[PATCH xserver] glamor: Paint first and last pixel of lines

[Adam Jackson]

On Wed, 2017-02-08 at 14:44 +0100, Max Staudt wrote:
> On 02/07/2017 09:06 PM, Keith Packard wrote:
> > > > Max Staudt <mstaudt at suse.de> writes:
> > 
> > > OpenGL implementations are allowed to be imprecise in drawing line caps.
> > > This patch expands on the original workaround in dc9fa908.
> > 
> > Yeah, finding a way to work around GL differences seems like a good
> > idea. In this case, however, I think you're fixing some drivers and
> > breaking others -- when drawing with non-idempotent raster
> > ops.
> > 
> > Idempotent raster ops are those for which multiple draws generate the
> > same result, like GXcopy; hence non-idempotent operations are those
> > which do not, such as GXxor.
> 
> What does this mean in practice - is the manual pixel painting the
> problem you're seeing here, because it may draw over the previously
> drawn lines?
> 
> How does a GXor line draw operation currently work in GLAMOR? Maybe it
> would be easier to limit acceleration to GXcopy (which I suppose is by
> far the most useful case) and do that case as correct as possible?

On desktop GL, non-GXcopy rops work by setting glLogicOp appropriately.
 So on drivers that _do_ draw lines like the server's zero-width
implementation, the extra points your patch adds for the caps would be
drawn twice, and with GXxor that would mean they're painted and then
immediately reset.

I think, practically speaking, that glamor needs to match fb even along
the corner cases where the X spec allows driver-dependent behavior. The
simplest way to do this for this case would be to fall down to the mi
line code for the case of non-trivial cap style and rop that reads the
destination; if done correctly this will still be "accelerated" in that
we'll still hit glamor's SetSpans path.

A better, but more complicated, solution would be to use the fragment
shader to draw the line, where the GL primitive is just some quad that
happens to extend a bit past the boundaries of the X primitive so we
definitely hit every pixel we need to.

- ajax