s_tritemp.h

Mesa is an open-source implementation of the OpenGL specification - a system for rendering interacti

/*
 * Mesa 3-D graphics library
 * Version:  7.0
 *
 * Copyright (C) 1999-2007  Brian Paul   All Rights Reserved.
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice 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 MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * BRIAN PAUL BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
 * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

/*
 * Triangle Rasterizer Template
 *
 * This file is #include'd to generate custom triangle rasterizers.
 *
 * The following macros may be defined to indicate what auxillary information
 * must be interpolated across the triangle:
 *    INTERP_Z        - if defined, interpolate integer Z values
 *    INTERP_RGB      - if defined, interpolate integer RGB values
 *    INTERP_ALPHA    - if defined, interpolate integer Alpha values
 *    INTERP_INDEX    - if defined, interpolate color index values
 *    INTERP_INT_TEX  - if defined, interpolate integer ST texcoords
 *                         (fast, simple 2-D texture mapping, without
 *                         perspective correction)
 *    INTERP_ATTRIBS  - if defined, interpolate arbitrary attribs (texcoords,
 *                         varying vars, etc)  This also causes W to be
 *                         computed for perspective correction).
 *
 * When one can directly address pixels in the color buffer the following
 * macros can be defined and used to compute pixel addresses during
 * rasterization (see pRow):
 *    PIXEL_TYPE          - the datatype of a pixel (GLubyte, GLushort, GLuint)
 *    BYTES_PER_ROW       - number of bytes per row in the color buffer
 *    PIXEL_ADDRESS(X,Y)  - returns the address of pixel at (X,Y) where
 *                          Y==0 at bottom of screen and increases upward.
 *
 * Similarly, for direct depth buffer access, this type is used for depth
 * buffer addressing (see zRow):
 *    DEPTH_TYPE          - either GLushort or GLuint
 *
 * Optionally, one may provide one-time setup code per triangle:
 *    SETUP_CODE    - code which is to be executed once per triangle
 *
 * The following macro MUST be defined:
 *    RENDER_SPAN(span) - code to write a span of pixels.
 *
 * This code was designed for the origin to be in the lower-left corner.
 *
 * Inspired by triangle rasterizer code written by Allen Akin.  Thanks Allen!
 *
 *
 * Some notes on rasterization accuracy:
 *
 * This code uses fixed point arithmetic (the GLfixed type) to iterate
 * over the triangle edges and interpolate ancillary data (such as Z,
 * color, secondary color, etc).  The number of fractional bits in
 * GLfixed and the value of SUB_PIXEL_BITS has a direct bearing on the
 * accuracy of rasterization.
 *
 * If SUB_PIXEL_BITS=4 then we'll snap the vertices to the nearest
 * 1/16 of a pixel.  If we're walking up a long, nearly vertical edge
 * (dx=1/16, dy=1024) we'll need 4 + 10 = 14 fractional bits in
 * GLfixed to walk the edge without error.  If the maximum viewport
 * height is 4K pixels, then we'll need 4 + 12 = 16 fractional bits.
 *
 * Historically, Mesa has used 11 fractional bits in GLfixed, snaps
 * vertices to 1/16 pixel and allowed a maximum viewport height of 2K
 * pixels.  11 fractional bits is actually insufficient for accurately
 * rasterizing some triangles.  More recently, the maximum viewport
 * height was increased to 4K pixels.  Thus, Mesa should be using 16
 * fractional bits in GLfixed.  Unfortunately, there may be some issues
 * with setting FIXED_FRAC_BITS=16, such as multiplication overflow.
 * This will have to be examined in some detail...
 *
 * For now, if you find rasterization errors, particularly with tall,
 * sliver triangles, try increasing FIXED_FRAC_BITS and/or decreasing
 * SUB_PIXEL_BITS.
 */


/*
 * Some code we unfortunately need to prevent negative interpolated colors.
 */
#ifndef CLAMP_INTERPOLANT
#define CLAMP_INTERPOLANT(CHANNEL, CHANNELSTEP, LEN)		\
do {								\
   GLfixed endVal = span.CHANNEL + (LEN) * span.CHANNELSTEP;	\
   if (endVal < 0) {						\
      span.CHANNEL -= endVal;					\
   }								\
   if (span.CHANNEL < 0) {					\
      span.CHANNEL = 0;						\
   }								\
} while (0)
#endif


static void NAME(GLcontext *ctx, const SWvertex *v0,
                                 const SWvertex *v1,
                                 const SWvertex *v2 )
{
   typedef struct {
      const SWvertex *v0, *v1;   /* Y(v0) < Y(v1) */
      GLfloat dx;	/* X(v1) - X(v0) */
      GLfloat dy;	/* Y(v1) - Y(v0) */
      GLfloat dxdy;	/* dx/dy */
      GLfixed fdxdy;	/* dx/dy in fixed-point */
      GLfloat adjy;	/* adjust from v[0]->fy to fsy, scaled */
      GLfixed fsx;	/* first sample point x coord */
      GLfixed fsy;
      GLfixed fx0;	/* fixed pt X of lower endpoint */
      GLint lines;	/* number of lines to be sampled on this edge */
   } EdgeT;

