Update Feature-Multitexture.md

docs/Feature-Multitexture.md

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+# Feature: Multitexturing
+
+synopsis: This feature allows to visualize multiple (opc) texture layers and control visualization properties.
+
+Instrument data and reconstruction information for example can be mapped onto the surface and blended with the albedo texture (here an accuracy map of the reconstruction):
+
+![image](https://github.com/pro3d-space/PRo3D/assets/513281/dc35a265-5432-4631-80d4-a8530e25bb0f)
+
+
+By using query parameters and a transfer function specific attributes can be filtered (e.g. here the elevation):
+
+![image](https://github.com/pro3d-space/PRo3D/assets/513281/1593df6d-2d63-49a1-a804-9e5bb06354c2)
+
+
+## UI
+
+The UI is implemented in https://github.com/pro3d-space/PRo3D/blob/045bafa5b167a5bc725b6dfa6519ac35e12f38e8/src/PRo3D.Core/Surface/Surface-Properties.fs#L175 and allows to specify what textures to use, and how to blend them.
+Datatypes for transfer functions is found in base: https://github.com/pro3d-space/PRo3D/blob/features/multitexture/src/PRo3D.Base/Multitexturing.fs
+The controls can be found in the surface properties when selecting a surface. 
+
+![image](https://github.com/pro3d-space/PRo3D/assets/513281/35f16a45-1f50-43ac-bdb6-311e9d826fb5)
+
+## Data & Rendering
+
+Rendering is handled in Aardvark.Geospatial.Opc which allows to customize what uniforms, textures and varyings are passed into the renderer.
+The entrypoints can be found here: https://github.com/pro3d-space/PRo3D/blob/045bafa5b167a5bc725b6dfa6519ac35e12f38e8/src/PRo3D.Core/Surface/Surface.Sg.fs#L205
+
+Inputs for the shader are prepared here: https://github.com/pro3d-space/PRo3D/blob/045bafa5b167a5bc725b6dfa6519ac35e12f38e8/src/PRo3D.Viewer/Viewer/Viewer-Utils.fs#L409
+The shader is here: https://github.com/pro3d-space/PRo3D/blob/045bafa5b167a5bc725b6dfa6519ac35e12f38e8/src/PRo3D.Base/Utilities.fs#L653
+
+In order to guide PRo3D how to handle the textures, opcx files are needed. Some logic finds it within the surface folder: https://github.com/pro3d-space/PRo3D/blob/045bafa5b167a5bc725b6dfa6519ac35e12f38e8/src/PRo3D.Viewer/Scene.fs#L123
+
+## Caveats and missing features
+
+ - Currently we only read opcx files as opposed to opcx.json files which also provide a friendly name for the texture layers. In favor of reduced complexity we opted for the simpler, established opcx solution which could also be extended to contain a friendly name for the layer.
+ - Textures are adressed via indices as opposed to by name (see https://github.com/pro3d-space/PRo3D/blob/045bafa5b167a5bc725b6dfa6519ac35e12f38e8/src/PRo3D.Viewer/Viewer/Viewer-Utils.fs#L382). This is historic and cannot be fixed easily. The 'Texture' record should be changed in aardvark.rendering, weights and textures separated into to arrays and so on..