Fix T81254: Incorrect calculation of EEVEE Transmittance Volumetrics

Regular rendering uses a custom blend mode, but render passes renders to
2 separate textures. This wasn't configured correctly inside the
fragment shaders. This patch adds a switch to configure the fragment
shader with the correct attachments.

Backport to Blender 2.83.

Reviewed By: Clément Foucault

Differential Revision: https://developer.blender.org/D9038

5 files changed, 21 insertions, 12 deletions

diff --git a/source/blender/draw/engines/eevee/eevee_private.h b/source/blender/draw/engines/eevee/eevee_private.h
index 10f3314c573..5330ea2f770 100644
--- a/source/blender/draw/engines/eevee/eevee_private.h
+++ b/source/blender/draw/engines/eevee/eevee_private.h
@@ -1119,7 +1119,7 @@ struct GPUShader *EEVEE_shaders_volumes_clear_sh_get(void);
 struct GPUShader *EEVEE_shaders_volumes_scatter_sh_get(void);
 struct GPUShader *EEVEE_shaders_volumes_scatter_with_lights_sh_get(void);
 struct GPUShader *EEVEE_shaders_volumes_integration_sh_get(void);
-struct GPUShader *EEVEE_shaders_volumes_resolve_sh_get(void);
+struct GPUShader *EEVEE_shaders_volumes_resolve_sh_get(bool accum);
 struct GPUShader *EEVEE_shaders_volumes_accum_sh_get(void);
 struct GPUShader *EEVEE_shaders_ggx_lut_sh_get(void);
 struct GPUShader *EEVEE_shaders_ggx_refraction_lut_sh_get(void);
diff --git a/source/blender/draw/engines/eevee/eevee_shaders.c b/source/blender/draw/engines/eevee/eevee_shaders.c
index 58897272425..f46a98ed845 100644
--- a/source/blender/draw/engines/eevee/eevee_shaders.c
+++ b/source/blender/draw/engines/eevee/eevee_shaders.c
@@ -137,7 +137,7 @@ static struct {
   struct GPUShader *scatter_sh;
   struct GPUShader *scatter_with_lights_sh;
   struct GPUShader *volumetric_integration_sh;
-  struct GPUShader *volumetric_resolve_sh;
+  struct GPUShader *volumetric_resolve_sh[2];
   struct GPUShader *volumetric_accum_sh;
 
   /* Shader strings */
@@ -854,13 +854,16 @@ struct GPUShader *EEVEE_shaders_volumes_integration_sh_get()
   return e_data.volumetric_integration_sh;
 }
 
-struct GPUShader *EEVEE_shaders_volumes_resolve_sh_get()
+struct GPUShader *EEVEE_shaders_volumes_resolve_sh_get(bool accum)
 {
-  if (e_data.volumetric_resolve_sh == NULL) {
-    e_data.volumetric_resolve_sh = DRW_shader_create_fullscreen_with_shaderlib(
-        datatoc_volumetric_resolve_frag_glsl, e_data.lib, SHADER_DEFINES);
+  const int index = accum ? 1 : 0;
+  if (e_data.volumetric_resolve_sh[index] == NULL) {
+    e_data.volumetric_resolve_sh[index] = DRW_shader_create_fullscreen_with_shaderlib(
+        datatoc_volumetric_resolve_frag_glsl,
+        e_data.lib,
+        accum ? "#define VOLUMETRICS_ACCUM\n" SHADER_DEFINES : SHADER_DEFINES);
   }
-  return e_data.volumetric_resolve_sh;
+  return e_data.volumetric_resolve_sh[index];
 }
 
