Spelling correction and additional content for texturing documentation.

documentation/texturemapping.txt

@@ -1,33 +1,62 @@
 ******************************
 Texture mapping PolyVox meshes
 ******************************
-The PolyVox library is only concerned with operations on volume data (such as extracting a mesh from from a volume) and deliberatly avoids the issue of rendering such data. This means PolyVox is not tied to any bparticualr graphics API or rendering engine, and makes it much easier to integrate PolyVox with existing technology, because in general a PolyVox mesh can be treated the same as any other mesh. However, the texturing of a PolyVox mesh is usually handled a little differently, and so the purpose of tis document is to provide some ideas about where to start with the process of tesxture mapping PolyVox meshes.
+The PolyVox library is only concerned with operations on volume data (such as extracting a mesh from from a volume) and deliberatly avoids the issue of rendering any resulting polygon meshes. This means PolyVox is not tied to any particualr graphics API or rendering engine, and makes it much easier to integrate PolyVox with existing technology, because in general a PolyVox mesh can be treated the same as any other mesh. However, the texturing of a PolyVox mesh is usually handled a little differently, and so the purpose of this document is to provide some ideas about where to start with the process of tesxture mapping PolyVox meshes.
 
 This document is aimed at readers in one of two positions:
 	1) You are trying to texture 'Minecraft-style' terrain with cubic blocks and a number of different materials.
-	2) You are trying to texture smooth terrain produce my the Marching Cubes (or similar) algoritm)
-These are absollutly not the limit of PolyVox, and you can choose much more advanced rendering approaches if you wish. For example, in the past we have texture mapped a voxel earth from a cube map, and used an animated procedural texture based on Perlin noise for the magma at the denter of the earth. However, if you are aiming for such advanced techniques then we assume you understand the basics in this document and have enough knowledge to expand the ideas youslf. But do feel free to drop by and ask questions on out forum.
+	2) You are trying to texture smooth terrain produced by the Marching Cubes (or similar) algoritm.
+These are absolutly not the limit of PolyVox, and you can choose much more advanced rendering approaches if you wish. For example, in the past we have texture mapped a voxel Earth from a cube map and used an animated *procedural* texture (based on Perlin noise) for the magma at the center of the earth. However, if you are aiming for such advanced techniques then we assume you understand the basics in this document and have enough knowledge to expand the ideas yourself. But do feel free to drop by and ask questions on our forum.
 
 Traditionally meshes are textured by providing a pair of UV texture coordinates for each vertex, and these UV coordinates determine which parts of a texture maps to each vertex. The process of texturing PolyVox meshes is more complex for a couple of reasons:
 1) PolyVox does not provide UV coordinates for each vertex.
 2) Voxel terrain (particulaly Minecraft-style) often involves many more textures than the GPU can read at a time.
-By readin this document you should learn how to work around the above problems.
+By reading this document you should learn how to work around the above problems.
 
 Mapping textures to mesh geometry
 ================================
-The lack of UV coordinates means some reativity is requried in order to apply texture maps to meshes. But before we get to that, we will first try to unserstand the rational behind PolyVox not providing UV coordinates in the first place. This rational is different for the smooth voxel meshes vs the cubic voxel meshes.
+The lack of UV coordinates means some creativity is requried in order to apply texture maps to meshes. But before we get to that, we will first try to unserstand the rational behind PolyVox not providing UV coordinates in the first place. This rational is different for the smooth voxel meshes vs the cubic voxel meshes.
 
 Rational
 --------
-The problem with texturing smooth voxel meshes is simply that the geometry can get very complex and it is not clear how the mapping between mesh geometry and a texture should be performed. In a traditional heightmap-based terrain this relationship is obvious as the texture map and heightmap simply line up diretly. But for more complex shapes some form of 'UV unwrapping' is usually perfomred to define this relationship. This is usually done by an artist with the help of a 3D modeling package and so is a semi-automatic process, but is time comsuming and driven by the artists idea of what looks right for their particualar scene. Even though fully automatic UV unwrapping is possibly it is usualy prohibitavly slow.
+The problem with texturing smooth voxel meshes is simply that the geometry can get very complex and it is not clear how the mapping between mesh geometry and a texture should be performed. In a traditional heightmap-based terrain this relationship is obvious as the texture map and heightmap simply line up diretly. But for more complex shapes some form of 'UV unwrapping' is usually perfomred to define this relationship. This is usually done by an artist with the help of a 3D modeling package and so is a semi-automatic process, but is time comsuming and driven by the artists idea of what looks right for their particualar scene. Even though fully automatic UV unwrapping is possible it is usualy prohibitavly slow.
 
