Render elements and black fringe

[beenyweenies]

Hello all,

I'm having difficulty compositing some render elements. The shot is a very simple one - a "floating" product with an alpha channel, with a really shallow, fairly strong depth of field baked into the render. I'm outputting the following passes:

- GI
- Lighting (usually split out into 2-5 individual Light Select passes)
- Reflection
- Refraction
- Self Illum
- Specular

I then composite these within After Effects by applying the Add blend mode to all layers. The end result is visually identical to the standalone RGBA output, however there is a heavy black fringe around the edges that is not present on the RGBA pass. I experimented with a few different file formats and alpha types (straight alpha PNGs, premult TIFs etc) all in 16 bit. Nothing changed the outcome.

Is there some additional compositing step needed when working with render elements with an alpha?

[beenyweenies]

Here's an example of what I'm referring to. In both images, the product is rendered with alpha (in Maya, there is no set, just a few lights) and composited against the background seen here in AE.

Thanks in advance for any help or advice!

[Joelaff]

Looks like you need to go to interpret footage in AE and set it to interpret your footage as premultiplied with black.

This only shows up when you enable the layers that are ADDed on top? Then the images must have negative values, as add mode can only add, unless the source is negative.

Are you using some type of sharpening image filter that might cause negative ringing?

[beenyweenies]

Looks like you need to go to interpret footage in AE and set it to interpret your footage as premultiplied with black.

Definitely not this, that was the first thing I checked. Good suggestion though. This occurs with PNGs with straight alpha as well as TIFs with premultiplied alphas.

This only shows up when you enable the layers that are ADDed on top? Then the images must have negative values, as add mode can only add, unless the source is negative.

If I understand your meaning then yes. I am outputting an RGBA sequence along with the individual render elements, and the RGBA sequence is perfect - no fringe. Same settings, same file format etc. But when added together, the render element passes have the fringe shown above.

If I set AE to ignore the alpha on the passes entirely, and use the RGBA pass as a matte, the fringing is less noticeable but still there.

Are you using some type of sharpening image filter that might cause negative ringing?

[Joelaff]

Oh, I seem to recall some cases where AE stupidly interprets the alpha channel as having a gamma of 2.2 rather than being linear (or vice versa). (We use Fusion Studio, not AE). Check the color management settings, which I think are on in their own tab in the interpret footage dialog.

Your project is 32b bit float? Your images are 32bit float (or 16bit half float) ? I think that issue I mentioned is only with floating point images when the source is, e.g., an EXR that is saved (non-standardly) in gamma 2.2 space rather than linear space.

See if you can determine which layer is the culprit... or does it get darker with each successive layer added?

[Joelaff]

What if you clip the added layers to the layers below? Or umultiply the RGBA (divide the RGB by the alpha) then add everything, then multiply back by the alpha. All of this is much more transparent in a node based compositor like Fusion, but you can do it in AE.

[beenyweenies]

Interesting. I did a bunch of additional testing and you were right, applying an unmult to every layer in the stack except for the bottom layer (same idea as multiplying back but without extra steps) gives the exact result of the RGBA pass. Makes sense, I was just surprised by this as I've never seen it documented as a required step in working with render elements.

Thanks for the help, problem solved!