"Whisky with Wodka" - The Learnings / mask channels for grading... kill the key

Every project has its own learnings. For "Whisky with Wodka" we delivered mask channels for the further DI and grading workflow at ARRI Schwarzfilm in Berlin. In theory it's a brilliant idea. We just pipe out the mask channels we created for our comps (matte painting, sky / sea extraction, etc. - we have to create them anyway) and provide it for the DI guys. You can deliver up to four channels within one TGA file (simply written into red, green, blue and alpha channel). Lustre can easily read them (as far as they aren't compressed). After that you got exact masks for all CG elements - a wonderful idea.

In practice it meant more work on the delivery side of us (simply because of logistic). You need to check five times more data (RGB + four mask channels), before go out with it. I must admit that we had some shots in return because at the very beginning we simply didn't understood, what it means for our quality control. On the other hand there is a huge limitation of that idea, which we hardly noticed. So if you want to use mask channels for your grading session take care of the following aspect:

Mask channels are fine for detailed, hard seperation of things. Just one example: seperate the sky from the sea. Seperate the matte painting from the original image. This just works fine. You deliver hard edged alpha mattes. Perhaps the colourist blurs them a bit. Everything will be fine. BUT don't load in greenscreen key mattes for grading. We got into a trap there. The movie isn't released yet so I can't provide in production examples. But just try to get this: we prepared a window with greenscreen to replace it with a view over the Baltic Sea (so funny, cause they got money from Saxony so the exterior shots we're shot in Binz, while the interior shots we're shot in Saxony). Markus Hering crosses it with his fine scrubby hairs. The DOP wanted us to make the Baltic Sea (exterior) darker than we would deliver it in a final comp, so he can bring it as close as possible to overexposure in his grading session. By using mask channels it should be no problem. But what we all didn't got is that a key isn't simply a black and white mask channel. You often blend parts of the background into the edges, to bring it more together. Moreover in this case you got very fine shades of transparency in the alpha because of the fine hairs. Now if you take this alpha from the keyer into grading and boost the brightness a big bunch up the composite simply breaks apart. The keyer blends the dark (wrong) exterior into the fine hairs. Let's say tranparency is 10% at this point. Now you lift brightness up by using the alpha channel. Make it simple and say it'll be 10 units up. The Baltic Sea is 100% visible so the alpha is white. The brightness shift will be 10 units as wished. But the hair is only 10% visible. So it will be only lifted 1 unit. In result you will see, that his (well keyed) hairs are simply far too dark and the whole composite blows apart.

I'll try to add some pictures if the movie is released. But this learning costed me and Thomas Kaufmann one night of our lives (an me as a lead a bunch of nerves too). So just remember: mask channels can be useful but are tricky. Keep in mind that your VFX vendor will need a lot more time for quality check. They won't work for complex keys and composites and should never be used for bringing the parts of the composite together in grading. The adjustment has to be done in compositing. Your range in grading is 10%. If you exceed it you run the risk of breaking your composite into pieces.