src/compiler/nir/nir_lower_wpos_center.c

/*
 * Copyright © 2015 Red Hat
 * Copyright © 2016 Intel Corporation
 *
 * 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 (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 MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS 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.
 */

#include "nir.h"
#include "nir_builder.h"

/**
 * This pass adds sample position to gl_FragCoord, intended for Vulkan drivers
 * on hardware which provides an integer pixel center. Vulkan mandates that the
 * pixel center must be half-integer, and also that the coordinate system's
 * origin must be upper left. This means that there's no need for a uniform - we
 * can always just add a constant. In the case that sample shading is enabled,
 * Vulkan expects FragCoord to include sample positions.
 *
 * Run before nir_lower_io().
 *
 * For a more full featured pass, consider using nir_lower_wpos_ytransform(),
 * which can handle pixel center integer / half integer, and origin lower
 * left / upper left transformations.
 */

static bool
lower_wpos_center_instr(nir_builder *b, nir_intrinsic_instr *intr, void *data)
{
   if (intr->intrinsic != nir_intrinsic_load_frag_coord)
      return false;

   nir_def *wpos = &intr->def;
   b->cursor = nir_after_instr(&intr->instr);

   nir_def *spos = nir_load_sample_pos_or_center(b);
   wpos = nir_fadd(b, wpos, nir_pad_vector_imm_int(b, spos, 0, 4));

   nir_def_rewrite_uses_after(&intr->def, wpos, wpos->parent_instr);
   return true;
}

bool
nir_lower_wpos_center(nir_shader *shader)
{
   assert(shader->info.stage == MESA_SHADER_FRAGMENT);

   return nir_shader_intrinsics_pass(shader, lower_wpos_center_instr,
                                     nir_metadata_control_flow,
                                     NULL);
}