Alpha matte in C4D?
  • Hey guys,

    I'm fairly new to C4D, but am loving it so far... I'm just trying to sort out a technical issue. I have a scene with a boat in some water, and want to get an alpha matte of just the boat, but minus the section that's underwater. I kind of understand object buffers, but am unsure how to get just the one section of the boat and not the entire thing. Any help would be great!! Thanks....

    Part of what might be making this more difficult is how I have the file set up - the boat is in a Cloner object which is allowing the boat to bob up and down on the surface of the water, plus the water has a Formula on it to get a rolling wave effect. Yeesh...
  • I would give the boat a solid white texture and the water (and anything else touching the boat) a solid black and render out, should render pretty quickly and you can use that as an alpha matte..
     I'm sure there is a more technical correct way to do it but it's worked for me in the past (also, I'm fairly inexperienced with c4d too.. just bought some training so hopefully that will change!)
  • Hmmm... Good idea! I'll do that. I think the best way to accomplish that is to just enable the luminance on the material and make it pure white/black, rather than using the color setting. I think that is somewhat the equivalent to a constant material from other 3D packages.
  • Unless your water is transparent, you should be able to use an object buffer. An object buffer only gives you a matte for what's visible to the camera. But I've done things similar to what @JamesDohertyEsq suggested to get clean mattes or quicker renders for certain passes.
  • Huh... The object buffer seems to be working now - maybe I was doing something differently before. Thanks for that!

    Mike
  • OK, so I got multipass working - pretty sweet, actually. I generated my regular RGB pass, plus 2 object buffers and a depth pass. 

    Unfortunately, there don't seem to be many options for saving those off, unless I'm missing something. I chose a layered Photoshop file, which gave me my RGB image plus 3 alphas. Unfortunately, After Effects (which I'm comping this in) only reads 1 alpha channel per file. I quickly batched an Action in Photoshop to save the other channels as different files, but that just seems silly for a project that supposed integrates so well with AE. Is there a better way to do this?
  • @mdouglas In your render settings under Save for multipass, uncheck Multi-Layer File. That will save each pass out to a separate file. Also, I tend to use TIFF or PNG sequences instead of PSD. Seems to save a bit of space. You can also look at saving to an RPF file, but those can get quite big and the channels aren't anti-aliased.
  • Ah - that makes sense! Guess I should have realized that myself - I think having it called "Multi-Layer" threw me off, as some of these end up as channels. Yeah, TIFFs and PNGs can be smaller - good point. I haven't used RPF files before, but the lack of anti-aliasing doesn't sound very useful...

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