GPU: Replace `GPUMaterialVolumeGrid` by `GPUMaterialAttribute`

This is to make the codegen and shading nodes object type agnostic. This
is essential for flexibility of the engine to use the nodetree as it see
fits.

The essential volume attributes struct properties are moved to the
`GPUMaterialAttribute` which see its final input name set on creation.

The binding process is centralized into `draw_volume.cc` to avoid
duplicating the code between multiple engines. It mimics the hair attributes
process.

Volume object grid transforms and other per object uniforms are packed into
one UBO per object. The grid transform is now based on object which simplify
the matrix preparations.

This also gets rid of the double transforms and use object info orco factors
for volume objects.

Tagging @brecht because he did the initial implementation of Volume Grids.

7 files changed, 124 insertions, 257 deletions

diff --git a/source/blender/draw/engines/eevee/eevee_shaders.c b/source/blender/draw/engines/eevee/eevee_shaders.c
index 4e4a2a9eb8e..85cc7f65126 100644
--- a/source/blender/draw/engines/eevee/eevee_shaders.c
+++ b/source/blender/draw/engines/eevee/eevee_shaders.c
@@ -1487,6 +1487,10 @@ struct GPUMaterial *EEVEE_material_get(
   GPUMaterial *mat = eevee_material_get_ex(scene, ma, wo, options, deferred);
 
   int status = GPU_material_status(mat);
+  /* Return null material and bypass drawing for volume shaders. */
+  if ((options & VAR_MAT_VOLUME) && status != GPU_MAT_SUCCESS) {
+    return NULL;
+  }
   switch (status) {
     case GPU_MAT_SUCCESS:
       break;
diff --git a/source/blender/draw/engines/eevee/eevee_shaders_extra.cc b/source/blender/draw/engines/eevee/eevee_shaders_extra.cc
index bb1a0b0abe4..7df26a95cfe 100644
--- a/source/blender/draw/engines/eevee/eevee_shaders_extra.cc
+++ b/source/blender/draw/engines/eevee/eevee_shaders_extra.cc
@@ -97,12 +97,26 @@ void eevee_shader_material_create_info_amend(GPUMaterial *gpumat,
     }
     attr_load << "};\n";
     attr_load << iface.name << " " << iface.instance_name << ";\n";
-    /* Global vars just to make code valid. Only Orco is supported. */
-    for (const ShaderCreateInfo::VertIn &in : info.vertex_inputs_) {
-      attr_load << in.type << " " << in.name << ";\n";
+    if (!is_volume) {
+      /* Global vars just to make code valid. Only Orco is supported. */
+      for (const ShaderCreateInfo::VertIn &in : info.vertex_inputs_) {
+        attr_load << in.type << " " << in.name << ";\n";
+      }
     }
     info.vertex_out_interfaces_.clear();
   }
+  if (is_volume) {
+    /** Volume grid attributes come from 3D textures. Transfer attributes to samplers. */
+    for (auto &input : info.vertex_inputs_) {
+      info.sampler(0, ImageType::FLOAT_3D, input.name, Frequency::BATCH);
+    }
+    info.additional_info("draw_volume_infos");
+    /* Do not add twice. */
+    if (!GPU_material_flag_get(gpumat, GPU_MATFLAG_OBJECT_INFO)) {
+      info.additional_info("draw_object_infos");
+    }
+    info.vertex_inputs_.clear();
+  }
 
   if (!is_volume) {
     info.define("EEVEE_GENERATED_INTERFACE");
diff --git a/source/blender/draw/engines/eevee/eevee_volumes.c b/source/blender/draw/engines/eevee/eevee_volumes.c
index 0ac6c6ad97c..b8bef61f8b1 100644
--- a/source/blender/draw/engines/eevee/eevee_volumes.c
+++ b/source/blender/draw/engines/eevee/eevee_volumes.c
@@ -45,30 +45,6 @@ static struct {
   GPUTexture *dummy_transmit;
 } e_data = {NULL}; /* Engine data */
 
