Vertex clipping and primitive types

Sam Spilsbury smspillaz at
Wed Sep 18 21:30:21 PDT 2013

Hi Bill,

On Sep 19, 2013 11:49 AM, "Bill Spitzak" <spitzak at> wrote:
> Sam Spilsbury wrote:
>> Hey all,
>> I've started playing around with trying to get weston surfaces (or
>> views as they will later be called) to use arbitrary vertex meshes.
> Do you have an explanation for "views"? I thought wayland already
supported putting the same buffer in more than one surface, so I don't see
the purpose of "views".

"Views" as in Jason Ekstrand's proposal. Its more or less orthogonal to
this (but just a clarification as to where things live in the stack).

> This proposal is for warping windows, not for just making non-rectangular
shapes, right? I think non-rectangular shapes are intended to be done by
using transparent pixels in the buffer.

Correct - although this is done on the compositor side and not by clients.
It is used to implement more complex animations like wobbly windows.

>> Currently, the vertex clipping implementation supports up to eight
>> vertices and assumes that they are arranged in a fan (eg
>> GL_TRIANGLE_FAN)[1]. I'm pretty sure that most geometry can be covered
>> as long as we support GL_TRIANGLES (disjunct triangles),
>> GL_TRIANGLE_STRIP (quadrilaterals or other sequential meshes) and
>> GL_TRIANGLE_FAN (circles).
> You don't intend to clip arbitrary shapes against each other, right? You
just want to go in after the clipping has produced these 8 triangles.

The point is to any arbitrary geometry against a rectangle. This is used to
ensure that redraw areas only stick to damage areas.


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