src/gallium/auxiliary/draw/draw_pipe_cull.c

/**************************************************************************
 *
 * Copyright 2007 VMware, Inc.
 * All Rights Reserved.
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 * permit persons to whom the Software is furnished to do so, subject to
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 *
 * The above copyright notice and this permission notice (including the
 * next paragraph) shall be included in all copies or substantial portions
 * of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
 * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
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/**
 * \brief  Drawing stage for polygon culling
 */

/* Authors:  Keith Whitwell <keithw@vmware.com>
 */


#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)
{
   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;
}