[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
2.8.4.1 Zero-Width (Cosmetic) Line Rasterization from volume 1f of the GEN3
docs.
The patch was tested on Intel Atom CPU N455.

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.
v2: Ian Romanick: comments fix.

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..897eb59 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
+      * volume 1f of the GEN3 docs,
+      * 2.8.4.1 Zero-Width (Cosmetic) Line Rasterization. 
+      */
+      width = 0;
+   }
    lis4 |= width << S4_LINE_WIDTH_SHIFT;
 
    if (lis4 != i915->state.Ctx[I915_CTXREG_LIS4]) {
-- 
1.9.1