View Full Version : Alpha filtering vs RGB filtering

Hugh Macdonald
08-13-2007, 05:13 AM
Hi all,

I'm a compositior, and am working with some images that one of our Lightwave guys has given me.... Something very strange has cropped up, and I said that I, as he's a little busy, would ask about it....

We're having some slight issues with the alpha in the renders that he is giving me - it doesn't quite tie in properly to the colour channels (we're generally working with RGBA EXRs)

We did a test, rendering some cards as pure white, with the alpha of the cards that we're working with.

I'd expect the RGB and A channels to all give identical results from something like this.

Here're a couple of (zoomed in 4x) images of the result:

(On the left is the colour channels, and on the right is the alpha)

When the colour channels have information other than white in, this will obviously cause problems around the edges, which is where this first cropped up.

I don't know if it has anything to do with this, but the black areas of the renders also have some kind of really low-level noise (I'm assuming that this noise is all-over, but just hugely less noticeable in the colour areas)

Here are the two images above, brightened x200:


With the renders that we're doing at the moment, the best workaround is to render it out twice, once with everything set to white, and then use the R channel from the second render as the alpha for the first, and forgetting about the in-built alpha, but this still leaves the problem of the low-level noise, and will slows down render-time.

Anyone have any ideas what is causing both or either of these problems?


08-13-2007, 05:29 AM
The noise comes from activating the "Dither Intensity" in the processing tab. This should be off when rendering HDR formats for compositing.

Your alpha looks allright, I'd need to comp it to check, but remember, LW by default renders pre-multiplied alphas, so elements should be rendered on black.
If you want straight alphas, the format can be changed in the "Render Globals->Output" tab (Alpha Format).


Hugh Macdonald
08-13-2007, 05:42 AM
Thanks for the tip about the Dithering - that makes sense to me - I've mentioned that one...

With the alpha, if it's a white card with premultiplied alpha, MDiv-ing it should give a hard white edge at the point where the alpha goes to 0.

This is what I'd expect (MDiv node in Shake, with ignoreZero on, and brightness halved):

However, this is what I'm getting when doing this:


08-13-2007, 05:55 AM
With the alpha, if it's a white card with premultiplied alpha, MDiv-ing it should give a hard white edge at the point where the alpha goes to 0.

I'm not familiar with shake... but you mention white card with pre-mult alpha... does that mean you are rendering out a pre-mult alpha mapped onto geometry?
If that is the case it won't work. Either un-mult the alpha before mapping it, or render two passes, one RGB only, the other with the pre-mult alpha set as the surface colour, then use the second render as the alpha for the first.