 struct GPUShader *EEVEE_shaders_volumes_accum_sh_get()
@@ -1409,7 +1412,8 @@ void EEVEE_shaders_free(void)
   DRW_SHADER_FREE_SAFE(e_data.scatter_sh);
   DRW_SHADER_FREE_SAFE(e_data.scatter_with_lights_sh);
   DRW_SHADER_FREE_SAFE(e_data.volumetric_integration_sh);
-  DRW_SHADER_FREE_SAFE(e_data.volumetric_resolve_sh);
+  DRW_SHADER_FREE_SAFE(e_data.volumetric_resolve_sh[0]);
+  DRW_SHADER_FREE_SAFE(e_data.volumetric_resolve_sh[1]);
   DRW_SHADER_FREE_SAFE(e_data.volumetric_accum_sh);
   DRW_SHADER_FREE_SAFE(e_data.probe_filter_glossy_sh);
   DRW_SHADER_FREE_SAFE(e_data.probe_filter_diffuse_sh);
diff --git a/source/blender/draw/engines/eevee/eevee_volumes.c b/source/blender/draw/engines/eevee/eevee_volumes.c
index 48e38a9c982..c6e6530830d 100644
--- a/source/blender/draw/engines/eevee/eevee_volumes.c
+++ b/source/blender/draw/engines/eevee/eevee_volumes.c
@@ -610,7 +610,7 @@ void EEVEE_volumes_cache_finish(EEVEE_ViewLayerData *sldata, EEVEE_Data *vedata)
         grp, NULL, USE_VOLUME_OPTI ? 1 : common_data->vol_tex_size[2]);
 
     DRW_PASS_CREATE(psl->volumetric_resolve_ps, DRW_STATE_WRITE_COLOR | DRW_STATE_BLEND_CUSTOM);
-    grp = DRW_shgroup_create(EEVEE_shaders_volumes_resolve_sh_get(), psl->volumetric_resolve_ps);
+    grp = DRW_shgroup_create(EEVEE_shaders_volumes_resolve_sh_get(false), psl->volumetric_resolve_ps);
     DRW_shgroup_uniform_texture_ref(grp, "inScattering", &txl->volume_scatter);
     DRW_shgroup_uniform_texture_ref(grp, "inTransmittance", &txl->volume_transmit);
     DRW_shgroup_uniform_texture_ref(grp, "inSceneDepth", &e_data.depth_src);
@@ -823,7 +823,7 @@ void EEVEE_volumes_output_init(EEVEE_ViewLayerData *sldata, EEVEE_Data *vedata,
   DRW_PASS_CREATE(psl->volumetric_accum_ps, DRW_STATE_WRITE_COLOR | DRW_STATE_BLEND_ADD_FULL);
   DRWShadingGroup *grp = NULL;
   if ((effects->enabled_effects & EFFECT_VOLUMETRIC) != 0) {
-    grp = DRW_shgroup_create(EEVEE_shaders_volumes_resolve_sh_get(), psl->volumetric_accum_ps);
+    grp = DRW_shgroup_create(EEVEE_shaders_volumes_resolve_sh_get(true), psl->volumetric_accum_ps);
     DRW_shgroup_uniform_texture_ref(grp, "inScattering", &txl->volume_scatter);
     DRW_shgroup_uniform_texture_ref(grp, "inTransmittance", &txl->volume_transmit);
     DRW_shgroup_uniform_texture_ref(grp, "inSceneDepth", &e_data.depth_src);
diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_accum_frag.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_accum_frag.glsl
index 1b6a7b33f42..d8ce7905a1a 100644
--- a/source/blender/draw/engines/eevee/shaders/volumetric_accum_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_accum_frag.glsl
@@ -1,8 +1,8 @@
 
 /* This shader is used to add default values to the volume accum textures.
  * so it looks similar (transmittance = 1, scattering = 0) */
-layout(location = 0, index = 0) out vec4 FragColor0;
-layout(location = 0, index = 1) out vec4 FragColor1;
+layout(location = 0) out vec4 FragColor0;
+layout(location = 1) out vec4 FragColor1;
 
 void main()
 {
diff --git a/source/blender/draw/engines/eevee/shaders/volumetric_resolve_frag.glsl b/source/blender/draw/engines/eevee/shaders/volumetric_resolve_frag.glsl
index 6ab21587c9a..665a34741ca 100644
--- a/source/blender/draw/engines/eevee/shaders/volumetric_resolve_frag.glsl
+++ b/source/blender/draw/engines/eevee/shaders/volumetric_resolve_frag.glsl
@@ -9,8 +9,13 @@
 uniform sampler2D inSceneDepth;
 
 /* Blend equation is : FragColor0 + FragColor1 * DstColor */
+#ifdef VOLUMETRICS_ACCUM
+layout(location = 0) out vec4 FragColor0;
+layout(location = 1) out vec4 FragColor1;
+#else
 layout(location = 0, index = 0) out vec4 FragColor0;
 layout(location = 0, index = 1) out vec4 FragColor1;
+#endif
 
 void main()
 {