[x3d-public] PBR > freewrl experience

[GPU Group]

My experience implementing some of the PBR v4 features in freewrl:
- took 16 days
- I started by defining the PhysicalMaterial, UnlitMaterial and extending
the Material node fields, using Michalis's documetation.
https://github.com/michaliskambi/x3d-tests/wiki/X3D-version-4:-New-features-of-materials,-lights-and-textures#new-x3dmaterialnode-node-with-emissive-and-normalmap-textures
- and internally extended structs in program and shader to hold all the new
fields
- then most of the 2 weeks was getting the information flowing to the
shader without breaking current functionality,
-- appearance.backMaterial - I spent a few days harmonizing
TwoMaterialShader and this new approach
-- for the texture maps I found there ware lots of issues with old code
assuming certain scenarios, and I spent about a week refactoring messy old
code
- I found the khronos glTF sample had some interesting hints beyond
physical, for example they use getter() functions in frag code that
hides/abstracts the issue of whether or not there's a texture map.

https://github.com/KhronosGroup/glTF-Sample-Viewer/tree/master/src/shaders

- see the metallic-roughness.frag

normal = getNormal();

And I refactored the frag shader to use getters like they do.

- I spent a few extra hours reading up on normal maps (which I learned are
in 'tangent space' so need a transform matrix called TBN applied):

https://learnopengl.com/Advanced-Lighting/Normal-Mapping

- normal map normals are expressed in tangent space


Once I had getters, and had all the info flowing to the frag shader, then
it only took a few days to do the metalic-roughness functionality.

If you have a loop where you are applying the lights, you probably have
something like:

diffuse += ...

specular +=

ambient +=

inside the loop.

For metalic-roughness, I found I could replace those 3 with

shade = getPointShade(pointToLight, materialInfo, normal, view);

from the khronos example to make a PBR equivalent light loop.


So in hindsight, it would have been easy to start with the scalar
PhyscialMaterial fields, and I could have added the texturemaps to the
getters() later.

For those implementing for GLES < 3.0 where texture units are in short
supply, the scalar part of PhysicalMaterial should be no problem.


I still have some minor tinkiering to do -when to apply gamma or its
inverse, Michalis has given some answers. And field naming,
textureTransform/textureCoordinate 'channel' field conventions, and I
haven't attempted EnvironmentLight.

Results:

1) PBR physics based rendering via PhysicalMaterial node
 - the lighting equations use roughness and metallic with baseColor
- (instead of specular, shininess and diffuseColor)
- and constrain to physics principle of 'conservation of energy'
http://dug9.users.sourceforge.net/web3d/tests/PBR/screenshot_pbr_teapots_view3dscene_freewrl.jpg
http://dug9.users.sourceforge.net/web3d/tests/PBR/metallic_roughness.x3dv

2) extended Material node - supports texture maps for normals, emissive,
diffuse, specularShininess, ambient
http://dug9.users.sourceforge.net/web3d/tests/PBR/screenshot_e_mat_sphere240_view3dscene_freewrl.jpg
http://dug9.users.sourceforge.net/web3d/tests/PBR/sphere_240_fw.x3dv

The PhysicalMaterial node also supports texturemap for metallicRoughness
although I haven't tested.

3) UnlitMaterial - roughed in plumbing but haven't tested thoroughly -
should work and support normal and emissive texture maps

freewrl code samples:
https://sourceforge.net/p/freewrl/git/ci/develop/tree/freex3d/src/lib/opengl/Compositing_Shaders.c

L.1106 MaterialInfo and gamma functions from Khronos glTF example
L.1155 getNormal() and getter() farm starts
L.1300-1340 getting ready for apply_lights_physical
L.1806 apply_lights_physical code with khronos glTF sample functions,
followed by freewrl light loop calling shade = pointShade(...) L.1917

https://sourceforge.net/p/freewrl/git/ci/develop/tree/freex3d/src/lib/scenegraph/Component_Shape.c

L.1417 compile_PhysicalMaterial - organizes into internal structs and flags
for sending to shader
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