draw: inject frontface info into wireframe outputs

Draw module can decompose primitives into wireframe models, which
is a fancy word for 'lines', unfortunately that decomposition means
that we weren't able to preserve the original front-face info which
could be derived from the original primitives (lines don't have a
'face'). To fix it allow draw module to inject a fake face semantic
into outputs from which the backends can figure out the original
frontfacing info of the primitives.

Signed-off-by: Zack Rusin <[email protected]>
Reviewed-by: Roland Scheidegger <[email protected]>
Reviewed-by: Jose Fonseca <[email protected]>

4 files changed, 101 insertions, 0 deletions

diff --git a/src/gallium/auxiliary/draw/draw_context.c b/src/gallium/auxiliary/draw/draw_context.c
index 29bbc94d0ab..377b852f052 100644
--- a/src/gallium/auxiliary/draw/draw_context.c
+++ b/src/gallium/auxiliary/draw/draw_context.c
@@ -39,6 +39,7 @@
 #include "util/u_helpers.h"
 #include "util/u_prim.h"
 #include "draw_context.h"
+#include "draw_pipe.h"
 #include "draw_vs.h"
 #include "draw_gs.h"
 
@@ -540,6 +541,22 @@ draw_get_shader_info(const struct draw_context *draw)
    }
 }
 
+/**
+ * Prepare outputs slots from the draw module
+ *
+ * Certain parts of the draw module can emit additional
+ * outputs that can be quite useful to the backends, a good
+ * example of it is the process of decomposing primitives
+ * into wireframes (aka. lines) which normally would lose
+ * the face-side information, but using this method we can
+ * inject another shader output which passes the original
+ * face side information to the backend.
+ */
+void
+draw_prepare_shader_outputs(struct draw_context *draw)
+{
+   draw_unfilled_prepare_outputs(draw, draw->pipeline.unfilled);
+}
 
 /**
  * Ask the draw module for the location/slot of the given vertex attribute in
@@ -1007,3 +1024,29 @@ draw_stats_clipper_primitives(struct draw_context *draw,
       }
    }
 }
+
+
+/**
+ * Returns true if the draw module will inject the frontface
+ * info into the outputs.
+ *
+ * Given the specified primitive and rasterizer state
+ * the function will figure out if the draw module
+ * will inject the front-face information into shader
+ * outputs. This is done to preserve the front-facing
+ * info when decomposing primitives into wireframes.
+ */
+boolean
+draw_will_inject_frontface(const struct draw_context *draw)
+{
+   unsigned reduced_prim = u_reduced_prim(draw->pt.prim);
+   const struct pipe_rasterizer_state *rast = draw->rasterizer;
+
+   if (reduced_prim != PIPE_PRIM_TRIANGLES) {
+      return FALSE;
+   }
+
+   return (rast &&
+           (rast->fill_front != PIPE_POLYGON_MODE_FILL ||
+            rast->fill_back != PIPE_POLYGON_MODE_FILL));
+}
diff --git a/src/gallium/auxiliary/draw/draw_context.h b/src/gallium/auxiliary/draw/draw_context.h
index c720806feca..46d0de6f853 100644
--- a/src/gallium/auxiliary/draw/draw_context.h
+++ b/src/gallium/auxiliary/draw/draw_context.h
@@ -126,10 +126,16 @@ draw_install_pstipple_stage(struct draw_context *draw, struct pipe_context *pipe
 struct tgsi_shader_info *
 draw_get_shader_info(const struct draw_context *draw);
 
+void
+draw_prepare_shader_outputs(struct draw_context *draw);
+
 int
 draw_find_shader_output(const struct draw_context *draw,
                         uint semantic_name, uint semantic_index);
 
+boolean
+draw_will_inject_frontface(const struct draw_context *draw);
+
 uint
 draw_num_shader_outputs(const struct draw_context *draw);
 
diff --git a/src/gallium/auxiliary/draw/draw_pipe.h b/src/gallium/auxiliary/draw/draw_pipe.h
index 547e4bf4d05..70c286f4330 100644
--- a/src/gallium/auxiliary/draw/draw_pipe.h
+++ b/src/gallium/auxiliary/draw/draw_pipe.h
@@ -103,6 +103,9 @@ void draw_pipe_passthrough_line(struct draw_stage *stage, struct prim_header *he
 void draw_pipe_passthrough_point(struct draw_stage *stage, struct prim_header *header);
 
