[Mesa-dev] [PATCH v4] i965/aa: fixing anti-aliasing bug for thinnest width lines - GEN7

[Predut, Marius]

Please "review by" feedback please :-)

-----Original Message-----
From: Predut, Marius 
Sent: Thursday, March 19, 2015 9:34 PM
To: mesa-dev at lists.freedesktop.org
Cc: Predut, Marius
Subject: [Mesa-dev][PATCH v4] i965/aa: fixing anti-aliasing bug for thinnest width lines - GEN7

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/gen7_sf_state.c |   21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/src/mesa/drivers/dri/i965/gen7_sf_state.c b/src/mesa/drivers/dri/i965/gen7_sf_state.c
index c9815b0..e33cb79 100644
--- a/src/mesa/drivers/dri/i965/gen7_sf_state.c
+++ b/src/mesa/drivers/dri/i965/gen7_sf_state.c
@@ -198,9 +198,24 @@ 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) {
+         /* 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;
+      }
       dw2 |= line_width_u3_7 << GEN6_SF_LINE_WIDTH_SHIFT;
    }
    if (ctx->Line.SmoothFlag) {
--
1.7.9.5