[Intel-gfx] [PATCH 0/4] Adding support for plane alpha/color blending through drm property

[Damien Lespiau]

On Fri, Mar 21, 2014 at 07:06:46PM +0530, Sagar Arun Kamble wrote:
> Hi Damien,
> 
> On Thu, 2014-03-20 at 14:45 +0000, Damien Lespiau wrote:
> > On Thu, Mar 20, 2014 at 02:11:40PM +0000, Damien Lespiau wrote:
> > > (source is premultiplied)
> > > 
> > > RGBA = ADD(SRC_COLOR*SRC_ALPHA, DST_COLOR*ONE_MINUS_SRC_ALPHA)
> > 
> > Grr, copy/paste error. If the source is already premultiplied:
> > 
> > RGBA = ADD(SRC_COLOR, DST_COLOR*ONE_MINUS_SRC_ALPHA)
> > 
> 
> 1. Currently there is no interface to advertise the DRM_FORMATS plane
> supportes to user mode? Should we add new IOCTL for that and include
> pre-multiplied alpha formats while advertising? Or am I missing any such
> API already available?

There's a 'formats' array in drmModePlane.

> 2. About constant alpha property - when we program constant alpha
> register will hardware be able to take care of the blending as per
> equations you have specified for non-premultiplied-alpha and
> premultiplied-alpha cases or we have to do any additional setting?
> Confusion is because of two combinations:
> a. pre-multiplied alpha+constant alpha
> b. non-pre-multiplied alpha+constant alpha

The first part of the question should be in the spec. I really do expect
the hw to work correctly with any combination of (premul, non-premult) x
(plane alpha, no plane alpha)

To be more clear: I don't think the glBlendFunc constants can represent
everything we want:

- It'd need to differentiate what we want do to between RGB and A (the
  same reason glBlendFuncSeparate was introduced).

- We can't represent blending of an unpremultied fb with a plane-global
  alpha with just the GL blending equation as it needs two
  multiplications (so in GL, you would do one of them in the fragment
  shader).

I would just proceed with a premultipled FB format and the alpha plane
property.

Let's wait until Ville returns, I may be totally not getting what he
meant.

-- 
Damien