Update README.md

Add `meshtopt_generateShadowIndexBuffer` documentation in a new "shadow indexing" section.

Author: zeux

README.md

@@ -41,7 +41,8 @@ When optimizing a mesh, you should typically feed it through a set of optimizati
 4. Overdraw optimization
 5. Vertex fetch optimization
 6. Vertex quantization
-7. (optional) Vertex/index buffer compression
+7. Shadow indexing
+8. (optional) Vertex/index buffer compression
 
 ## Indexing
 
@@ -124,6 +125,29 @@ unsigned short pz = meshopt_quantizeHalf(v.z);
 
 Since quantized vertex attributes often need to remain in their compact representations for efficient transfer and storage, they are usually dequantized during vertex processing by configuring the GPU vertex input correctly to expect normalized integers or half precision floats, which often needs no or minimal changes to the shader code. When CPU dequantization is required instead, `meshopt_dequantizeHalf` can be used to convert half precision values back to single precision; for normalized integer formats, the dequantization just requires dividing by 2^N-1 for unorm and 2^(N-1)-1 for snorm variants, for example manually reversing `meshopt_quantizeUnorm(v, 10)` can be done by dividing by 1023.
 
+## Shadow indexing
+
+Many rendering pipelines require meshes to be rendered to depth-only targets, such as shadow maps or during a depth pre-pass, in addition to color/G-buffer targets. While using the same geometry data for both cases is possible, reducing the number of unique vertices for depth-only rendering can be beneficial, especially when the source geometry has many attribute seams due to faceted shading or lightmap texture seams.
+To achieve this, this library provides the `meshopt_generateShadowIndexBuffer` algorithm, which generates a second (shadow) index buffer that can be used with the original vertex data:
+
+
+
+This is possible to achieve using `meshopt_generateShadowIndexBuffer` algorithm, which will generate a second (shadow) index buffer that can be used with the original vertex data. Because the vertex data is shared, this should be done after other optimizations of the vertex/index data, but it is possible (and recommended) to optimize the resulting index buffer for vertex cache at the end:
+
+```c++
+std::vector<unsigned int> shadow_indices(index_count);
+// note: this assumes Vertex starts with float3 positions and should be adjusted accordingly for quantized positions
+meshopt_generateShadowIndexBuffer(&shadow_indices[0], indices, index_count, &vertices[0].x, vertex_count, sizeof(float) * 3, sizeof(Vertex));
+```
+
+Because the vertex data is shared, shadow indexing should be done after other optimizations of the vertex/index data. However, it's possible (and recommended) to optimize the resulting shadow index buffer for vertex cache:
+
+```c++
+meshopt_optimizeVertexCache(&shadow_indices[0], &shadow_indices[0], index_count, vertex_count);
+```
+
+In some cases, it may be beneficial to split the vertex positions into a separate buffer to maximize efficiency for depth-only rendering. Note that the example above assumes only positions are relevant for shadow rendering, but more complex materials may require adding texture coordinates (for alpha testing) or skinning data to the vertex portion used as a key. `meshopt_generateShadowIndexBufferMulti` can be useful for these cases if the relevant data is not contiguous.
+
 ## Vertex/index buffer compression
 
 In case storage size or transmission bandwidth is of importance, you might want to additionally compress vertex and index data. While several mesh compression libraries, like Google Draco, are available, they typically are designed to maximize the compression ratio at the cost of disturbing the vertex/index order (which makes the meshes inefficient to render on GPU) or decompression performance. They also frequently don't support custom game-ready quantized vertex formats and thus require to re-quantize the data after loading it, introducing extra quantization errors and making decoding slower.
@@ -233,7 +257,7 @@ std::vector<unsigned int> remap(index_count);
 size_t vertex_count = meshopt_generateVertexRemapMulti(&remap[0], NULL, index_count, index_count, streams, sizeof(streams) / sizeof(streams[0]));
 ```
 
-After this `meshopt_remapVertexBuffer` needs to be called once for each vertex stream to produce the correctly reindexed stream.
+After this `meshopt_remapVertexBuffer` needs to be called once for each vertex stream to produce the correctly reindexed stream. For shadow indexing, similarly `meshopt_generateShadowIndexBufferMulti` is available as a replacement.
 
 Instead of calling `meshopt_optimizeVertexFetch` for reordering vertices in a single vertex buffer for efficiency, calling `meshopt_optimizeVertexFetchRemap` and then calling `meshopt_remapVertexBuffer` for each stream again is recommended.