[cairo] [PATCH 1/3] gl/msaa: Prevent stroke overlap

[Taekyun Kim]

We can avoid using stencil test for cases where drawing twice will 
produce same result.
For example, opaque solid source with op OVER.

Clearing entire stencil buffer for every stroke might be quite expensive 
for most of drivers
and GPUs. It would be good to restrict clearing area within bounded 
rectangle.

And have you measured the effect of glColorMask() in clipping code?
AFAIK, it was extremely expensive. (But I didn't measure it with recent 
GPUs and drivers.)

Here's my suggestion.

if (! used_stencil_buffer_for_clip) {
     glStencilFunc (GL_NEVER, ...); // Nothing will be drawn to the 
color buffer
     glStencilOp (GL_ONE, GL_ONE, GL_ONE);
     drawBoundedRectangle (); // Stroke extent
}

One last thing is that you consumes stencil buffer to prevent overlap 
thus it would be hard
to cache stencil buffer for a clip.

On 10/17/2011 11:56 PM, Henry (Yu) Song - SISA wrote:
>
> When stroking we do not send our polygon to the tessellator, so
>
> it may have overlapping stroke components. Use the stencil buffer
>
> to prevent stroke components from overlapping.
>
> ---
>
> src/cairo-gl-msaa-compositor.c |   26 ++++++++++++++++++++++++++
>
> 1 files changed, 26 insertions(+), 0 deletions(-)
>
> diff --git a/src/cairo-gl-msaa-compositor.c 
> b/src/cairo-gl-msaa-compositor.c
>
> index 7a0e828..01ef40f 100644
>
> --- a/src/cairo-gl-msaa-compositor.c
>
> +++ b/src/cairo-gl-msaa-compositor.c
>
> @@ -247,6 +247,27 @@ _stroke_shaper_add_quad (void                
> *closure,
>
>                                          quad);
>
> }
>
> +static void
>
> +_prevent_overlapping_drawing (cairo_clip_t     *clip,
>
> +                       cairo_bool_t      used_stencil_buffer_for_clip)
>
> +{
>
> +   if (used_stencil_buffer_for_clip == FALSE) {
>
> +     /* Enable the stencil buffer, even if we have no clip so that
>
> +        we can use it below to prevent overlapping shapes. We initialize
>
> +        it all to one here which represents infinite clip. */
>
> +     glDepthMask (GL_TRUE);
>
> +     glEnable (GL_STENCIL_TEST);
>
> +     glClearStencil(1);
>
> +     glClear (GL_STENCIL_BUFFER_BIT);
>
> +     glStencilFunc (GL_EQUAL, 1, 1);
>
> +    }
>
> +
>
> +    /* This means that once we draw to a particular pixel nothing 
> else can
>
> +       be drawn there until the stencil buffer is reset or the 
> stencil test
>
> +       is disabled. */
>
> +    glStencilOp (GL_ZERO, GL_ZERO, GL_ZERO);
>
> +}
>
> +
>
> static cairo_int_status_t
>
> _cairo_gl_msaa_compositor_stroke (const cairo_compositor_t *compositor,
>
>                          cairo_composite_rectangles_t      *composite,
>
> @@ -261,6 +282,8 @@ _cairo_gl_msaa_compositor_stroke (const 
> cairo_compositor_t      *compositor,
>
>      cairo_gl_surface_t *dst = (cairo_gl_surface_t *) composite->surface;
>
>      struct _tristrip_composite_info info;
>
> +    cairo_bool_t used_stencil_buffer_for_clip = FALSE;
>
> +
>
>      if (antialias != CAIRO_ANTIALIAS_NONE)
>
>      return CAIRO_INT_STATUS_UNSUPPORTED;
>
> @@ -296,11 +319,14 @@ _cairo_gl_msaa_compositor_stroke (const 
> cairo_compositor_t      *compositor,
>
>      if (! _cairo_composite_rectangles_can_reduce_clip (composite,
>
>                                           composite->clip))
>
>      {
>
> +     used_stencil_buffer_for_clip = TRUE;
>
>      status = _draw_clip_to_stencil_buffer (info.ctx, &info.setup, 
> composite->clip);
>
>      if (unlikely (status))
>
>          goto finish;
>
>      }
>
> +    _prevent_overlapping_drawing (composite->clip, 
> used_stencil_buffer_for_clip);
>
> +
>
>      status = _cairo_path_fixed_stroke_to_shaper ((cairo_path_fixed_t 
> *) path,
>
>                                     style,
>
>                                     ctm,
>
> -- 
>
> 1.7.4.1
>
> --
>
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>
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>
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>
>
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