[Mesa-dev] [PATCH v4] i965/aa: fixing anti-aliasing bug for thinnest width lines - GEN6

[Ian Romanick]

On 03/19/2015 01:23 PM, Marius Predut wrote:
> On SNB and IVB hw, for 1 pixel line thickness or less,
> the general anti-aliasing algorithm give up - 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 section 6.3.12.1 Zero-Width (Cosmetic) Line Rasterization.
> 
> v2: Daniel Stone: Fix = used instead of == in an if-statement.
> v3: Ian Romanick: Use "._Enabled" flag insteed ".Enabled".
>     Add code comments. re-word wrap the commit message.
>     Add a complete bugzillia list.
>     Improve the hardcoded values to produce better results.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=28832
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=9951
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=27007
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60797
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=15006
> 
> Signed-off-by: Marius Predut <marius.predut at intel.com>
> ---
>  src/mesa/drivers/dri/i965/gen6_sf_state.c |   22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/gen6_sf_state.c b/src/mesa/drivers/dri/i965/gen6_sf_state.c
> index f9d8d27..91b46d9 100644
> --- a/src/mesa/drivers/dri/i965/gen6_sf_state.c
> +++ b/src/mesa/drivers/dri/i965/gen6_sf_state.c
> @@ -367,9 +367,25 @@ upload_sf_state(struct brw_context *brw)
>        float line_width =
>           roundf(CLAMP(ctx->Line.Width, 0.0, ctx->Const.MaxLineWidth));
>        uint32_t line_width_u3_7 = U_FIXED(line_width, 7);
> -      /* TODO: line width of 0 is not allowed when MSAA enabled */
> -      if (line_width_u3_7 == 0)
> -         line_width_u3_7 = 1;
> +
> +      /* Line width of 0 is not allowed when MSAA enabled */
> +      if (ctx->Multisample._Enabled) {
> +         if (line_width_u3_7 == 0)
> +             line_width_u3_7 = 1;
> +      } else if (ctx->Line.SmoothFlag && ctx->Line.Width <= 1.49) {
                                                               ^^^^
Based on the previous version of the patch, isn't that supposed to be
1.0?  Does this really not cause any piglit regressions?

> +         /* For lines less than 1 pixel thick, 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 section 6.3.12.1 Zero-Width (Cosmetic) Line
> +          * Rasterization.
> +          */
> +         line_width_u3_7 = 0;
> +      }
>        dw3 |= line_width_u3_7 << GEN6_SF_LINE_WIDTH_SHIFT;
>     }
>     if (ctx->Line.SmoothFlag) {
>