Sorry for the non-descript subject. I'm just not sure what's wrong. The following test fails: generated_tests/spec/glsl-1.30/execution/interpolation/interpolation-noperspective-gl_FrontColor-flat-fixed.shader_test as do many of its friends. Note that the color is meant to be non-perspective-interpolated. Also note that the flat-none.shader_test variant of this works as expected. As does the flat-vertex.shader_test variant after my perspective-interpolation for clip distances fix is applied. So it's something wrong with the clipper. Comparing to generated_tests/spec/glsl-1.30/execution/interpolation/interpolation-smooth-gl_FrontColor-flat-fixed.shader_test, it looks different (which smoothly-interpolates the color). And there's no reason to think that color is being perspective-interpolated, as that is decided in the user frag shader. However I'm not sure what's going on - 1/w is getting messed up perhaps? Not sure. Furthermore, note that generated_tests/spec/glsl-1.30/execution/interpolation/interpolation-smooth-gl_FrontColor-flat-fixed.shader_test does pass with swr, and the only difference is that the color is perspective-interpolated vs no-perspective-interpolated.
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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