[PATCH weston] compositor: add a masking mechanism to weston_layer

[Giulio Camuffo]

But the visible region of a surface wouldn't be a rect minus a rect,
but the intersection of two rects, which is always a rect as long as
they are both axis aligned, and surface rotation isn't supported
anyway by the rpi renderer, right? What's a rect minus a rect is the
weston_view::clip region, which is not the visible part of the surface
but the hidden part. So maybe having clip be the visible part would be
better for the rpi renderer, but anyway layer masking could still be
supported.

Still, this patch makes use of a mechanism already present in weston,
and which the rpi renderer doesn't respect. So it is a problem in the
rpi renderer or in the mechanism itself, not in the user of the
mechanism, imho.

Giulio

2013/12/10 Pekka Paalanen <ppaalanen at gmail.com>:
> On Tue, 10 Dec 2013 14:13:50 +0100
> Giulio Camuffo <giuliocamuffo at gmail.com> wrote:
>
>> 2013/12/10 Pekka Paalanen <ppaalanen at gmail.com>:
>> > On Tue, 10 Dec 2013 11:13:42 +0100
>> > Giulio Camuffo <giuliocamuffo at gmail.com> wrote:
>> >
>> >> 2013/12/10 Jason Ekstrand <jason at jlekstrand.net>:
>> >> > Giulio,
>> >> > Couple thoughts.  First, you don't provide an implementation of
>> >> > the clipping in  any of the renderers.  Probably have to wait on
>> >> > the Collabora people for the RPi renderer, but we should have
>> >> > pixman and gl implementations of this.
>> >>
>> >> There is no need to add support in the renderers for that. The
>> >> masking is done in view_accumulate_damage(): the part of the view's
>> >> boundingbox that doesn't fit in the mask is added to the view's
>> >> clip, and the renderers then clip that away already.
>> >
>> > Does this work if the renderer paints the whole surface regardless
>> > of damage? Rpi-renderer does that, since every surface is on its own
>> > overlay.
>> >
>> > The whole damage tracking is kind of unused on the rpi-renderer,
>> > since the firmware will probably redraw everything anyway.
>> > Prohibiting damage will not prevent parts of a surface from being
>> > painted. Damage is just a hint saying what is not necessary to
>> > repaint.
>>
>> No, the masking isn't done by not damaging the parts not in the mask,
>> but by adding the opposite of the mask in weston_view::clip. But i see
>> that the rpi renderer doesn't clip away that, so no, currently this
>> wouldn't work with it. I guess it could work though, right?
>
> We do not have arbitrary clip rectangles. We can only pick one
> arbitrary but axis-aligned rectangle from within the buffer to map onto
> the screen and that's it, just like your average overlay hardware.
>
> We could do more complex clips by using one dispmanx element (overlay)
> per rectangle of the final paint region, but that gets complicated, and
> even though there can be many elements, they are still a limited
> resource. I do not think this would be feasible.
>
> So if the surface can still be just a single rect on screen, overlaid
> with other surfaces in the z-order, then it would be possible. But I
> guess you'd have to separate occlusion clips from mask clips. Note
> though, that a rect minus a rect is not in general a rect but a region.
>
>
> Thanks,
> pq