[Bf-committers] Color Unpremultiply

[Brecht Van Lommel]

Hi,

The problem is that it's not actually true that this is always the
correct thing to do. Ideally, you should never do color space
conversion on images with alpha, because you can only really do it
right after it has been composited over some background.

If you're compositing over a black background, leaving this option off
will give the exact correct result. If you're compositing over a
lighter background, enabling it will likely give better results, but
it's actually still not quite right then. It could be made so that
it's enabled by default, since maybe it's more common to composite
alpha images over a light background (e.g. for the web), but I don't
think we should remove the option.

Brecht.

On Fri, Feb 17, 2012 at 9:04 PM, gespertino at gmail.com
<gespertino at gmail.com> wrote:
> Hi. I have some questions regarding the new "Color Unpremultiply" option in
> Blender.
> I understand what it does, but I wonder if there isn't a more elegant or
> accurate way to expose it.
>
> First of all, the text in the tooltip is a little bit misleading. The
> operation performed isn't done to avoid a fringe on light backgrounds. It's
> done because colorspace conversions shouldn't be performed on premultiplied
> data.
> The resulting effect when doing it wrong is indeed a fringe, but what the
> operation does is to deliver a properly converted image, not avoiding a
> fringe.
> This may sound silly, but I think that using a more accurate tooltip would
> have an educational effect because people would know what it does or would
> investigate more rather than "this is for reducing a fringe".
> A fringe can come from a wrong compositing earlier, and believing that
> color unpremultiply will fix it is wrong.
>
> Apart from that, I wonder if it's really necessary to have that option
> instead of just applying it whenever premultiplied alpha is selected and
> the output file is gamma-corrected.
>
> Actually, I wonder if Blender shouldn't use premultiplied or straight
> (graying out sky) when output is RGBA and conversely offer just sky when
> output is RGB.
>
> I don't know if blender's internals allow this, but it would simplify the
> UI a little and help to avoid some common mistakes caused by mere
> distraction.
> Apart from that it would have an educational value making easier to new
> users to choose the right values.
>
> Makes sense?
> _______________________________________________
> Bf-committers mailing list
> Bf-committers at blender.org
> http://lists.blender.org/mailman/listinfo/bf-committers