USD visOption pass through and manual

docs/source/manual/index.rst

@@ -15,6 +15,7 @@ User Manual
    combining
    comparing
    analysing
+   rendering
 
 
 Indices and tables

docs/source/manual/rendering.rst

@@ -0,0 +1,40 @@
+.. _rendering:
+
+==================
+Rendering Geometry
+==================
+
+There are many reasons to render geometry beyond the VTK based
+pyg44omety viewer. Potential targets are 3D asset packages (e.g.
+blender, houdini), games engines (unreal engine, unity). A common
+format is Universal Scene Description (USD). USD can be loaded in
+many packages and so a dedicated UDS converter/viewer is available
+in pyg4ometry. For example to load and convert a file to USD
+
+  .. code-block:: python
+    :linenos:
+
+    r = pyg4ometry.gdml.Reader("lht.gdml")
+    l = r.getRegistry().getWorldVolume()
+    v = pyg4ometry.visualisation.UsdViewer("lht.usd")
+    v.traverseHierarchy(l)
+    v.save()
+
+This will write a file called lht.gdml which can be viewed in
+a USD viewer like usdview. USD has a preview shader. The settings
+can set via the `VisualisationOptions` stored in logical volumes
+(`visOption`), so for example
+
+  .. code-block:: python
+    :linenos:
+
+    r = pyg4ometry.gdml.Reader("lht.gdml")
+    reg = r.getRegistry()
+    reg.logicalVolumeDict["vCalorimeter"].visOptions.colour = [1, 0, 0]
+    v = pyg4ometry.visualisation.UsdViewer("lht.usd")
+    v.traverseHierarchy(reg.getWorldVolume())
+    v.save()
+
+`visOptions.color` and `visOptions.alpha` are passed though to the
+USD shader. There are many more options for shading in USD which can
+be accessed via `visOptions.usdOptions`

src/pyg4ometry/visualisation/VisualisationOptions.py

@@ -54,6 +54,8 @@ def __init__(
 
         # vis options for pbr shaders
         self.usdOptions = UsdPreviewSurfaceOptions()
+        self.usdOptions.diffuseColor = colour
+        self.usdOptions.opacity = alpha
 
     def __repr__(self):
         rgba = [*self.getColour(), self.alpha]