[Mesa-dev] [PATCH v2] i915/aa: fixing anti-aliasing bug for thinnest width lines

[Ian Romanick]

On 09/09/2015 06:59 AM, Marius Predut wrote:
> On PNV platform, for 1 pixel line thickness or less,
> the general anti-aliasing algorithm gives up, and a garbage line is generated.
> Setting a Line Width of 0.0 specifies the rasterization
> of the "thinnest" (one-pixel-wide), non-antialiased lines.
> Lines rendered with zero Line Width are rasterized using
> Grid Intersection Quantization rules as specified by bspec G45: Volume 2: 3D/Media,
> 7.3.13.1 Zero-Width (Cosmetic) Line Rasterization section.
> 
> This patch follow the same rules as patches fixing the
> https://bugs.freedesktop.org/show_bug.cgi?id=28832
> bug.
> 
> v1: Eduardo Lima Mitev:  Wrong indentation inside the if clause.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90367
> 
> Signed-off-by: Marius Predut <marius.predut at intel.com>
> ---
>  src/mesa/drivers/dri/i915/i915_state.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/src/mesa/drivers/dri/i915/i915_state.c b/src/mesa/drivers/dri/i915/i915_state.c
> index 4c83073..ebb4e9a 100644
> --- a/src/mesa/drivers/dri/i915/i915_state.c
> +++ b/src/mesa/drivers/dri/i915/i915_state.c
> @@ -599,6 +599,21 @@ i915LineWidth(struct gl_context * ctx, GLfloat widthf)
>     
>     width = (int) (widthf * 2);
>     width = CLAMP(width, 1, 0xf);
> +
> +   if (ctx->Line.Width < 1.5 || widthf < 1.5) {
> +     /* For 1 pixel line thickness or less, the general
> +      * anti-aliasing algorithm gives up, and a garbage line is
> +      * generated.  Setting a Line Width of 0.0 specifies the
> +      * rasterization of the "thinnest" (one-pixel-wide),
> +      * non-antialiased lines.
> +      *
> +      * Lines rendered with zero Line Width are rasterized using
> +      * Grid Intersection Quantization rules as specified by
> +      * bspec G45: Volume 2: 3D/Media,

Eh... Isn't G45 actually i965-class hardware?  GEN4.5, right?  I don't
see how a GEN4 bspec reference is useful (or correct?) in the GEN3
driver.  I think you want "2.8.4.1 Zero-Width (Cosmetic) Line
Rasterization" from volume 1f of the GEN3 docs.

I looked at section 2.8.4.3 Anti-aliased Line Rasterization of volume 1f
of the GEN3 docs that I have.  I don't see any mention of this
restriction.  Is it documented anywhere?

It also seems like this will affect non-antialised wide lines.  Is that
correct?

> +      * 7.3.13.1 Zero-Width (Cosmetic) Line Rasterization section
> +      */
> +      width = 0;
> +   }
>     lis4 |= width << S4_LINE_WIDTH_SHIFT;
>  
>     if (lis4 != i915->state.Ctx[I915_CTXREG_LIS4]) {