 
+void draw_unfilled_prepare_outputs(struct draw_context *context,
+                                   struct draw_stage *stage);
+
 
 /**
  * Get a writeable copy of a vertex.
diff --git a/src/gallium/auxiliary/draw/draw_pipe_unfilled.c b/src/gallium/auxiliary/draw/draw_pipe_unfilled.c
index d87741b91e7..d8a603f61cb 100644
--- a/src/gallium/auxiliary/draw/draw_pipe_unfilled.c
+++ b/src/gallium/auxiliary/draw/draw_pipe_unfilled.c
@@ -47,6 +47,8 @@ struct unfilled_stage {
     * and PIPE_POLYGON_MODE_POINT,
     */
    unsigned mode[2];
+
+   int face_slot;
 };
 
 
@@ -55,8 +57,31 @@ static INLINE struct unfilled_stage *unfilled_stage( struct draw_stage *stage )
    return (struct unfilled_stage *)stage;
 }
 
+static void
+inject_front_face_info(struct draw_stage *stage,
+                       struct prim_header *header)
+{
+   struct unfilled_stage *unfilled = unfilled_stage(stage);
+   unsigned ccw = header->det < 0.0;
+   boolean is_front_face = (
+      (stage->draw->rasterizer->front_ccw && ccw) ||
+      (!stage->draw->rasterizer->front_ccw && !ccw));
+   unsigned slot = unfilled->face_slot;
+   struct vertex_header *v0 = header->v[0];
+   struct vertex_header *v1 = header->v[1];
+   struct vertex_header *v2 = header->v[2];
+
+   /* In case the backend doesn't care about it */
+   if (slot < 0) {
+      return;
+   }
 
+   v0->data[slot][0] = is_front_face;
+   v1->data[slot][0] = is_front_face;
+   v2->data[slot][0] = is_front_face;
+}
 
+   
 static void point( struct draw_stage *stage,
 		   struct vertex_header *v0 )
 {
@@ -83,6 +108,8 @@ static void points( struct draw_stage *stage,
    struct vertex_header *v1 = header->v[1];
    struct vertex_header *v2 = header->v[2];
 
+   inject_front_face_info(stage, header);
+
    if ((header->flags & DRAW_PIPE_EDGE_FLAG_0) && v0->edgeflag) point( stage, v0 );
    if ((header->flags & DRAW_PIPE_EDGE_FLAG_1) && v1->edgeflag) point( stage, v1 );
    if ((header->flags & DRAW_PIPE_EDGE_FLAG_2) && v2->edgeflag) point( stage, v2 );
@@ -99,6 +126,8 @@ static void lines( struct draw_stage *stage,
    if (header->flags & DRAW_PIPE_RESET_STIPPLE)
       stage->next->reset_stipple_counter( stage->next );
 
+   inject_front_face_info(stage, header);
+
    if ((header->flags & DRAW_PIPE_EDGE_FLAG_2) && v2->edgeflag) line( stage, v2, v0 );
    if ((header->flags & DRAW_PIPE_EDGE_FLAG_0) && v0->edgeflag) line( stage, v0, v1 );
    if ((header->flags & DRAW_PIPE_EDGE_FLAG_1) && v1->edgeflag) line( stage, v1, v2 );
@@ -192,6 +221,26 @@ static void unfilled_destroy( struct draw_stage *stage )
    FREE( stage );
 }
 
+/*
+ * Try to allocate an output slot which we can use
+ * to preserve the front face information.
+ */
+void
+draw_unfilled_prepare_outputs( struct draw_context *draw,
+                               struct draw_stage *stage )
+{
+   struct unfilled_stage *unfilled = unfilled_stage(stage);
+   const struct pipe_rasterizer_state *rast = draw ? draw->rasterizer : 0;
+   if (rast &&
+       (rast->fill_front != PIPE_POLYGON_MODE_FILL ||
+        rast->fill_back != PIPE_POLYGON_MODE_FILL)) {
+      unfilled->face_slot = draw_alloc_extra_vertex_attrib(
+         stage->draw, TGSI_SEMANTIC_FACE, 0);
+   } else {
+      unfilled->face_slot = -1;
+   }
+}
+
 
 /**
  * Create unfilled triangle stage.