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.

4 files changed, 89 insertions, 30 deletions

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;
+}