Trouble with Blender Cookie's compositing tutorial (the Bartek Skorupa ones)

So I feel like I’ve learned a lot from the latest round of Bartek Skorupa’s compositing tutorials on Blender Cookie, but I’ve run into an issue with the refraction pass that I just don’t understand. I thought I’d set it up exactly the same as in the tutorial, but when I add the refraction pass, it seems to be adding black - how is it even possible to “add” black?

I’m a little perplexed. Attached is the .blend file. I’m not new to the compositor so much, but I am new to some of the techniques he’s prescribed in the tutorial, so I’m really not sure at which step along the way I did something incorrectly.

Any help would be much appreciated.


This issue will be covered in the next round. As you noticed - this video is almost an hour long and I simply didn’t manage to cover all of the things - refraction included.
First the good news: You didn’t do anything wrong.
Second - the bad news: There is not much you can do here. It simply won’t mix properly because of alpha and the way refraction pass is created in Blender’s internal engine.
Some users keep trying to find the way to deal with it, but in my opinion it’s not worth it.
My advice? If combining passes yourself - avoid using transparency. It will simply not mix in.
In my scene everything worked because I used alpha of 100%. Transparency was set via “frasnel”.
Every time when you use alpha different then 1 - your passes may not properly mix together.

We unfortunately have to choose: Either transparency or combining passes. We can’t have both.

aha, thank you so much for replying. i really appreciate getting this kind of information directly from the source! that’s really good to know, i was ready to resign myself to simply not using transparency at all in scenes where i thought i’d be doing extensive compositing. i’ll play around with changing only fresnel instead of alpha. and i guess i’ll need to bear this bit about alpha in mind when setting up my scenes to begin with from now on.

by the way, thank you so much for the compositing tutorials. they’ve been really enlightening, and i’ve really been looking forward for a while now to something compositor-related that’s this advanced.

also, Bartek, i was curious, now that i know you’re planning to do another round of tutorials on this topic - i run into very strange issues when compositing surfaces with SSS. will this be covered?

is there even a fix for that, or is this another one of those things we just need to find a way around or prepare for in advance?

Thank you very much.
By the way: Did you notice how it’s tagged? :slight_smile:

Re your last question: I don’t think I’ll say anything about SSS. From my experience - it also needs separate treatment. No pass store the SSS information, so again - no simple solution.

lol, i did not notice that; but i personally feel that’s being a bit modest. i realize there are a lot of tips and tricks to make the workflow faster for a beginner, but a few of these solutions offer a lot more control than any blender compositing tutorials i’ve seen elsewhere. i mean, ‘relighting’ the scene in the compositor? i’ve assumed there was a way, but haven’t seen anyone else effectively explain how.

i’m looking forward to the next round!

Well… It was not me who gave the tags :slight_smile: