[Bf-committers] Alpha clamping in render layers node

[Brecht Van Lommel]

Hi,

On Tue, Jan 17, 2012 at 10:09 AM, Lukas Tönne
<lukas.toenne at googlemail.com> wrote:
> Troy Sobotka has asked me in IRC if it would be feasible to clamp
> alpha values output in the render layers node.
>
> The problem is that AA filtering leads to alpha values with both a<0
> and a>1, which usually requires the artist to add additional math
> nodes to clamp those values to 0..1 range. While Brecht's alleged
> opinion (correct me if i'm wrong) is that this is not a problem since
> compositing should be done with full samples, it is apparently a nasty
> workflow.

My opinion is not that's a not a problem, but that it's not a bug so
should not be in the bug tracker.

> So here's a patch that adds a "clamp alpha" option to render layers
> node. The button for this setting is located in the node editor side
> bar, instead of the node itself to avoid clutter (case of "set once,
> don't touch again"). It is enabled by default for new nodes, but
> disabled for old files (backward & forward compatible).

I think clamping alpha to a valid range way can always be done, no
option is needed. However, it needs to be done on any render output,
not in a compositing node. For example saving directly to EXR without
nodes enabled or doing full sample re-compositing also needs this, so
the clamping code should elsewhere. Also note that both the combined
and color passes have an alpha channel, so ideally both would be
clamped.

Brecht.