multiplied output instead of copying the finished alpha back to the original plate.



Most of the time we want to remove the noise/grain from our plates before attempting to a pull a key. This makes a cleaner alpha and we usually try to restore that grain later on by matching the plate’s original grain.



The problem is no matter how much a fine match you have done, you probably still lost some details and you end up with considerable softness in your plate.



Let’s take a look on how we can approach this problem. We have a green screen plate courtesy of Hollywod Camera Works website.




We then add a Denoise node and sample a flat area of the green screen. 




Notice now the difference and how we introduced some softening. 











I put in a pre-made key for this shot which is basically a set of nodes grouped together. This gives me a pre-multiplied result straight from the keyer that I’m happy with.




We now want to add 2 Copy nodes and copy our finished alpha to our original plate as well as the denoised version and finally premult both.



Notice the difference between the 2 approaches.




Copying the alpha back requires some additional manual work to despill and refine the edges. (Which in some cases is the best approach)


Now let’s add a merge node with a minus operation for our original pre-multiplied plate and the denoised one.



If we view this with pumping up the viewer gamma a bit we see what we originally took out and lost by denoising our plate. It’s not just pure noise/grain, but also details.



We can add another minus node for the denoised plate and the key. This is actually NOT needed but for viewing purposes as it shows us what the keyer took out automatically for our edges.




Add a merge node and plus the lost noise/grain and details back on to our key.




This not only works on keys but on some paint tasks. I say some because if there were lots of details on what was painted out you will probably add that back which I’m sure is not what you want.


This was all grouped in to a gizmo and can be found here.


Thanks and share if you think it's useful.







-2 # Joel Skeete 2016-11-17 16:06
Thanks for the tip. I was just wondering if you found this gives better results than flipping the 'Denoise' output to 'Noise' instead of 'Result'?
-1 # Mohamed Selim 2016-12-14 21:04
Sorry for not seeing this. Did not get a notification for some reason.

This just guarantees that your only dealing with the resulting pixels after you've done your key and not before. In other words, any pixels changed during the keying process will remain changed after adding the noise.

If you use the noise output as you mention this will be of course before you have pulled your key so they would introduce unwanted noise.

Also as mentioned above this technique only is useful if you plan on using your keyer's premultiplied output instead of copying the alpha to the original plate.

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