-Even if such an unwrapping was possible in a reasonable timeframe, the next problem is that it would be invalidated as soon as the mesh changed. Enabling dynamic terrain manipulation is one of the appealing factors of voxel terrain terrain, and is this use case were discarded then the user may as well just model their terrain in an existing 3D modelling package and texture there. For these reasons we do not attempt to generate and UV coordinates for smooth voxel meshes.
+Even if such an unwrapping was possible in a reasonable timeframe, the next problem is that it would be invalidated as soon as the mesh changed. Enabling dynamic terrain manipulation is one of the appealing factors of voxel terrain, and if this use case were discarded then the user may as well just model their terrain in an existing 3D modelling package and texture there. For these reasons we do not attempt to generate and UV coordinates for smooth voxel meshes.
 
 The rational in the cubic case is almost the opposite. For Minecraft style terrain you want to simply line up an instance of a texture with each face of a cube, and generating the texture coorordinates for this is very easy. In fact it's so easy that there's no point in doing it - the logic can instead be implemented in a shader which in turn allows the size of each vertex to be much smaller.
 
 Triplanar Texturing
 -------------------
-The most common approach to texture mapping smooth voxel terrain is to use Triplanar Texturing. Actually we do not need to go into too much detail here as there are a number of other references available [references] but we will summarise the process. 
+The most common approach to texture mapping smooth voxel terrain is to use *triplanar texturing*. Actually we do not need to go into too much detail here as there are a number of other references available [references] but we will summarise the process. 
+
+The basic idea is to project a texture along all three main axes and blend between the three texture samples according to the surface normal. As an example, suppose that we wish to write a fragment shader to apply a single texture to our terrain, and that we have access to both the world space position of the fragment and also it's normal. Also, note that your textures should be set to wrap because the world space position will quickly go outside the bound of 0.0-1.0. The world space position will need to have been passed through from earlier in the pipeline while the normal can be computed using one of the approaches in the lighting (link) document. The shader code would then look something like this [footnote: code is untested is is simplified compared to real world code. hopefully it compiles, but if not it should still give you an idea of how it works]:
+
+// Take the three testure samples
+vec4 sampleX = texture2d(inputTexture, worldSpacePos.yz); // Project along x axis
+vec4 sampleY = texture2d(inputTexture, worldSpacePos.xz); // Project along y axis
+vec4 sampleZ = texture2d(inputTexture, worldSpacePos.xy); // Project along z axis
+
+// Blend the samples according to the normal
+vec4 blendedColour = sampleX * normal.x + sampleY * normal.y + sampleZ * normal.z; 
+
+Note that this approach will lead to the texture repeating once every world unit, and so in practice you may wish to scale the world space positions to make the texture appear the desired size. Also this technique can be extended to work with normal mapping though we won't go into the details here.
+
+This idea of triplanar texturing can also be applied to the cubic meshes, though in some ways it can be considered to be even simpler. With cubic meshes the normal always points exactly along one of the main axes, and so it is not necessary to sample the texture three time and nor to blend the results. Instead you can use conditional branching in the fragment shader to determine which pair of values out of {x,y,z} should be used as the texture coordintes. Something like:
+
+vec4 sample = vec4(0, 0, 0, 0); // We'll fill this in below
+// Assume the normal is normalised.
+if(normal.x > 0.9) // x must be one while y and z are zero
+{
+	//Project onto yz plane
+	sample = texture2D(inputTexture, worldSpacePos.yz);
+}
+// Now similar logic for the other two axes.
+.
+.
+.
+
+You might also choose to sample a different texture for each of the axes, in order to apply a different texture to each face of your cubes. If so, you probably want to pack your differnt face textures together using an approach like thos described later in this document for multiple material textures.
+
 
 Blending between materials
 --------------------------