[RFC v2] surface crop & scale protocol extension

Rafael Antognolli rafael.antognolli at intel.com
Fri Nov 8 17:00:28 PST 2013

On Fri, Nov 08, 2013 at 10:59:07AM -0800, Bill Spitzak wrote:
> Pekka Paalanen wrote:
> >Hi all,
> >
> >this is the v2 of the crop and scale extension, as RFC.
> I get the impression that the result of crop+scale is supposed to be exactly
> the same as though the client made a second buffer of the scale size, scaled
> the crop region from the first buffer to this second buffer, then attached
> it with the normal wayland mechanism. Correct?

>From what I understood, the visual result might be that, but it's not
what should go on inside the renderer.

> So that compositors are allowed to only keep the cropped area of a buffer,
> there will have to be limitations on changing the crop+scale without also
> doing another attach. Maybe it does not work unless there is an attach in
> the same commit, or you might even require the attach to be after the
> crop+scale and before the commit.

IMHO the compositor would keep the entire buffer, just like it already
does. So when a buffer is attached to a surface, in the case of the gl
renderer, it would get entirely uploaded as a texture, and just in the
end when the texture is going to be rendered on the screen, only the
cropped area would be presented (in a scaled version, if that's the
case). This would allow to change the crop & scale parameters without
the need to a new attach.

Unless I'm wrong regarding the "buffer being entirely uploaded to the
compositor", but that's how I was implementing it.

> The big problem I see with this api is that if buffer_scale is not one, the
> client is unable to specify the crop or scale rectangles in pixels. However
> this is a general problem with buffer_scale everywhere. (actually you seem
> to be using fixed, not integers, so it is possible if buffer_scale is a
> power of 2). I would change the crop to be in pixels. The scale rectangle
> requires fixing or replacing the buffer scale mechanism.
> You need to define what happens if the crop region extends outside the
> buffer. I think the edge pixels will have to be replicated to fill, since
> using transparency here will turn an opaque buffer into one with
> transparency, and produce an anti-aliased transparent edge, both of which
> may defeat the ability to use hardware scaling.

It's defined as "undefined/dirty" pixels, isn't it? I think that's good
enough for now, at least.

> I think there may also have to be rules about whether filters are allowed to
> sample outside the crop region (there are things other than box filters, you
> know). For security reasons this must not be allowed outside the actual
> buffer, so that adjacent memory does not leak into the displayed image, but
> it could be left undefined for pixels between the crop and the buffer edge.

More information about the wayland-devel mailing list