[RFC weston] compositor: optimize/simplify shaders

[Pekka Paalanen]

On Mon, 27 Aug 2012 17:03:10 +0300
Pekka Paalanen <ppaalanen at gmail.com> wrote:

> Hi Rob,
> 
> I've started reviewing your patch and fixing the remaining bugs. So far
> I think I got most of the blend/opaque region stuff sorted out. I
> haven't still gotten to the geometry, where I can trigger visual bugs
> with and without getting your TODO printouts.
> 
> I'm guessing the xwayland surfaces will come and haunt us, because I
> think they contain both an opaque region with undefined alpha values,
> and opaque and non-opaque regions with valid alpha values.I haven't
> even tested them yet, but I guess a quick fix would be to paint opaque
> regions always with a shader that forces texture alpha to 1.0. Would it
> better as a different shader program or a uniform flag for a shader, I
> don't know.
> 
> I'm not sure we should have nested functions in Weston code base. Also
> like you noted, compositor_wayland.c does not build anymore.
> 
> Here's my WIP tree that may be rebased!

The tree is still here and updated:
http://cgit.collabora.com/git/user/pq/wayland-demos.git/log/?h=shaders
I don't think I will be rebasing it anymore.

I merged Rob's updates there.

Rob, I will leave cleaning up the series for you, once we are done. You
probably want to squash some things etc.

> I should get to reviewing the geometry tomorrow.

I started on geometry, and wrote a simple debug feature:
http://people.collabora.com/~pq/geometry-debug-2.png
http://people.collabora.com/~pq/geometry-debug-3.png
http://people.collabora.com/~pq/geometry-debug-4.png

It draws the triangle edges. It's quite fun in realtime, apart from
leaving a mess on the screen.

When I was reading through the x1==x2 and y1==y2 special cases in
calculate_edges(), I had the feeling the polygon winding order might
get wrong sometimes. Also looking at it on paper I was certain I should
be getting those TODO messages.

Turned out the (!es->transform.enabled) special case was hiding all
that. When forcing the transformed path, there's obviously something
not quite right:
http://people.collabora.com/~pq/geometry-debug-5.png

That is in my git branch, too.

There is also room for a simplification: as the intersection of two
rectangles is guaranteed to be convex, we don't need the "center point"
vertex, we can simply use any vertex on the perimeter. This should make
the usual case of an aligned rectangle to collapse into two triangles
instead of the current four, and work fine for all cases.

And I still haven't gotten to the complex parts yet :-P


Cheers!
- pq