src/compiler/nir/nir_lower_viewport_transform.c

   1 /*
   2  * Copyright (C) 2019 Alyssa Rosenzweig
   3  *
   4  * Permission is hereby granted, free of charge, to any person obtaining a
   5  * copy of this software and associated documentation files (the "Software"),
   6  * to deal in the Software without restriction, including without limitation
   7  * the rights to use, copy, modify, merge, publish, distribute, sublicense,
   8  * and/or sell copies of the Software, and to permit persons to whom the
   9  * Software is furnished to do so, subject to the following conditions:
  10  *
  11  * The above copyright notice and this permission notice (including the next
  12  * paragraph) shall be included in all copies or substantial portions of the
  13  * Software.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  16  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
  18  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  19  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  20  * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
  21  * IN THE SOFTWARE.
  22  */
  23
  24 /* On some hardware (particularly, all current versions of Mali GPUs),
  25  * vertex shaders do not output gl_Position in world-space. Instead, they
  26  * output gl_Position in transformed screen space via the "pseudo"
  27  * position varying. Thus, this pass finds writes to gl_Position and
  28  * changes them to transformed writes, still to gl_Position. The
  29  * outputted screen space is still written back to VARYING_SLOT_POS,
  30  * which is semantically ambiguous but nevertheless a good match for
  31  * Gallium/NIR/Mali.
  32  *
  33  * Implements coordinate transformation as defined in section 12.5
  34  * "Coordinate Transformation" of the OpenGL ES 3.2 full specification.
  35  *
  36  * This pass must run before lower_vars/lower_io such that derefs are
  37  * still in place.
  38  */
  39
  40 #include "nir/nir.h"
  41 #include "nir/nir_builder.h"
  42
  43 void
  44 nir_lower_viewport_transform(nir_shader *shader)
  45 {
  46    assert(shader->info.stage == MESA_SHADER_VERTEX);
  47
  48    nir_foreach_function(func, shader) {
  49       nir_foreach_block(block, func->impl) {
  50          nir_foreach_instr_safe(instr, block) {
  51             if (instr->type != nir_instr_type_intrinsic)
  52                continue;
  53
  54             nir_intrinsic_instr *intr = nir_instr_as_intrinsic(instr);
  55             if (intr->intrinsic != nir_intrinsic_store_deref)
  56                continue;
  57
  58             nir_variable *var = nir_intrinsic_get_var(intr, 0);
  59             if (var->data.mode != nir_var_shader_out ||
  60                 var->data.location != VARYING_SLOT_POS)
  61                continue;
  62
  63             nir_builder b;
  64             nir_builder_init(&b, func->impl);
  65             b.cursor = nir_before_instr(instr);
  66
  67             /* Grab the source and viewport */
  68             nir_ssa_def *input_point = nir_ssa_for_src(&b, intr->src[1], 4);
  69             nir_ssa_def *scale = nir_load_viewport_scale(&b);
  70             nir_ssa_def *offset = nir_load_viewport_offset(&b);
  71
  72             /* World space to normalised device coordinates to screen space */
  73
  74             nir_ssa_def *w_recip = nir_frcp(&b, nir_channel(&b, input_point, 3));
  75
  76             nir_ssa_def *ndc_point = nir_fmul(&b,
  77                   nir_channels(&b, input_point, 0x7), w_recip);
  78
  79             nir_ssa_def *screen = nir_fadd(&b,
  80                   nir_fmul(&b, ndc_point, scale), offset);
  81
  82             /* gl_Position will be written out in screenspace xyz, with w set to
  83              * the reciprocal we computed earlier. The transformed w component is
  84              * then used for perspective-correct varying interpolation. The
  85              * transformed w component must preserve its original sign; this is
  86              * used in depth clipping computations
  87              */
  88
  89             nir_ssa_def *screen_space = nir_vec4(&b,
  90                              nir_channel(&b, screen, 0),
  91                              nir_channel(&b, screen, 1),
  92                              nir_channel(&b, screen, 2),
  93                              w_recip);
  94
  95             nir_instr_rewrite_src(instr, &intr->src[1],
  96                                   nir_src_for_ssa(screen_space));
  97          }
  98       }
  99
 100       nir_metadata_preserve(func->impl, nir_metadata_block_index |
 101                             nir_metadata_dominance);
 102    }
 103 }