src/compiler/nir/nir_lower_bitmap.c

/*
 * Copyright © 2015 Red Hat
 *
 * 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"

/* Lower glBitmap().
 *
 * This is based on the logic in st_get_bitmap_shader() in TGSI compiler.
 * From st_cb_bitmap.c:
 *
 *    glBitmaps are drawn as textured quads.  The user's bitmap pattern
 *    is stored in a texture image.  An alpha8 texture format is used.
 *    The fragment shader samples a bit (texel) from the texture, then
 *    discards the fragment if the bit is off.
 *
 *    Note that we actually store the inverse image of the bitmap to
 *    simplify the fragment program.  An "on" bit gets stored as texel=0x0
 *    and an "off" bit is stored as texel=0xff.  Then we kill the
 *    fragment if the negated texel value is less than zero.
 *
 * Note that the texture format will be, according to what driver supports,
 * in order of preference (with swizzle):
 *
 *    I8_UNORM - .xxxx
 *    A8_UNORM - .000x
 *    L8_UNORM - .xxx1
 *
 * If L8_UNORM, options->swizzle_xxxx is true.  Otherwise we can just use
 * the .w comp.
 *
 * Run before nir_lower_io.
 */

static nir_variable *
get_texcoord(nir_shader *shader)
{
   nir_variable *texcoord = NULL;

   /* find gl_TexCoord, if it exists: */
   nir_foreach_variable(var, &shader->inputs) {
      if (var->data.location == VARYING_SLOT_TEX0) {
         texcoord = var;
         break;
      }
   }

   /* otherwise create it: */
   if (texcoord == NULL) {
      texcoord = nir_variable_create(shader,
                                     nir_var_shader_in,
                                     glsl_vec4_type(),
                                     "gl_TexCoord");
      texcoord->data.location = VARYING_SLOT_TEX0;
   }

   return texcoord;
}

static void
lower_bitmap(nir_shader *shader, nir_builder *b,
             const nir_lower_bitmap_options *options)
{
   nir_ssa_def *texcoord;
   nir_tex_instr *tex;
   nir_ssa_def *cond;
   nir_intrinsic_instr *discard;

   texcoord = nir_load_var(b, get_texcoord(shader));

   tex = nir_tex_instr_create(shader, 1);
   tex->op = nir_texop_tex;
   tex->sampler_dim = GLSL_SAMPLER_DIM_2D;
   tex->coord_components = 2;
   tex->sampler_index = options->sampler;
   tex->texture_index = options->sampler;
   tex->dest_type = nir_type_float;
   tex->src[0].src_type = nir_tex_src_coord;
   tex->src[0].src = nir_src_for_ssa(texcoord);

   nir_ssa_dest_init(&tex->instr, &tex->dest, 4, 32, NULL);
   nir_builder_instr_insert(b, &tex->instr);

   /* kill if tex != 0.0.. take .x or .w channel according to format: */
   cond = nir_f2b(b, nir_channel(b, &tex->dest.ssa,
                  options->swizzle_xxxx ? 0 : 3));

   discard = nir_intrinsic_instr_create(shader, nir_intrinsic_discard_if);
   discard->src[0] = nir_src_for_ssa(cond);
   nir_builder_instr_insert(b, &discard->instr);

   shader->info->fs.uses_discard = true;
}

static void
lower_bitmap_impl(nir_function_impl *impl,
                  const nir_lower_bitmap_options *options)
{
   nir_builder b;

   nir_builder_init(&b, impl);
   b.cursor = nir_before_cf_list(&impl->body);

   lower_bitmap(impl->function->shader, &b, options);

   nir_metadata_preserve(impl, nir_metadata_block_index |
                               nir_metadata_dominance);
}

void
nir_lower_bitmap(nir_shader *shader,
                 const nir_lower_bitmap_options *options)
{
   assert(shader->stage == MESA_SHADER_FRAGMENT);

   lower_bitmap_impl(nir_shader_get_entrypoint(shader), options);
}