-static void eevee_create_textures_volumes(void)
-{
-  const float zero[4] = {0.0f, 0.0f, 0.0f, 0.0f};
-  e_data.dummy_zero = DRW_texture_create_3d(1, 1, 1, GPU_RGBA8, DRW_TEX_WRAP, zero);
-
-  const float one[4] = {1.0f, 1.0f, 1.0f, 1.0f};
-  e_data.dummy_one = DRW_texture_create_3d(1, 1, 1, GPU_RGBA8, DRW_TEX_WRAP, one);
-
-  const float flame = 0.0f;
-  e_data.dummy_flame = DRW_texture_create_3d(1, 1, 1, GPU_R8, DRW_TEX_WRAP, &flame);
-}
-
-static GPUTexture *eevee_volume_default_texture(eGPUVolumeDefaultValue default_value)
-{
-  switch (default_value) {
-    case GPU_VOLUME_DEFAULT_0:
-      return e_data.dummy_zero;
-    case GPU_VOLUME_DEFAULT_1:
-      return e_data.dummy_one;
-  }
-
-  return e_data.dummy_zero;
-}
-
 void EEVEE_volumes_set_jitter(EEVEE_ViewLayerData *sldata, uint current_sample)
 {
   EEVEE_CommonUniformBuffer *common_data = &sldata->common_data;
@@ -224,11 +200,6 @@ void EEVEE_volumes_cache_init(EEVEE_ViewLayerData *sldata, EEVEE_Data *vedata)
   Scene *scene = draw_ctx->scene;
   DRWShadingGroup *grp = NULL;
 
-  /* Textures */
-  if (!e_data.dummy_zero) {
-    eevee_create_textures_volumes();
-  }
-
   /* Quick breakdown of the Volumetric rendering:
    *
    * The rendering is separated in 4 stages:
@@ -265,7 +236,7 @@ void EEVEE_volumes_cache_init(EEVEE_ViewLayerData *sldata, EEVEE_Data *vedata)
       !LOOK_DEV_STUDIO_LIGHT_ENABLED(draw_ctx->v3d)) {
     struct GPUMaterial *mat = EEVEE_material_get(vedata, scene, NULL, wo, VAR_MAT_VOLUME);
 
-    if (GPU_material_has_volume_output(mat)) {
+    if (mat && GPU_material_has_volume_output(mat)) {
       grp = DRW_shgroup_material_create(mat, psl->volumetric_world_ps);
     }
 
@@ -280,11 +251,7 @@ void EEVEE_volumes_cache_init(EEVEE_ViewLayerData *sldata, EEVEE_Data *vedata)
       DRW_shgroup_uniform_block(grp, "renderpass_block", sldata->renderpass_ubo.combined);
 
       /* Fix principle volumetric not working with world materials. */
-      ListBase gpu_grids = GPU_material_volume_grids(mat);
-      LISTBASE_FOREACH (GPUMaterialVolumeGrid *, gpu_grid, &gpu_grids) {
-        DRW_shgroup_uniform_texture(
-            grp, gpu_grid->sampler_name, eevee_volume_default_texture(gpu_grid->default_value));
-      }
+      grp = DRW_shgroup_volume_create_sub(NULL, NULL, grp, mat);
 
       DRW_shgroup_call_procedural_triangles(grp, NULL, common_data->vol_tex_size[2]);
 
@@ -296,186 +263,17 @@ void EEVEE_volumes_cache_init(EEVEE_ViewLayerData *sldata, EEVEE_Data *vedata)
     /* If no world or volume material is present just clear the buffer with this drawcall */
     grp = DRW_shgroup_create(EEVEE_shaders_volumes_clear_sh_get(), psl->volumetric_world_ps);
     DRW_shgroup_uniform_block(grp, "common_block", sldata->common_ubo);
+    DRW_shgroup_uniform_block(grp, "grid_block", sldata->grid_ubo);
     DRW_shgroup_uniform_block(grp, "probe_block", sldata->probe_ubo);
+    DRW_shgroup_uniform_block(grp, "planar_block", sldata->planar_ubo);
     DRW_shgroup_uniform_block(grp, "light_block", sldata->light_ubo);
+    DRW_shgroup_uniform_block(grp, "shadow_block", sldata->shadow_ubo);
     DRW_shgroup_uniform_block(grp, "renderpass_block", sldata->renderpass_ubo.combined);
 
