[Mesa-dev] [PATCH] i965: do not round line width when multisampling or antialiasing are enabled

[Kenneth Graunke]

On Tuesday, June 09, 2015 08:49:54 AM Iago Toral Quiroga wrote:
> In commit fe74fee8fa721a we rounded the line width to the nearest integer to
> match the GLES3 spec requirements stated in section 13.4.2.1, but that seems
> to break a dEQP test that renders wide lines in some multisampling scenarios.
> 
> Ian noted that the Open 4.4 spec has the following similar text:
> 
>     "The actual width of non-antialiased lines is determined by rounding the
>     supplied width to the nearest integer, then clamping it to the
>     implementation-dependent maximum non-antialiased line width."
> 
> and suggested that when ES removed antialiased lines, they removed
> "non-antialised" from that paragraph but probably should not have.
> 
> Going by that note, this patch restricts the quantization implemented in
> fe74fee8fa721a only to regular aliased lines. This seems to fix the broken
> test without causing regressions in any of the dEQP line rasterization tests
> (dEQP-GLES3.functional.rasterization.*line*).
> 
> Fixes:
> dEQP-GLES3.functional.rasterization.fbo.rbo_multisample_max.primitives.lines_wide
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90749
> ---
>  src/mesa/drivers/dri/i965/gen6_sf_state.c | 5 +++--
>  src/mesa/drivers/dri/i965/gen7_sf_state.c | 6 ++++--
>  src/mesa/drivers/dri/i965/gen8_sf_state.c | 5 +++--
>  3 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/src/mesa/drivers/dri/i965/gen6_sf_state.c b/src/mesa/drivers/dri/i965/gen6_sf_state.c
> index e445ce2..b674268 100644
> --- a/src/mesa/drivers/dri/i965/gen6_sf_state.c
> +++ b/src/mesa/drivers/dri/i965/gen6_sf_state.c
> @@ -364,8 +364,9 @@ upload_sf_state(struct brw_context *brw)
>        /* OpenGL dictates that line width should be rounded to the nearest
>         * integer
>         */
> -      float line_width =
> -         roundf(CLAMP(ctx->Line.Width, 0.0, ctx->Const.MaxLineWidth));
> +      float line_width = CLAMP(ctx->Line.Width, 0.0, ctx->Const.MaxLineWidth);
> +      if (!ctx->Multisample._Enabled && !ctx->Line.SmoothFlag)
> +         line_width = CLAMP(roundf(line_width), 0.0, ctx->Const.MaxLineWidth);
>        uint32_t line_width_u3_7 = U_FIXED(line_width, 7);
>  
>        /* Line width of 0 is not allowed when MSAA enabled */
> diff --git a/src/mesa/drivers/dri/i965/gen7_sf_state.c b/src/mesa/drivers/dri/i965/gen7_sf_state.c
> index 58e3337..07b4394 100644
> --- a/src/mesa/drivers/dri/i965/gen7_sf_state.c
> +++ b/src/mesa/drivers/dri/i965/gen7_sf_state.c
> @@ -195,9 +195,11 @@ upload_sf_state(struct brw_context *brw)
>        /* OpenGL dictates that line width should be rounded to the nearest
>         * integer
>         */
> -      float line_width =
> -         roundf(CLAMP(ctx->Line.Width, 0.0, ctx->Const.MaxLineWidth));
> +      float line_width = CLAMP(ctx->Line.Width, 0.0, ctx->Const.MaxLineWidth);
> +      if (!ctx->Multisample._Enabled && !ctx->Line.SmoothFlag)
> +         line_width = CLAMP(roundf(line_width), 0.0, ctx->Const.MaxLineWidth);
>        uint32_t line_width_u3_7 = U_FIXED(line_width, 7);
> +
>        /* Line width of 0 is not allowed when MSAA enabled */
>        if (ctx->Multisample._Enabled) {
>           if (line_width_u3_7 == 0)
> diff --git a/src/mesa/drivers/dri/i965/gen8_sf_state.c b/src/mesa/drivers/dri/i965/gen8_sf_state.c
> index 52a21b6..6c765f0 100644
> --- a/src/mesa/drivers/dri/i965/gen8_sf_state.c
> +++ b/src/mesa/drivers/dri/i965/gen8_sf_state.c
> @@ -157,8 +157,9 @@ upload_sf(struct brw_context *brw)
>     /* OpenGL dictates that line width should be rounded to the nearest
>      * integer
>      */
> -   float line_width =
> -      roundf(CLAMP(ctx->Line.Width, 0.0, ctx->Const.MaxLineWidth));
> +   float line_width = CLAMP(ctx->Line.Width, 0.0, ctx->Const.MaxLineWidth);
> +   if (!ctx->Multisample._Enabled && !ctx->Line.SmoothFlag)
> +      line_width = CLAMP(roundf(line_width), 0.0, ctx->Const.MaxLineWidth);
>     uint32_t line_width_u3_7 = U_FIXED(line_width, 7);
>     if (line_width_u3_7 == 0)
>        line_width_u3_7 = 1;
> 

Why not drop the second CLAMP?  Given our current MaxLineWidth values,
roundf() of a value clamped to [0, MaxLineWidth] will remain in-range.

To enforce this, it would probably be good to add an assertion to
brw_initialize_context_constants():

   /* For non-antialiased lines, we have to round the line width to the
    * nearest whole number.  Make sure that we don't advertise a line
    * width that, when rounded, will be beyond the actual hardware
    * maximum.
    */
   assert(roundf(ctx->Const.MaxLineWidth) <= ctx->Const.MaxLineWidth));

With the second CLAMP dropped (or the suggestion refuted),
Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>

Thanks for fixing this, Iago!  Thanks for clarifying the spec, Ian!
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