Updated MaterialX documentation for recent changes

doc/MaterialX.md

@@ -18,27 +18,38 @@ We currently support exporting to MaterialX-compatible UsdShade networks:
     - Connections to color components and alpha/luminance
     - Normal maps
     - Custom color spaces
+- Place2dTexture
+- The ColorCorrect nodes from LookdevKit
+- The clamp utility node
+- Inserts explicit type conversion nodes where Maya uses implicit conversion
+- Exports connections at the component level
+
+### Partially supported:
+
+- Lambert, Blinn, and Phong shader nodes are exported as MaterialX approximations. This allows roundtripping.
 
 ### Not supported:
 
-- Lambert, Blinn, and Phong shader nodes
+- Layered and ramp surface shaders
+- Most of the Maya shader utility nodes
 - MaterialXSurface shader nodes from MaterialX Maya contrib
 
 ### To enable:
 
 Two options:
-- Install MayaUSD v0.16.0
+- Install MayaUSD v0.16.0 or later
 - Rebuild tip of MayaUSD repo using a MaterialX-enabled build of USD
 
 ## Import
 
 We can import MaterialX networks.
 - UsdShade networks consisting of MaterialX nodes used for shading in the `mtlx` render context
 - `.mtlx` files referenced in a USD stage as converted by the `usdMtlx` plugin in USD
+- Anything that was exported from Maya
 
 ### Not supported:
 
-- MaterialX networks containing surface shaders other than standard surface or preview surface
+- MaterialX networks containing surface shaders other than those exported
 - MaterialX networks containing unexpected procedural nodes
 - Importing to MaterialXSurface shader nodes
 
@@ -61,11 +72,13 @@ Maya spheres with standard surface shading:
 
 The same spheres exported with MaterialX shading and loaded as a USD stage: ![alt text](./USDMaterialXStandardSurfaceSampler.JPG "USD MaterialX sampler")
 
+- Some of the recently exportable Maya nodes require additional MaterialX nodegraph implementations supplied with MayaUSD (Phong, place2dTexture, LookdevKit::ColorCorrect)
+- Tangents will be shader-generated. By default they are arbitrarily oriented along the X derivative in screen space, but if a texcoord stream is used in the shader, the tangents will be aligned with the U direction.
+- Textures marked as sRGB will be automatically converted to Maya working color space. Limited to these working spaces: "scene-linear Rec.709-sRGB", "ACEScg", "ACES2065-1", "scene-linear DCI-P3 D65", and "scene-linear Rec.2020".
+
 ### Not supported:
 
 - DirectX 11 viewport
-- [Issue 1523](https://github.com/PixarAnimationStudios/USD/issues/1523): Color spaces
-- [Issue 1538](https://github.com/PixarAnimationStudios/USD/issues/1538): MaterialX networks containing surface shaders other than standard surface or preview surface
 
 ### To enable:
 
@@ -77,16 +90,19 @@ Once the updated plugin is in use, the viewport will automatically select Materi
 
 ### Building a MaterialX-enabled USD compatible with MayaUSD
 
-We currently use MaterialX 1.38.3 and USD 21.11. Combining these two together requires patching USD:
+Requires patching USD:
 
+For USD 22.05b:
 ```
-git checkout tags/v21.11
-# Fetch updates selected for next USD release
-git cherry-pick 08a5644770d063fe355aff1a2069b22b84d8402a
-git cherry-pick adfc04eea92b91965b0da68503539b079a5d30d9
-git cherry-pick 4ac8a7e9a440695f0392a94e27db1683773fce76
+git checkout tags/v22.05b
+# Fetch updates for MaterialX 1.38.4:
+git cherry-pick ac3ca253e643a095e0985264238be28b95109a44
+git cherry-pick 034df39a78a5fc4cada49b545ad2a87fd6666e1d
+git cherry-pick aaf20de564e29c3e59bcd32b3539add08b7597c6
+git cherry-pick 8a1ad41d27e2f1b9595a7404a7f8a89dce5cb5bc
+git cherry-pick e6edb7e8fd74f3f0d5cd981450df0c937a809257
 ```
 
-Then you need to merge the pending MaterialX v1.38.3 update submitted as [pull request 1738](https://github.com/PixarAnimationStudios/USD/pull/1738)
+Tip of USD dev branch is already using MaterialX 1.38.4
 
 Then you build USD using the `build_usd.py` script and the `--materialx` option.