gallium: added draw_set_mrd() function to fix polygon offset

The Minimum Resolvable Depth factor depends on the driver and can't just
be computed from the number of Z buffer bits.
Glean's polygon offset test now passes with softpipe.
Still need to determine the MRD factor for other gallium drivers, if they use
the draw module's polygon offset stage...

4 files changed, 16 insertions, 2 deletions

diff --git a/src/gallium/auxiliary/draw/draw_context.c b/src/gallium/auxiliary/draw/draw_context.c
index 74deb44bd2..fab8fc95fc 100644
--- a/src/gallium/auxiliary/draw/draw_context.c
+++ b/src/gallium/auxiliary/draw/draw_context.c
@@ -103,6 +103,18 @@ void draw_flush( struct draw_context *draw )
 }
 
 
+/**
+ * Specify the Minimum Resolvable Depth factor for polygon offset.
+ * This factor potentially depends on the number of Z buffer bits,
+ * the rasterization algorithm and the arithmetic performed on Z
+ * values between vertex shading and rasterization.  It will vary
+ * from one driver to another.
+ */
+void draw_set_mrd(struct draw_context *draw, double mrd)
+{
+   draw->mrd = mrd;
+}
+
 
 /**
  * Register new primitive rasterization/rendering state.
diff --git a/src/gallium/auxiliary/draw/draw_context.h b/src/gallium/auxiliary/draw/draw_context.h
index 1d40c6c3be..a29bb01d81 100644
--- a/src/gallium/auxiliary/draw/draw_context.h
+++ b/src/gallium/auxiliary/draw/draw_context.h
@@ -72,6 +72,7 @@ void draw_enable_line_stipple(struct draw_context *draw, boolean enable);
 
 void draw_enable_point_sprites(struct draw_context *draw, boolean enable);
 
+void draw_set_mrd(struct draw_context *draw, double mrd);
 
 boolean
 draw_install_aaline_stage(struct draw_context *draw, struct pipe_context *pipe);
diff --git a/src/gallium/auxiliary/draw/draw_pipe_offset.c b/src/gallium/auxiliary/draw/draw_pipe_offset.c
index 1fea5e6dcb..8ddc4ee617 100644
--- a/src/gallium/auxiliary/draw/draw_pipe_offset.c
+++ b/src/gallium/auxiliary/draw/draw_pipe_offset.c
@@ -122,9 +122,8 @@ static void offset_first_tri( struct draw_stage *stage,
 			      struct prim_header *header )
 {
    struct offset_stage *offset = offset_stage(stage);
-   float mrd = 1.0f / 65535.0f; /* XXX this depends on depthbuffer bits! */
 
-   offset->units = stage->draw->rasterizer->offset_units * mrd;
+   offset->units = stage->draw->rasterizer->offset_units * stage->draw->mrd;
    offset->scale = stage->draw->rasterizer->offset_scale;
 
    stage->tri = offset_tri;
diff --git a/src/gallium/auxiliary/draw/draw_private.h b/src/gallium/auxiliary/draw/draw_private.h
index 37c4c87f87..a16b45d340 100644
--- a/src/gallium/auxiliary/draw/draw_private.h
+++ b/src/gallium/auxiliary/draw/draw_private.h
@@ -172,6 +172,8 @@ struct draw_context
 
    boolean force_passthrough; /**< never clip or shade */
 
+   double mrd;  /**< minimum resolvable depth value, for polygon offset */
+
    /* pipe state that we need: */
    const struct pipe_rasterizer_state *rasterizer;
    struct pipe_viewport_state viewport;