     DRW_shgroup_call_procedural_triangles(grp, NULL, common_data->vol_tex_size[2]);
   }
 }
 
-static bool eevee_volume_object_grids_init(Object *ob, ListBase *gpu_grids, DRWShadingGroup *grp)
-{
-  Volume *volume = ob->data;
-  BKE_volume_load(volume, G.main);
-
-  /* Test if we need to use multiple transforms. */
-  DRWVolumeGrid *first_drw_grid = NULL;
-  bool multiple_transforms = true;
-
-  LISTBASE_FOREACH (GPUMaterialVolumeGrid *, gpu_grid, gpu_grids) {
-    const VolumeGrid *volume_grid = BKE_volume_grid_find_for_read(volume, gpu_grid->name);
-    DRWVolumeGrid *drw_grid = (volume_grid) ?
-                                  DRW_volume_batch_cache_get_grid(volume, volume_grid) :
-                                  NULL;
-
-    if (drw_grid) {
-      if (first_drw_grid == NULL) {
-        first_drw_grid = drw_grid;
-      }
-      else if (drw_grid &&
-               !equals_m4m4(drw_grid->object_to_texture, first_drw_grid->object_to_texture)) {
-        multiple_transforms = true;
-        break;
-      }
-    }
-  }
-
-  /* Bail out of no grids to render. */
-  if (first_drw_grid == NULL) {
-    return false;
-  }
-
-  /* Set transform matrix for the volume as a whole. This one is also used for
-   * clipping so must map the entire bounding box to 0..1. */
-  float bounds_to_object[4][4];
-
-  if (multiple_transforms) {
-    /* For multiple grids with different transform, we first transform from object space
-     * to bounds, then for each individual grid from bounds to texture. */
-    const BoundBox *bb = BKE_volume_boundbox_get(ob);
-    float bb_size[3];
-    sub_v3_v3v3(bb_size, bb->vec[6], bb->vec[0]);
-    size_to_mat4(bounds_to_object, bb_size);
-    copy_v3_v3(bounds_to_object[3], bb->vec[0]);
-
-    invert_m4_m4(first_drw_grid->object_to_bounds, bounds_to_object);
-    DRW_shgroup_uniform_mat4(grp, "volumeObjectToTexture", first_drw_grid->object_to_bounds);
-  }
-  else {
-    /* All grid transforms are equal, we can transform to texture space immediately. */
-    DRW_shgroup_uniform_mat4(grp, "volumeObjectToTexture", first_drw_grid->object_to_texture);
-  }
-
-  /* Don't use orco transform here, only matrix. */
-  DRW_shgroup_uniform_vec3_copy(grp, "volumeOrcoLoc", (float[3]){0.5f, 0.5f, 0.5f});
-  DRW_shgroup_uniform_vec3_copy(grp, "volumeOrcoSize", (float[3]){0.5f, 0.5f, 0.5f});
-
-  /* Set density scale. */
-  const float density_scale = BKE_volume_density_scale(volume, ob->obmat);
-  DRW_shgroup_uniform_float_copy(grp, "volumeDensityScale", density_scale);
-
-  /* Bind volume grid textures. */
-  LISTBASE_FOREACH (GPUMaterialVolumeGrid *, gpu_grid, gpu_grids) {
-    const VolumeGrid *volume_grid = BKE_volume_grid_find_for_read(volume, gpu_grid->name);
-    DRWVolumeGrid *drw_grid = (volume_grid) ?
-                                  DRW_volume_batch_cache_get_grid(volume, volume_grid) :
-                                  NULL;
-
-    /* Handle 3 cases here:
-     * - Grid exists and texture was loaded -> use texture.
-     * - Grid exists but has zero size or failed to load -> use zero.
-     * - Grid does not exist -> use default value. */
-    GPUTexture *grid_tex = (drw_grid)    ? drw_grid->texture :
-                           (volume_grid) ? e_data.dummy_zero :
-                                           eevee_volume_default_texture(gpu_grid->default_value);
-
-    DRW_shgroup_uniform_texture(grp, gpu_grid->sampler_name, grid_tex);
-
-    if (drw_grid && multiple_transforms) {
-      /* Specify per-volume transform matrix that is applied after the
-       * transform from object to bounds. */
-      mul_m4_m4m4(drw_grid->bounds_to_texture, drw_grid->object_to_texture, bounds_to_object);
-      DRW_shgroup_uniform_mat4(grp, gpu_grid->transform_name, drw_grid->bounds_to_texture);
-    }
-  }
-
-  return true;
