i965: fix point rasterization when rendering to FBO

The FBO pixel coordinate system, with (0,0) as the
upper-left pixel, is inverted in Y compared to the
normal OpenGL pixel coordinate system, which has
(0,0) as its lower-left pixel.

Viewport and polygon stipple are sensitive to this
inversion; so is point rasterization.  The basic
fix is simple: when rendering to an FBO, instead
of the normal RASTRULE_UPPER_RIGHT that's
appropriate for OpenGL windows, use the Y inversion
RASTRULE_LOWER_RIGHT.

Unfortunately, current Intel documentation has this
value listed as "Reserved, but not seen as useful".
It does work on at least some i965-class devices,
though; and the worst that could happen if an
older device didn't support it would be incorrect
point rasterization to FBOs, which is what happens
already, so this fix is at least no worse than what
happens presently, and is better for some (and possibly
all) i965-class devices.

2 files changed, 45 insertions, 1 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_defines.h b/src/mesa/drivers/dri/i965/brw_defines.h
index 590b064c7ef..74dbba4fdd1 100644
--- a/src/mesa/drivers/dri/i965/brw_defines.h
+++ b/src/mesa/drivers/dri/i965/brw_defines.h
@@ -225,6 +225,24 @@
 
 #define BRW_RASTRULE_UPPER_LEFT  0    
 #define BRW_RASTRULE_UPPER_RIGHT 1
+/* These are listed as "Reserved, but not seen as useful"
+ * in Intel documentation (page 212, "Point Rasterization Rule",
+ * section 7.4 "SF Pipeline State Summary", of document
+ * "Intel® 965 Express Chipset Family and Intel® G35 Express
+ * Chipset Graphics Controller Programmer's Reference Manual,
+ * Volume 2: 3D/Media", Revision 1.0b as of January 2008,
+ * available at 
+ *     http://intellinuxgraphics.org/documentation.html
+ * at the time of this writing).
+ *
+ * These appear to be supported on at least some
+ * i965-family devices, and the BRW_RASTRULE_LOWER_RIGHT
+ * is useful when using OpenGL to render to a FBO
+ * (which has the pixel coordinate Y orientation inverted
+ * with respect to the normal OpenGL pixel coordinate system).
+ */
+#define BRW_RASTRULE_LOWER_LEFT  2
+#define BRW_RASTRULE_LOWER_RIGHT 3
 
 #define BRW_RENDERTARGET_CLAMPRANGE_UNORM    0
 #define BRW_RENDERTARGET_CLAMPRANGE_SNORM    1
diff --git a/src/mesa/drivers/dri/i965/brw_sf_state.c b/src/mesa/drivers/dri/i965/brw_sf_state.c
index 93a9686f718..fc4eddda0a5 100644
--- a/src/mesa/drivers/dri/i965/brw_sf_state.c
+++ b/src/mesa/drivers/dri/i965/brw_sf_state.c
@@ -231,7 +231,33 @@ sf_unit_create_from_key(struct brw_context *brw, struct brw_sf_unit_key *key,
        sf.sf6.line_width = 0;
 
    /* _NEW_POINT */
-   sf.sf6.point_rast_rule = BRW_RASTRULE_UPPER_RIGHT;	/* opengl conventions */
+   key->render_to_fbo = brw->intel.ctx.DrawBuffer->Name != 0;
+   if (!key->render_to_fbo) {
+      /* Rendering to an OpenGL window */
+      sf.sf6.point_rast_rule = BRW_RASTRULE_UPPER_RIGHT;
+   }
+   else {
+      /* If rendering to an FBO, the pixel coordinate system is
+       * inverted with respect to the normal OpenGL coordinate
+       * system, so BRW_RASTRULE_LOWER_RIGHT is correct.
+       * But this value is listed as "Reserved, but not seen as useful"
+       * in Intel documentation (page 212, "Point Rasterization Rule",
+       * section 7.4 "SF Pipeline State Summary", of document
+       * "Intel® 965 Express Chipset Family and Intel® G35 Express
+       * Chipset Graphics Controller Programmer's Reference Manual,
+       * Volume 2: 3D/Media", Revision 1.0b as of January 2008,
+       * available at 
+       *     http://intellinuxgraphics.org/documentation.html
+       * at the time of this writing).
+       *
+       * It does work on at least some devices, if not all;
+       * if devices that don't support it can be identified,
+       * the likely failure case is that points are rasterized
+       * incorrectly, which is no worse than occurs without
+       * the value, so we're using it here.
+       */
+      sf.sf6.point_rast_rule = BRW_RASTRULE_LOWER_RIGHT;
+   }
    /* XXX clamp max depends on AA vs. non-AA */
 
    sf.sf7.sprite_point = key->point_sprite;