[Mesa-dev] [PATCH] i965: increase the antialiased line width by 0.25

[Samuel Iglesias Gonsalvez]

The line antialiasing support in the HW doesn't compute properly
the pixel coverage by the line when line width is equal to 1.0.

This patch increases a little bit the line width making the piglit's
line-aa-width pass and fix some related bugs in applications like
1 pixel lines in octave plot aliased to 0.

This patch is based on Eric Anholt's work on bug 28832.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=28832
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=60797

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias at igalia.com>
---
 src/mesa/drivers/dri/i965/gen6_sf_state.c | 7 ++++++-
 src/mesa/drivers/dri/i965/gen7_sf_state.c | 7 ++++++-
 src/mesa/drivers/dri/i965/gen8_sf_state.c | 7 ++++++-
 3 files changed, 18 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 ec14be4..b46f09d 100644
--- a/src/mesa/drivers/dri/i965/gen6_sf_state.c
+++ b/src/mesa/drivers/dri/i965/gen6_sf_state.c
@@ -323,7 +323,12 @@ upload_sf_state(struct brw_context *brw)
 
    /* _NEW_LINE */
    {
-      uint32_t line_width_u3_7 = U_FIXED(CLAMP(ctx->Line.Width, 0.0, 7.99), 7);
+      float line_width = ctx->Line.Width;
+      if (ctx->Line.SmoothFlag)
+         line_width += 0.25;
+
+      uint32_t line_width_u3_7 = U_FIXED(CLAMP(line_width, ctx->Const.MinLineWidth, ctx->Const.MaxLineWidth), 7);
+
       /* TODO: line width of 0 is not allowed when MSAA enabled */
       if (line_width_u3_7 == 0)
          line_width_u3_7 = 1;
diff --git a/src/mesa/drivers/dri/i965/gen7_sf_state.c b/src/mesa/drivers/dri/i965/gen7_sf_state.c
index 7fe1435..49f85e9 100644
--- a/src/mesa/drivers/dri/i965/gen7_sf_state.c
+++ b/src/mesa/drivers/dri/i965/gen7_sf_state.c
@@ -191,7 +191,12 @@ upload_sf_state(struct brw_context *brw)
 
    /* _NEW_LINE */
    {
-      uint32_t line_width_u3_7 = U_FIXED(CLAMP(ctx->Line.Width, 0.0, 7.99), 7);
+      float line_width = ctx->Line.Width;
+      if (ctx->Line.SmoothFlag)
+         line_width += 0.25;
+
+      uint32_t line_width_u3_7 = U_FIXED(CLAMP(line_width, ctx->Const.MinLineWidth, ctx->Const.MaxLineWidth), 7);
+
       /* TODO: line width of 0 is not allowed when MSAA enabled */
       if (line_width_u3_7 == 0)
          line_width_u3_7 = 1;
diff --git a/src/mesa/drivers/dri/i965/gen8_sf_state.c b/src/mesa/drivers/dri/i965/gen8_sf_state.c
index 0693fee..eb3d8a3 100644
--- a/src/mesa/drivers/dri/i965/gen8_sf_state.c
+++ b/src/mesa/drivers/dri/i965/gen8_sf_state.c
@@ -120,7 +120,12 @@ upload_sf(struct brw_context *brw)
    dw1 = GEN6_SF_STATISTICS_ENABLE | GEN6_SF_VIEWPORT_TRANSFORM_ENABLE;
 
    /* _NEW_LINE */
-   uint32_t line_width_u3_7 = U_FIXED(CLAMP(ctx->Line.Width, 0.0, 7.99), 7);
+   float line_width = ctx->Line.Width;
+   if (ctx->Line.SmoothFlag)
+      line_width += 0.25;
+
+   uint32_t line_width_u3_7 = U_FIXED(CLAMP(line_width, ctx->Const.MinLineWidth, ctx->Const.MaxLineWidth), 7);
+
    if (line_width_u3_7 == 0)
       line_width_u3_7 = 1;
    dw2 |= line_width_u3_7 << GEN6_SF_LINE_WIDTH_SHIFT;
-- 
1.9.0