Updated texture mapping docs.

documentation/TextureMapping.rst

@@ -98,25 +98,13 @@ Blending between materials
 --------------------------
 An additional complication when working with smooth voxel terrain is that it is usually desirable to blend smoothly between adjacent voxels with different materials. This situation does not occur with cubic meshes because the texture is considered to be per-face instead of per-vertex, and PolyVox enforces this by ensuring that all the vertices of a given face have the same material.
 
-With a smooth mesh it is possible for each of the three vertices of any given triangle to have different material identifiers (see figure below). If this is not explicitely handled then the graphics hardware will interpolate these material values across the face of the triangle. Fundamentally, the concept of interpolating between material identifiers does not make sense, because if we have 1='grass', 2='rock' and 3='sand' then it does not make sense to say rock is the average of grass and sand.
+With a smooth mesh it is possible for each of the three vertices of any given triangle to have different material identifiers. If this is not explicitely handled then the graphics hardware will interpolate these material values across the face of the triangle. Fundamentally, the concept of interpolating between material identifiers does not make sense, because if we have (for example) 1='grass', 2='rock' and 3='sand' then it does not make sense to say rock is the average of grass and sand.
 
-ADD FIGURE HERE
+Correctly handling of this is a suprising difficult problem. For now, the best approach is discribed in our article 'Volumetric representation of virtual terrain' which appeared in Game Engine Gems Volume 1 and which is freely available through the Google Books preview here: http://books.google.com/books?id=WNfD2u8nIlIC&lpg=PR1&dq=game%20engine%20gems&pg=PA39#v=onepage&q&f=false
 
-There are a couple approaches we can adopt to combat this problem. However, the solutions do have some performance and/or memory cost so we should first realise that this issue only applies to a small number of the tringles in the mesh. Typically most triangles will be using the same material for all three vertices, and so it is almost certainly worth splitting the mesh into two pieces. One peice will contain all the triangles which have the same material and can be rendered as normal. The other peice will contain all the triangles which have a mix of materials, and for these we use the more complex rendering techniques below. Note that splitting the mesh like this will also increase the batch count so it may not be desirable in all circumstances.
+As off October 2012 we are actively researching alternative solutions to this problem though it will be some time before the results become available.
 
-NOTE - THESE SOLUTIONS ARE WORK IN PROGRESS. CORRECTLY BLENDING MATERIAL IS AN AREA WHICH WE ARE STILL RESEARCHING, AND IN THE MEAN TIME YOU MIGHT ALSO BE INTERESTED IN OUR ARTICLE IN GAME ENGINE GEMS. See: http://books.google.com/books?id=WNfD2u8nIlIC&lpg=PR1&dq=game%20engine%20gems&pg=PA39#v=onepage&q&f=false
+Actual implementation of these material blending approaches is left as an excercise to the reader, though it is possible that in the future we will add some utility functions to PolyVox to assist with tasks such as splitting the mesh or adding the required extra vertex attributes. Our test implementations have performed the mesh processing on the CPU before the mesh is uploaded to the graphics card, but it does seem like there is a lot of potential for implementing these approaches in the geometry shader.
-
-One approach is to attach an alpha value to each vertex so that corners of a triangle can optionally be faded out. If a triangle has the same material value at each vertex then we also give it full alpha at each vertex and the triangle draws normally, but if it has a differnt material for each vertex then we duplicate the triangle three times (once for each material). Each new triangle should then use the same material at each vertex, this material being one of those from the original triangle. The alpha values of the vertices of the new triangles are set such that when the three triangles are drawn on top of each other with additive alpha blending, the desired smoothly shaded triangle results.
-
-One drawback of this approach is that the mesh needs to be drawn with alpha blending enabled, which is both costly and also allows previously drawn geometry to show through. Therefore, before any alpha blended geometry is drawn, you also need to draw the triangle solidly in black (which in turn means one of your material identifier need to be reserved as solid black - we use material zero below). This whole process is rather difficult to explain, but hopefully this diagram of the inputs and outputs makes it clearer:
-
-//MaterialBlending diagram
-
-//NOTE - Actually this doesn't work. The black triangles need different blending so must be drawn seperatly. we should recommend first off all that the mesh is split into single/multi material parts, and then come on to the approaches for multimaterial handling. Also consider the issue of whether the rendering order of triangles is guarenteed... but does this matter if black is a seperate pass? Also, what about drawing white triangles and then using multiplicative blending? No, this doesn't help because the white triangles would still need to be drawn seperatly as mutiplying by white is no better than adding black. Subtractive blending maybe? I don't think so... Custom blend process? Using alpha and colour sperately? Check the options here.
-
-//Add kers approach based on extra vertex properties.
-
-Actual implementation of these material blending approaches is left as an excercise to the reader, though it is possible that in the future we will add some utility functions to PolyVox to assist with tasks such as splitting the mesh or adding the extra vertex attributes. Our test implementations have performed the mesh processing on the CPU before the mesh is uploaded to the graphics card, but it does seem like there is a lot of potential for implementing these approaches in the geometry shader.
 
 Storage of textures
 ===================