   const SWcontext *swrast = SWRAST_CONTEXT(ctx);
#ifdef INTERP_Z
   const GLint depthBits = ctx->DrawBuffer->Visual.depthBits;
   const GLint fixedToDepthShift = depthBits <= 16 ? FIXED_SHIFT : 0;
   const GLfloat maxDepth = ctx->DrawBuffer->_DepthMaxF;
#define FixedToDepth(F)  ((F) >> fixedToDepthShift)
#endif
   EdgeT eMaj, eTop, eBot;
   GLfloat oneOverArea;
   const SWvertex *vMin, *vMid, *vMax;  /* Y(vMin)<=Y(vMid)<=Y(vMax) */
   GLfloat bf = SWRAST_CONTEXT(ctx)->_BackfaceSign;
   const GLint snapMask = ~((FIXED_ONE / (1 << SUB_PIXEL_BITS)) - 1); /* for x/y coord snapping */
   GLfixed vMin_fx, vMin_fy, vMid_fx, vMid_fy, vMax_fx, vMax_fy;

   SWspan span;

   (void) swrast;

   INIT_SPAN(span, GL_POLYGON);
   span.y = 0; /* silence warnings */

#ifdef INTERP_Z
   (void) fixedToDepthShift;
#endif

   /*
   printf("%s()\n", __FUNCTION__);
   printf("  %g, %g, %g\n",
          v0->attrib[FRAG_ATTRIB_WPOS][0],
          v0->attrib[FRAG_ATTRIB_WPOS][1],
          v0->attrib[FRAG_ATTRIB_WPOS][2]);
   printf("  %g, %g, %g\n",
          v1->attrib[FRAG_ATTRIB_WPOS][0],
          v1->attrib[FRAG_ATTRIB_WPOS][1],
          v1->attrib[FRAG_ATTRIB_WPOS][2]);
   printf("  %g, %g, %g\n",
          v2->attrib[FRAG_ATTRIB_WPOS][0],
          v2->attrib[FRAG_ATTRIB_WPOS][1],
          v2->attrib[FRAG_ATTRIB_WPOS][2]);
   */

   /* Compute fixed point x,y coords w/ half-pixel offsets and snapping.
    * And find the order of the 3 vertices along the Y axis.
    */
   {
      const GLfixed fy0 = FloatToFixed(v0->attrib[FRAG_ATTRIB_WPOS][1] - 0.5F) & snapMask;
      const GLfixed fy1 = FloatToFixed(v1->attrib[FRAG_ATTRIB_WPOS][1] - 0.5F) & snapMask;
      const GLfixed fy2 = FloatToFixed(v2->attrib[FRAG_ATTRIB_WPOS][1] - 0.5F) & snapMask;
      if (fy0 <= fy1) {
         if (fy1 <= fy2) {
            /* y0 <= y1 <= y2 */
            vMin = v0;   vMid = v1;   vMax = v2;
            vMin_fy = fy0;  vMid_fy = fy1;  vMax_fy = fy2;
         }
         else if (fy2 <= fy0) {
            /* y2 <= y0 <= y1 */
            vMin = v2;   vMid = v0;   vMax = v1;
            vMin_fy = fy2;  vMid_fy = fy0;  vMax_fy = fy1;
         }
         else {
            /* y0 <= y2 <= y1 */
            vMin = v0;   vMid = v2;   vMax = v1;
            vMin_fy = fy0;  vMid_fy = fy2;  vMax_fy = fy1;
            bf = -bf;
         }
      }
      else {
         if (fy0 <= fy2) {
            /* y1 <= y0 <= y2 */
            vMin = v1;   vMid = v0;   vMax = v2;
            vMin_fy = fy1;  vMid_fy = fy0;  vMax_fy = fy2;
            bf = -bf;
         }
         else if (fy2 <= fy1) {
            /* y2 <= y1 <= y0 */
            vMin = v2;   vMid = v1;   vMax = v0;
            vMin_fy = fy2;  vMid_fy = fy1;  vMax_fy = fy0;
            bf = -bf;
         }
         else {
            /* y1 <= y2 <= y0 */
            vMin = v1;   vMid = v2;   vMax = v0;
            vMin_fy = fy1;  vMid_fy = fy2;  vMax_fy = fy0;
         }
      }

      /* fixed point X coords */
      vMin_fx = FloatToFixed(vMin->attrib[FRAG_ATTRIB_WPOS][0] + 0.5F) & snapMask;
      vMid_fx = FloatToFixed(vMid->attrib[FRAG_ATTRIB_WPOS][0] + 0.5F) & snapMask;
      vMax_fx = FloatToFixed(vMax->attrib[FRAG_ATTRIB_WPOS][0] + 0.5F) & snapMask;
   }

