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

[Marius Predut]

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.

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 5f10b84..6cd342c 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,
+             * 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]) {
-- 
1.9.1