-}
-
-static bool eevee_volume_object_mesh_init(Scene *scene,
-                                          Object *ob,
-                                          ListBase *gpu_grids,
-                                          DRWShadingGroup *grp)
-{
-  static const float white[3] = {1.0f, 1.0f, 1.0f};
-  ModifierData *md = NULL;
-
-  /* Smoke Simulation */
-  if ((md = BKE_modifiers_findby_type(ob, eModifierType_Fluid)) &&
-      (BKE_modifier_is_enabled(scene, md, eModifierMode_Realtime)) &&
-      ((FluidModifierData *)md)->domain != NULL) {
-    FluidModifierData *fmd = (FluidModifierData *)md;
-    FluidDomainSettings *fds = fmd->domain;
-
-    /* Don't try to show liquid domains here. */
-    if (!fds->fluid || !(fds->type == FLUID_DOMAIN_TYPE_GAS)) {
-      return false;
-    }
-
-    /* Don't show smoke before simulation starts, this could be made an option in the future. */
-    /* (sebbas): Always show smoke for manta */
-#if 0
-    const DRWContextState *draw_ctx = DRW_context_state_get();
-    const bool show_smoke = ((int)DEG_get_ctime(draw_ctx->depsgraph) >=
-                             *fds->point_cache[0]->startframe);
-#endif
-
-    if (fds->fluid && (fds->type == FLUID_DOMAIN_TYPE_GAS) /* && show_smoke */) {
-      DRW_smoke_ensure(fmd, fds->flags & FLUID_DOMAIN_USE_NOISE);
-    }
-
-    LISTBASE_FOREACH (GPUMaterialVolumeGrid *, gpu_grid, gpu_grids) {
-      if (STREQ(gpu_grid->name, "density")) {
-        DRW_shgroup_uniform_texture_ref(
-            grp, gpu_grid->sampler_name, fds->tex_density ? &fds->tex_density : &e_data.dummy_one);
-      }
-      else if (STREQ(gpu_grid->name, "color")) {
-        DRW_shgroup_uniform_texture_ref(
-            grp, gpu_grid->sampler_name, fds->tex_color ? &fds->tex_color : &e_data.dummy_one);
-      }
-      else if (STR_ELEM(gpu_grid->name, "flame", "temperature")) {
-        DRW_shgroup_uniform_texture_ref(
-            grp, gpu_grid->sampler_name, fds->tex_flame ? &fds->tex_flame : &e_data.dummy_flame);
-      }
-      else {
-        DRW_shgroup_uniform_texture(
-            grp, gpu_grid->sampler_name, eevee_volume_default_texture(gpu_grid->default_value));
-      }
-    }
-
-    /* Constant Volume color. */
-    bool use_constant_color = ((fds->active_fields & FLUID_DOMAIN_ACTIVE_COLORS) == 0 &&
-                               (fds->active_fields & FLUID_DOMAIN_ACTIVE_COLOR_SET) != 0);
-
-    DRW_shgroup_uniform_vec3(
-        grp, "volumeColor", (use_constant_color) ? fds->active_color : white, 1);
-
-    /* Output is such that 0..1 maps to 0..1000K */
-    DRW_shgroup_uniform_vec2(grp, "volumeTemperature", &fds->flame_ignition, 1);
-  }
-  else {
-    LISTBASE_FOREACH (GPUMaterialVolumeGrid *, gpu_grid, gpu_grids) {
-      DRW_shgroup_uniform_texture(
-          grp, gpu_grid->sampler_name, eevee_volume_default_texture(gpu_grid->default_value));
-    }
-  }
-
-  /* Transform for mesh volumes. */
-  static const float unit_mat[4][4] = {{1.0f, 0.0f, 0.0f, 0.0f},
-                                       {0.0f, 1.0f, 0.0f, 0.0f},
-                                       {0.0f, 0.0f, 1.0f, 0.0f},
-                                       {0.0f, 0.0f, 0.0f, 1.0f}};
-  float *texco_loc, *texco_size;
-  BKE_mesh_texspace_get_reference((struct Mesh *)ob->data, NULL, &texco_loc, &texco_size);
-
-  DRW_shgroup_uniform_mat4(grp, "volumeObjectToTexture", unit_mat);
-  DRW_shgroup_uniform_vec3(grp, "volumeOrcoLoc", texco_loc, 1);
-  DRW_shgroup_uniform_vec3(grp, "volumeOrcoSize", texco_size, 1);
-
-  return true;
-}
-
 void EEVEE_volumes_cache_object_add(EEVEE_ViewLayerData *sldata,
                                     EEVEE_Data *vedata,
                                     Scene *scene,
@@ -502,15 +300,22 @@ void EEVEE_volumes_cache_object_add(EEVEE_ViewLayerData *sldata,
 
