[Bf-committers] Alpha changes

David erwin94 at gmx.net
Sun Jan 27 18:03:58 CET 2013

Hi all,

first of all, I want to say that I like the more unified approach to alpha
handling that is currently in trunk. But I feel there are still some issues
that will cause confusion for users:

Quick recap first:

- float buffers are assumed to be premultiplied/associated alpha
- byte buffers are assumed to be unpremultiplied/unassociated/straight alpha

This works well for most calculations and file outputs, and where it does not
fit a conversion can be added.

The issues arise only for display: If you want to have a valid composite
output, it should be a premultiplied image (as the compositor works with float buffers).
This leads to the correct display in the image viewer "Color and Alpha" mode,
and all file outputs are correct as well. But the "Color" mode of the
image viewer is weird: currently it premultiplies the unpremultiplied byte buffer,
but what it really should display is the rgb color
values without taking alpha into account at all. This is slightly inconsistent
with the rules above: the image viewer uses a byte buffer for display, but what
I expect to see is the unmodified premultiplied values. This is for a couple of

- backwards compatibility: 2.65 and before showed this in the "Color" mode.

- actually seeing whats in your image: at the moment you have no chance to
  see what is in your image in areas where alpha is 0 (while this might seem
  intuitive at first there are valid reasons to keep colors in 0 alpha areas
  and this is a common compositing technique for illuminating but transparent

- not distorting colors for alpha > 1: if alpha > 1 the unpremultiply on the
  way to the byte buffer darkens this part of the image and the premultiply
  of the image viewer cannot undo this, as alpha is clamped by then.

- and at last: I would expect a "Color" display mode (as opposed to a "Color and Alpha" mode)
  to NOT take alpha into account

My current workaround is to have an "set alpha" with a alpha value of 1 at the
end of my composites that I disable for "Color and Alpha" mode and enable for
"Color" mode. While I like the changes to alpha handling, this is not much better
than the unpremultiply that was necessary in 2.65 and before for a correct "Color
and Alpha" mode. And as this has been refactored anyway for 2.66, we should strive
to get it correct.

till then, David.

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