Bug 98851

One thing I wonder about is whether the clipper needs to know that it's a no-perspective attribute. There's an interpolation step done in clip.h:intersect, and I'm not sure that's right for the non-perspective-correcting case. (I'm also not sure that it's wrong...)

Ah yeah, looks like that's exactly what's going on -

src/gallium/auxiliary/draw/draw_pipe_clip.c:interp has the following logic:

   /**
    * Compute the t in screen-space instead of 3d space to use
    * for noperspective interpolation.
    *
    * The points can be aligned with the X axis, so in that case try
    * the Y.  When both points are at the same screen position, we can
    * pick whatever value (the interpolated point won't be in front
    * anyway), so just use the 3d t.
    */
   if (clip->num_linear_attribs) {
      int k;
      t_nopersp = t;
      /* find either in.x != out.x or in.y != out.y */
      for (k = 0; k < 2; k++) {
         if (in->clip_pos[k] != out->clip_pos[k]) {
            /* do divide by W, then compute linear interpolation factor */
            float in_coord = in->clip_pos[k] / in->clip_pos[3];
            float out_coord = out->clip_pos[k] / out->clip_pos[3];
            float dst_coord = dst->clip_pos[k] / dst->clip_pos[3];
            t_nopersp = (dst_coord - out_coord) / (in_coord - out_coord);
            break;
         }
      }
      for (j = 0; j < clip->num_linear_attribs; j++) {
         const unsigned attr = clip->linear_attribs[j];
         interp_attr(dst->data[attr], t_nopersp, in->data[attr], out->data[attr]);
      }
   }

swr needs to grow a linear-interpolated attribute mask and incorporate the above logic for those.

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