   /* vertex/edge relationship */
   eMaj.v0 = vMin;   eMaj.v1 = vMax;   /*TODO: .v1's not needed */
   eTop.v0 = vMid;   eTop.v1 = vMax;
   eBot.v0 = vMin;   eBot.v1 = vMid;

   /* compute deltas for each edge:  vertex[upper] - vertex[lower] */
   eMaj.dx = FixedToFloat(vMax_fx - vMin_fx);
   eMaj.dy = FixedToFloat(vMax_fy - vMin_fy);
   eTop.dx = FixedToFloat(vMax_fx - vMid_fx);
   eTop.dy = FixedToFloat(vMax_fy - vMid_fy);
   eBot.dx = FixedToFloat(vMid_fx - vMin_fx);
   eBot.dy = FixedToFloat(vMid_fy - vMin_fy);

   /* compute area, oneOverArea and perform backface culling */
   {
      const GLfloat area = eMaj.dx * eBot.dy - eBot.dx * eMaj.dy;

      if (IS_INF_OR_NAN(area) || area == 0.0F)
         return;

      if (area * bf * swrast->_BackfaceCullSign < 0.0)
         return;

      oneOverArea = 1.0F / area;

      /* 0 = front, 1 = back */
      span.facing = oneOverArea * bf > 0.0F;
   }

   /* Edge setup.  For a triangle strip these could be reused... */
   {
      eMaj.fsy = FixedCeil(vMin_fy);
      eMaj.lines = FixedToInt(FixedCeil(vMax_fy - eMaj.fsy));
      if (eMaj.lines > 0) {
         eMaj.dxdy = eMaj.dx / eMaj.dy;
         eMaj.fdxdy = SignedFloatToFixed(eMaj.dxdy);
         eMaj.adjy = (GLfloat) (eMaj.fsy - vMin_fy);  /* SCALED! */
         eMaj.fx0 = vMin_fx;
         eMaj.fsx = eMaj.fx0 + (GLfixed) (eMaj.adjy * eMaj.dxdy);
      }
      else {
         return;  /*CULLED*/
      }

      eTop.fsy = FixedCeil(vMid_fy);
      eTop.lines = FixedToInt(FixedCeil(vMax_fy - eTop.fsy));
      if (eTop.lines > 0) {
         eTop.dxdy = eTop.dx / eTop.dy;
         eTop.fdxdy = SignedFloatToFixed(eTop.dxdy);
         eTop.adjy = (GLfloat) (eTop.fsy - vMid_fy); /* SCALED! */
         eTop.fx0 = vMid_fx;
         eTop.fsx = eTop.fx0 + (GLfixed) (eTop.adjy * eTop.dxdy);
      }

      eBot.fsy = FixedCeil(vMin_fy);
      eBot.lines = FixedToInt(FixedCeil(vMid_fy - eBot.fsy));
      if (eBot.lines > 0) {
         eBot.dxdy = eBot.dx / eBot.dy;
         eBot.fdxdy = SignedFloatToFixed(eBot.dxdy);
         eBot.adjy = (GLfloat) (eBot.fsy - vMin_fy);  /* SCALED! */
         eBot.fx0 = vMin_fx;
         eBot.fsx = eBot.fx0 + (GLfixed) (eBot.adjy * eBot.dxdy);
      }
   }

   /*
    * Conceptually, we view a triangle as two subtriangles
    * separated by a perfectly horizontal line.  The edge that is
    * intersected by this line is one with maximal absolute dy; we
    * call it a ``major'' edge.  The other two edges are the
    * ``top'' edge (for the upper subtriangle) and the ``bottom''
    * edge (for the lower subtriangle).  If either of these two
    * edges is horizontal or very close to horizontal, the
    * corresponding subtriangle might cover zero sample points;
    * we take care to handle such cases, for performance as well
    * as correctness.
    *
    * By stepping rasterization parameters along the major edge,
    * we can avoid recomputing them at the discontinuity where
    * the top and bottom edges meet.  However, this forces us to
    * be able to scan both left-to-right and right-to-left.
    * Also, we must determine whether the major edge is at the
    * left or right side of the triangle.  We do this by
    * computing the magnitude of the cross-product of the major
    * and top edges.  Since this magnitude depends on the sine of
    * the angle between the two edges, its sign tells us whether
    * we turn to the left or to the right when travelling along
    * the major edge to the top edge, and from this we infer
    * whether the major edge is on the left or the right.
    *
    * Serendipitously, this cross-product magnitude is also a
    * value we need to compute the iteration parameter
    * derivatives for the triangle, and it can be used to perform
    * backface culling because its sign tells us whether the
    * triangle is clockwise or counterclockwise.  In this code we
    * refer to it as ``area'' because it's also proportional to
    * the pixel area of the triangle.
    */

   {
      GLint scan_from_left_to_right;  /* true if scanning left-to-right */
#ifdef INTERP_INDEX
      GLfloat didx, didy;
#endif

      /*
       * Execute user-supplied setup code