   int mat_options = VAR_MAT_VOLUME | VAR_MAT_MESH;
   struct GPUMaterial *mat = EEVEE_material_get(vedata, scene, ma, NULL, mat_options);
-  eGPUMaterialStatus status = GPU_material_status(mat);
 
   /* If shader failed to compile or is currently compiling. */
-  if (status != GPU_MAT_SUCCESS) {
+  if (mat == NULL) {
     return;
   }
 
+  /* TODO(fclem): Reuse main shading group to avoid shading binding cost just like for surface
+   * shaders. */
   DRWShadingGroup *grp = DRW_shgroup_material_create(mat, vedata->psl->volumetric_objects_ps);
 
+  grp = DRW_shgroup_volume_create_sub(scene, ob, grp, mat);
+
+  if (grp == NULL) {
+    return;
+  }
+
   /* TODO(fclem): remove those "unnecessary" UBOs */
   DRW_shgroup_uniform_block(grp, "planar_block", sldata->planar_ubo);
   DRW_shgroup_uniform_block(grp, "probe_block", sldata->probe_ubo);
@@ -518,22 +323,7 @@ void EEVEE_volumes_cache_object_add(EEVEE_ViewLayerData *sldata,
   DRW_shgroup_uniform_block(grp, "light_block", sldata->light_ubo);
   DRW_shgroup_uniform_block(grp, "grid_block", sldata->grid_ubo);
   DRW_shgroup_uniform_block(grp, "renderpass_block", sldata->renderpass_ubo.combined);
-
   DRW_shgroup_uniform_block(grp, "common_block", sldata->common_ubo);
-
-  ListBase gpu_grids = GPU_material_volume_grids(mat);
-
-  if (ob->type == OB_VOLUME) {
-    if (!eevee_volume_object_grids_init(ob, &gpu_grids, grp)) {
-      return;
-    }
-  }
-  else {
-    if (!eevee_volume_object_mesh_init(scene, ob, &gpu_grids, grp)) {
-      return;
-    }
-  }
-
   /* TODO: Reduce to number of slices intersecting. */
   /* TODO: Preemptive culling. */
   DRW_shgroup_call_procedural_triangles(grp, ob, sldata->common_data.vol_tex_size[2]);
diff --git a/source/blender/draw/engines/eevee/shaders/shadow_vert.glsl b/source/blender/draw/engines/eevee/shaders/shadow_vert.glsl
index 0e8e8dd9d01..ab0f4d6bec8 100644
--- a/source/blender/draw/engines/eevee/shaders/shadow_vert.glsl
+++ b/source/blender/draw/engines/eevee/shaders/shadow_vert.glsl
@@ -144,3 +144,13 @@ vec3 attr_load_uv(vec3 attr)
   return attr;
 }
 #endif
+
+/* Passthrough. */
+float attr_load_temperature_post(float attr)
+{
+  return attr;
+}
+vec4 attr_load_color_post(vec4 attr)
+{
+  return attr;
+}
diff --git a/source/blender/draw/engines/eevee/shaders/surface_frag.glsl b/source/blender/draw/engines/eevee/shaders/surface_frag.glsl
index 9ad7a4fdbc1..79ec3807d0b 100644
--- a/source/blender/draw/engines/eevee/shaders/surface_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/surface_frag.glsl
