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List Info Thread: Clarification of colour alpha + alpha channel blending semantics
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Clarification of colour alpha + alpha channel blending semantics |
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List Info Thread: Clarification of colour alpha + alpha channel blending semantics
[1]
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about | contact Other archives ( Real Estate discussion Medical topics ) |
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st.com>
babbled:
> Denis,
>
> I am currently trying to support a customer who wants
to use
> the EVAS (enlightenment) library on top of DirectFB, on
an
> STMicroelectronics set top box SoC. One of the things
that
> EVAS does when blitting images with alpha is switch on
> _both_ DSBLIT_BLEND_COLORALPHA and
> DSBLIT_BLEND_ALPHACHANNEL.
>
> I have been asked to support this blend flag
combination
> in hardware, in order to get the performance the
customer
> wants with the EVAS library, and am having difficulty
> producing the same results as the software rasterizer.
I have
> to admit that up to now, based on the one line
descriptions
> of these flags in the API doc, I had assumed they were
> mutually exclusive. But they are clearly not.
>
> Could you please clarify for me exactly what the
behaviour
> of DSBLIT_BLEND_COLORALPHA is, both on its own
> and in combination with DSBLIT_BLEND_ALPHACHANNEL.
> I am particularly interested in the case where the
source
> has an alpha channel and the RGB components are already
> pre-multiplied by the source alpha.
really? evas doesn't use premul-alpha in its software or gl
engines, and the
xrender engine converts to premul to keep xrender happy. (nb
- this is on the
todo list to change to premul internally).
> As an aside, is there a diagram anywhere that shows the
> whole pipeline and the operations done at each stage.
> That would be a very useful reference for driver
writers.
>
> Best Regards,
> -stephen
>
>
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