Rework of volume shading

After code review and experimentation, this commit makes some changes to the way that volumes are shaded. Previously, there were problems with the 'scattering' component, in that it wasn't physically correct - it didn't conserve energy and was just acting as a brightness multiplier. This has been changed to be more correct, so that as the light is scattered out of the volume, there is less remaining to penetrate through.

Since this behaviour is very similar to absorption but more useful, absorption has been removed and has been replaced by a 'transmission colour' - controlling the colour of light penetrating through the volume after it has been scattered/absorbed. As well as this, there's now 'reflection', a non-physically correct RGB multiplier for out-scattered light. This is handy for tweaking the overall colour of the volume, without having to worry about wavelength dependent absorption, and its effects on transmitted light. Now at least, even though there is the ability to tweak things non-physically, volume shading is physically based by default, and has a better combination of correctness and ease of use.

There's more detailed information and example images here:
http://wiki.blender.org/index.php/User:Broken/VolumeRendering

Also did some tweaks/optimisation:
* Removed shading step size (was a bit annoying, if it comes back, it will be in a different form)
* Removed phase function options, now just one asymmetry slider controls the range between back-scattering, isotropic scattering, and forward scattering. (note, more extreme values gives artifacts with light cache, will fix...)
* Disabled the extra 'bounce lights' from the preview render for volumes, speeds updates significantly
* Enabled voxeldata texture in preview render
* Fixed volume shadows (they were too dark, fixed by avoiding using the shadfac/AddAlphaLight stuff)

More revisions to come later...

1 files changed, 51 insertions, 56 deletions

diff --git a/source/blender/makesrna/intern/rna_material.c b/source/blender/makesrna/intern/rna_material.c
index 9f7b97dfa05..593fc2e5935 100644
--- a/source/blender/makesrna/intern/rna_material.c
+++ b/source/blender/makesrna/intern/rna_material.c
@@ -537,14 +537,19 @@ static void rna_def_material_mtex(BlenderRNA *brna)
 	/* volume material */
 	prop= RNA_def_property(srna, "map_coloremission", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_EMISSION_COL);
-	RNA_def_property_ui_text(prop, "Emission Color", "Causes the texture to affect the colour of emission");
+	RNA_def_property_ui_text(prop, "Emission Color", "Causes the texture to affect the color of emission");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
-	prop= RNA_def_property(srna, "map_colorabsorption", PROP_BOOLEAN, PROP_NONE);
-	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_ABSORPTION_COL);
-	RNA_def_property_ui_text(prop, "Absorption Color", "Causes the texture to affect the result colour after absorption");
+	prop= RNA_def_property(srna, "map_colorreflection", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_REFLECTION_COL);
+	RNA_def_property_ui_text(prop, "Reflection Color", "Causes the texture to affect the color of scattered light");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
+	prop= RNA_def_property(srna, "map_colortransmission", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_TRANSMISSION_COL);
+	RNA_def_property_ui_text(prop, "Transmission Color", "Causes the texture to affect the result color after other light has been scattered/absorbed");
+	RNA_def_property_update(prop, NC_TEXTURE, NULL);
+	
 	prop= RNA_def_property(srna, "map_density", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_DENSITY);
 	RNA_def_property_ui_text(prop, "Density", "Causes the texture to affect the volume's density");
@@ -555,28 +560,34 @@ static void rna_def_material_mtex(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Emission", "Causes the texture to affect the volume's emission");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
-	prop= RNA_def_property(srna, "map_absorption", PROP_BOOLEAN, PROP_NONE);
-	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_ABSORPTION);
-	RNA_def_property_ui_text(prop, "Absorption", "Causes the texture to affect the volume's absorption");
-	RNA_def_property_update(prop, 0, "rna_Material_update");
-	
 	prop= RNA_def_property(srna, "map_scattering", PROP_BOOLEAN, PROP_NONE);
 	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_SCATTERING);
 	RNA_def_property_ui_text(prop, "Scattering", "Causes the texture to affect the volume's scattering");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
+	prop= RNA_def_property(srna, "map_reflection", PROP_BOOLEAN, PROP_NONE);
+	RNA_def_property_boolean_sdna(prop, NULL, "mapto", MAP_REFLECTION);
+	RNA_def_property_ui_text(prop, "Reflection", "Causes the texture to affect the reflected light's brightness");
+	RNA_def_property_update(prop, NC_TEXTURE, NULL);
+	
 	prop= RNA_def_property(srna, "coloremission_factor", PROP_FLOAT, PROP_NONE);
 	RNA_def_property_float_sdna(prop, NULL, "colfac");
 	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
 	RNA_def_property_ui_text(prop, "Emission Color Factor", "Amount texture affects emission color.");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
-	prop= RNA_def_property(srna, "colorabsorption_factor", PROP_FLOAT, PROP_NONE);
+	prop= RNA_def_property(srna, "colorreflection_factor", PROP_FLOAT, PROP_NONE);
 	RNA_def_property_float_sdna(prop, NULL, "colfac");
 	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
-	RNA_def_property_ui_text(prop, "Absorpion Color Factor", "Amount texture affects diffuse color.");
+	RNA_def_property_ui_text(prop, "Reflection Color Factor", "Amount texture affects color of out-scattered light");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
+	prop= RNA_def_property(srna, "colortransmission_factor", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_float_sdna(prop, NULL, "colfac");
+	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
+	RNA_def_property_ui_text(prop, "Transmission Color Factor", "Amount texture affects result color after light has been scattered/absorbed.");
+	RNA_def_property_update(prop, NC_TEXTURE, NULL);
+	
 	prop= RNA_def_property(srna, "density_factor", PROP_FLOAT, PROP_NONE);
 	RNA_def_property_float_sdna(prop, NULL, "varfac");
 	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
@@ -589,16 +600,16 @@ static void rna_def_material_mtex(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Emission Factor", "Amount texture affects emission.");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
-	prop= RNA_def_property(srna, "absorption_factor", PROP_FLOAT, PROP_NONE);
+	prop= RNA_def_property(srna, "scattering_factor", PROP_FLOAT, PROP_NONE);
 	RNA_def_property_float_sdna(prop, NULL, "varfac");
 	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
-	RNA_def_property_ui_text(prop, "Absorption Factor", "Amount texture affects absorption.");
+	RNA_def_property_ui_text(prop, "Scattering Factor", "Amount texture affects scattering.");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
-	
-	prop= RNA_def_property(srna, "scattering_factor", PROP_FLOAT, PROP_NONE);
+								  
+	prop= RNA_def_property(srna, "reflection_factor", PROP_FLOAT, PROP_NONE);
 	RNA_def_property_float_sdna(prop, NULL, "varfac");
 	RNA_def_property_ui_range(prop, 0, 1, 10, 3);
-	RNA_def_property_ui_text(prop, "Scattering Factor", "Amount texture affects scattering.");
+	RNA_def_property_ui_text(prop, "Reflection Factor", "Amount texture affects brightness of out-scattered light.");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
 	/* end volume material */
@@ -955,15 +966,6 @@ static void rna_def_material_volume(BlenderRNA *brna)
 		//{MA_VOL_STEP_ADAPTIVE, "ADAPTIVE", 0, "Adaptive", ""},
 		{0, NULL, 0, NULL, NULL}};
 		