@@ -179,3 +179,13 @@ vec3 attr_load_uv(vec3 attr)
 {
   return vec3(0);
 }
+
+/* Passthrough. */
+float attr_load_temperature_post(float attr)
+{
+  return attr;
+}
+vec4 attr_load_color_post(vec4 attr)
+{
+  return attr;
+}
diff --git a/source/blender/draw/engines/eevee/shaders/surface_vert.glsl b/source/blender/draw/engines/eevee/shaders/surface_vert.glsl
index 6c6b810422b..49c18832f72 100644
--- a/source/blender/draw/engines/eevee/shaders/surface_vert.glsl
+++ b/source/blender/draw/engines/eevee/shaders/surface_vert.glsl
@@ -157,3 +157,13 @@ vec3 attr_load_uv(vec3 attr)
   return attr;
 }
 #endif
+
+/* Passthrough. */
+float attr_load_temperature_post(float attr)
+{
+  return attr;
+}
+vec4 attr_load_color_post(vec4 attr)
+{
+  return attr;
+}
diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
index e0a79872928..914261d7f59 100644
--- a/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_frag.glsl
@@ -4,17 +4,13 @@
 /* Based on Frosbite Unified Volumetric.
  * https://www.ea.com/frostbite/news/physically-based-unified-volumetric-rendering-in-frostbite */
 
-#ifdef MESH_SHADER
-uniform vec3 volumeOrcoLoc;
-uniform vec3 volumeOrcoSize;
-uniform mat4 volumeObjectToTexture;
-uniform float volumeDensityScale = 1.0;
-#endif
+/* Store volumetric properties into the froxel textures. */
 
 flat in int slice;
 
 /* Warning: these are not attributes, these are global vars. */
 vec3 worldPosition = vec3(0.0);
+vec3 objectPosition = vec3(0.0);
 vec3 viewPosition = vec3(0.0);
 vec3 viewNormal = vec3(0.0);
 vec3 volumeOrco = vec3(0.0);
@@ -24,9 +20,9 @@ layout(location = 1) out vec4 volumeExtinction;
 layout(location = 2) out vec4 volumeEmissive;
 layout(location = 3) out vec4 volumePhase;
 
-/* Store volumetric properties into the froxel textures. */
+int attr_id;
 
-#ifdef MESH_SHADER
+#ifndef CLEAR
 GlobalData init_globals(void)
 {
   GlobalData surf;
@@ -80,10 +76,8 @@ void main()
   viewPosition = get_view_space_from_depth(ndc_cell.xy, ndc_cell.z);
   worldPosition = point_view_to_world(viewPosition);
 #ifdef MESH_SHADER
-  volumeOrco = point_world_to_object(worldPosition);
-  /* TODO: redundant transform */
-  volumeOrco = (volumeOrco - volumeOrcoLoc + volumeOrcoSize) / (volumeOrcoSize * 2.0);
-  volumeOrco = (volumeObjectToTexture * vec4(volumeOrco, 1.0)).xyz;
+  objectPosition = point_world_to_object(worldPosition);
+  volumeOrco = OrcoTexCoFactors[0].xyz + objectPosition * OrcoTexCoFactors[1].xyz;
 
