draw_pipe_cull.c

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 * Copyright 2007 VMware, Inc.
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/**
 * \brief  Drawing stage for polygon culling
 */

/* Authors:  Keith Whitwell <[email protected]>
 */


#include "util/u_math.h"
#include "util/u_memory.h"
#include "pipe/p_defines.h"
#include "draw_pipe.h"


struct cull_stage {
   struct draw_stage stage;
   unsigned cull_face;  /**< which face(s) to cull (one of PIPE_FACE_x) */
   unsigned front_ccw;
};


static inline struct cull_stage *cull_stage( struct draw_stage *stage )
{
   return (struct cull_stage *)stage;
}

/*
 * Triangles can be culled using regular face cull.
 */
static void cull_tri( struct draw_stage *stage,
		      struct prim_header *header )
{
   /* Do the regular face culling */
   {
      const unsigned pos = draw_current_shader_position_output(stage->draw);
      /* Window coords: */
      const float *v0 = header->v[0]->data[pos];
      const float *v1 = header->v[1]->data[pos];
      const float *v2 = header->v[2]->data[pos];

      /* edge vectors: e = v0 - v2, f = v1 - v2 */
      const float ex = v0[0] - v2[0];
      const float ey = v0[1] - v2[1];
      const float fx = v1[0] - v2[0];
      const float fy = v1[1] - v2[1];


      /* det = cross(e,f).z */
      header->det = ex * fy - ey * fx;

      if (header->det != 0) {
         /* if det < 0 then Z points toward the camera and the triangle is
          * counter-clockwise winding.
          */
         unsigned ccw = (header->det < 0);
         unsigned face = ((ccw == cull_stage(stage)->front_ccw) ?
                          PIPE_FACE_FRONT :
                          PIPE_FACE_BACK);

         if ((face & cull_stage(stage)->cull_face) == 0) {
            /* triangle is not culled, pass to next stage */
            stage->next->tri( stage->next, header );
         }
      } else {
         /*
          * With zero area, this is back facing (because the spec says
          * it's front facing if sign is positive?).
          * Some apis apparently do not allow us to cull zero area tris
          * here, in case of fill mode line (which is rather lame).
          */
         if ((PIPE_FACE_BACK & cull_stage(stage)->cull_face) == 0) {
            stage->next->tri( stage->next, header );
         }
      }
   }
}

static void cull_first_tri( struct draw_stage *stage,
			    struct prim_header *header )
{
   struct cull_stage *cull = cull_stage(stage);

   cull->cull_face = stage->draw->rasterizer->cull_face;
   cull->front_ccw = stage->draw->rasterizer->front_ccw;

   stage->tri = cull_tri;
   stage->tri( stage, header );
}


static void cull_flush( struct draw_stage *stage, unsigned flags )
{
   stage->tri = cull_first_tri;
   stage->next->flush( stage->next, flags );
}


static void cull_reset_stipple_counter( struct draw_stage *stage )
{
   stage->next->reset_stipple_counter( stage->next );
}


static void cull_destroy( struct draw_stage *stage )
{
   draw_free_temp_verts( stage );
   FREE( stage );
}


/**
 * Create a new polygon culling stage.
 */
struct draw_stage *draw_cull_stage( struct draw_context *draw )
{
   struct cull_stage *cull = CALLOC_STRUCT(cull_stage);
   if (!cull)
      goto fail;

   cull->stage.draw = draw;
   cull->stage.name = "cull";
   cull->stage.next = NULL;
   cull->stage.point = draw_pipe_passthrough_point;
   cull->stage.line = draw_pipe_passthrough_line;
   cull->stage.tri = cull_first_tri;
   cull->stage.flush = cull_flush;
   cull->stage.reset_stipple_counter = cull_reset_stipple_counter;
   cull->stage.destroy = cull_destroy;

   if (!draw_alloc_temp_verts( &cull->stage, 0 ))
      goto fail;

   return &cull->stage;

fail:
   if (cull)
      cull->stage.destroy( &cull->stage );

   return NULL;
}