-	static EnumPropertyItem prop_phasefunction_items[] = {
-		{MA_VOL_PH_ISOTROPIC, "ISOTROPIC", 0, "Isotropic", ""},
-		{MA_VOL_PH_MIEHAZY, "MIE_HAZY", 0, "Mie Hazy", ""},
-		{MA_VOL_PH_MIEMURKY, "MIE_MURKY", 0, "Mie Murky", ""},
-		{MA_VOL_PH_RAYLEIGH, "RAYLEIGH", 0, "Rayleigh", ""},
-		{MA_VOL_PH_HG, "HENYEY-GREENSTEIN", 0, "Henyey-Greenstein", ""},
-		{MA_VOL_PH_SCHLICK, "SCHLICK", 0, "Schlick", ""},
-		{0, NULL, 0, NULL, NULL}};
-
 	srna= RNA_def_struct(brna, "MaterialVolume", NULL);
 	RNA_def_struct_sdna(srna, "VolumeSettings");
 	RNA_def_struct_nested(brna, srna, "Material");
@@ -982,13 +984,6 @@ static void rna_def_material_volume(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Step Size", "Distance between subsequent volume depth samples.");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
-	prop= RNA_def_property(srna, "shading_step_size", PROP_FLOAT, PROP_NONE);
-	RNA_def_property_float_sdna(prop, NULL, "shade_stepsize");
-	RNA_def_property_range(prop, 0.0f, FLT_MAX);
-	RNA_def_property_ui_range(prop, 0.0f, 1.0f, 1, 3);
-	RNA_def_property_ui_text(prop, "Shading Step Size", "Distance between subsequent volume shading samples.");
-	RNA_def_property_update(prop, 0, "rna_Material_update");
-	
 	prop= RNA_def_property(srna, "scattering_mode", PROP_ENUM, PROP_NONE);
 	RNA_def_property_enum_sdna(prop, NULL, "shade_type");
 	RNA_def_property_enum_items(prop, prop_scattering_items);
@@ -1043,31 +1038,30 @@ static void rna_def_material_volume(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Density Scale", "Multiplier for the material's density");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
-	prop= RNA_def_property(srna, "absorption", PROP_FLOAT, PROP_NONE);
-	RNA_def_property_float_sdna(prop, NULL, "absorption");
+	prop= RNA_def_property(srna, "scattering", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_float_sdna(prop, NULL, "scattering");
 	RNA_def_property_range(prop, 0.0f, FLT_MAX);
-	RNA_def_property_ui_range(prop, 0.0f, 10.0f, 1, 3);
-	RNA_def_property_ui_text(prop, "Absorption", "Amount of light that gets absorbed by the volume - higher values mean light travels less distance");
+	RNA_def_property_ui_range(prop, 0.0f, 10.0f, 1 ,3);
+	RNA_def_property_ui_text(prop, "Scattering", "Amount of light that gets scattered out by the volume - the more out-scattering, the shallower the light will penetrate ");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
-	prop= RNA_def_property(srna, "absorption_color", PROP_FLOAT, PROP_COLOR);
-	RNA_def_property_float_sdna(prop, NULL, "absorption_col");
+	prop= RNA_def_property(srna, "transmission_color", PROP_FLOAT, PROP_COLOR);
+	RNA_def_property_float_sdna(prop, NULL, "transmission_col");
 	RNA_def_property_array(prop, 3);
-	RNA_def_property_ui_text(prop, "Absorption Color", "");
+	RNA_def_property_ui_text(prop, "Transmission Color", "Result color of the volume, after other light has been scattered/absorbed");
 	RNA_def_property_update(prop, 0, "rna_Material_draw_update");
 	