   if (any(lessThan(volumeOrco, vec3(0.0))) || any(greaterThan(volumeOrco, vec3(1.0)))) {
     /* Note: Discard is not an explicit return in Metal prior to versions 2.3.
@@ -100,15 +94,13 @@ void main()
   volumeEmissive = vec4(0.0, 0.0, 0.0, 1.0);
   volumePhase = vec4(0.0, 0.0, 0.0, 0.0);
 #else
-#  ifdef MESH_SHADER
   g_data = init_globals();
   attrib_load();
-#  endif
   Closure cl = nodetree_exec();
 #  ifdef MESH_SHADER
-  cl.scatter *= volumeDensityScale;
-  cl.absorption *= volumeDensityScale;
-  cl.emission *= volumeDensityScale;
+  cl.scatter *= drw_volume.density_scale;
+  cl.absorption *= drw_volume.density_scale;
+  cl.emission *= drw_volume.density_scale;
 #  endif
 
   volumeScattering = vec4(cl.scatter, 1.0);
@@ -124,35 +116,72 @@ void main()
 #endif
 }
 
-vec3 attr_load_orco(vec4 orco)
+vec3 grid_coordinates()
+{
+  vec3 co = volumeOrco;
+#ifdef MESH_SHADER
+  /* Optional per-grid transform. */
+  if (drw_volume.grids_xform[attr_id][3][3] != 0.0) {
+    co = (drw_volume.grids_xform[attr_id] * vec4(objectPosition, 1.0)).xyz;
+  }
+#endif
+  attr_id += 1;
+  return co;
+}
+
+vec3 attr_load_orco(sampler3D orco)
 {
+  attr_id += 1;
   return volumeOrco;
 }
-vec4 attr_load_tangent(vec4 tangent)
+vec4 attr_load_tangent(sampler3D tangent)
 {
+  attr_id += 1;
   return vec4(0);
 }
-vec4 attr_load_vec4(vec4 attr)
+vec4 attr_load_vec4(sampler3D tex)
 {
-  return vec4(0);
+  return texture(tex, grid_coordinates());
 }
-vec3 attr_load_vec3(vec3 attr)
+vec3 attr_load_vec3(sampler3D tex)
 {
-  return vec3(0);
+  return texture(tex, grid_coordinates()).rgb;
 }
-vec2 attr_load_vec2(vec2 attr)
+vec2 attr_load_vec2(sampler3D tex)
 {
-  return vec2(0);
+  return texture(tex, grid_coordinates()).rg;
 }
-float attr_load_float(float attr)
+float attr_load_float(sampler3D tex)
 {
-  return 0.0;
+  return texture(tex, grid_coordinates()).r;
 }
-vec4 attr_load_color(vec4 attr)
+vec4 attr_load_color(sampler3D tex)
 {
-  return vec4(0);
+  return texture(tex, grid_coordinates());
 }
-vec3 attr_load_uv(vec3 attr)
+vec3 attr_load_uv(sampler3D attr)
 {
+  attr_id += 1;
   return vec3(0);
 }
+
+/* TODO(@fclem): These implementation details should concern the DRWManager and not be a fix on
+ * the engine side. But as of now, the engines are reponsible for loading the attributes. */
+float attr_load_temperature_post(float attr)
+{
+#ifdef MESH_SHADER
+  /* Bring the into standard range without having to modify the grid values */
+  attr = (attr > 0.01) ? (attr * drw_volume.temperature_mul + drw_volume.temperature_bias) : 0.0;
+#endif
+  return attr;
+}
+vec4 attr_load_color_post(vec4 attr)
+{
+#ifdef MESH_SHADER
+  /* Density is premultiplied for interpolation, divide it out here. */
+  attr.rgb *= safe_rcp(attr.a);
+  attr.rgb *= drw_volume.color_mul.rgb;
+  attr.a = 1.0;
+#endif
+  return attr;
+}