-	prop= RNA_def_property(srna, "scattering", PROP_FLOAT, PROP_NONE);
-	RNA_def_property_float_sdna(prop, NULL, "scattering");
-	RNA_def_property_range(prop, 0.0f, FLT_MAX);
-	RNA_def_property_ui_range(prop, 0.0f, 10.0f, 1 ,3);
-	RNA_def_property_ui_text(prop, "Scattering", "Amount of light that gets scattered by the volume - values > 1.0 are non-physical");
-	RNA_def_property_update(prop, 0, "rna_Material_update");
+	prop= RNA_def_property(srna, "reflection_color", PROP_FLOAT, PROP_COLOR);
+	RNA_def_property_float_sdna(prop, NULL, "reflection_col");
+	RNA_def_property_array(prop, 3);
+	RNA_def_property_ui_text(prop, "Reflection Color", "Colour of light scattered out of the volume (does not affect transmission)");
+	RNA_def_property_update(prop, NC_MATERIAL|ND_SHADING_DRAW, NULL);
 	
-	prop= RNA_def_property(srna, "emission", PROP_FLOAT, PROP_NONE);
-	RNA_def_property_float_sdna(prop, NULL, "emission");
+	prop= RNA_def_property(srna, "reflection", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_float_sdna(prop, NULL, "reflection");
 	RNA_def_property_range(prop, 0.0f, FLT_MAX);
-	RNA_def_property_ui_range(prop, 0.0f, 10.0f, 1, 3);
-	RNA_def_property_ui_text(prop, "Emission", "Amount of light that gets emitted by the volume");
+	RNA_def_property_ui_range(prop, 0.0f, 100.0f, 1 ,3);
+	RNA_def_property_ui_text(prop, "Reflection", "Multiplier to make out-scattered light brighter or darker (non-physically correct)");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 	
 	prop= RNA_def_property(srna, "emission_color", PROP_FLOAT, PROP_COLOR);
@@ -1076,16 +1070,17 @@ static void rna_def_material_volume(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Emission Color", "");
 	RNA_def_property_update(prop, 0, "rna_Material_draw_update");
 	
-	prop= RNA_def_property(srna, "phase_function", PROP_ENUM, PROP_NONE);
-	RNA_def_property_enum_sdna(prop, NULL, "phasefunc_type");
-	RNA_def_property_enum_items(prop, prop_phasefunction_items);
-	RNA_def_property_ui_text(prop, "Phase Function", "Isotropic/Anisotropic scattering");
+	prop= RNA_def_property(srna, "emission", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_float_sdna(prop, NULL, "emission");
+	RNA_def_property_range(prop, 0.0f, FLT_MAX);
+	RNA_def_property_ui_range(prop, 0.0f, 10.0f, 1, 3);
+	RNA_def_property_ui_text(prop, "Emission", "Amount of light that gets emitted by the volume");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
-	
+		
 	prop= RNA_def_property(srna, "asymmetry", PROP_FLOAT, PROP_NONE);
-	RNA_def_property_float_sdna(prop, NULL, "phasefunc_g");
+	RNA_def_property_float_sdna(prop, NULL, "asymmetry");
 	RNA_def_property_range(prop, -1.0f, 1.0f);
-	RNA_def_property_ui_text(prop, "Asymmetry", "Continuum between forward scattering and back scattering");
+	RNA_def_property_ui_text(prop, "Asymmetry", "Back scattering (-1.0) to Forward scattering (1.0) and the range in between.");
 	RNA_def_property_update(prop, 0, "rna_Material_update");
 }