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# Usage
## Concept
* Sparse Conv Tensor: like hybird [torch.sparse_coo_tensor](https://pytorch.org/docs/stable/sparse.html#sparse-coo-docs) but only have two difference: 1. SparseConvTensor only have one dense dim, 2. indice of SparseConvTensor is transposed. see torch doc for more details.
* Sparse Convolution: equivalent to perform dense convolution when you convert SparseConvTensor to dense. Sparse Convolution only run calculation on valid data.
* Submanifold Convolution (SubMConv): like Sparse Convolution but indices keeps same. imagine that you copy same spatial structure to output, then iterate them, get input coordinates by conv rule, finally apply convolution **ONLY** in these output coordinates.
## SparseConvTensor
* features: ```[N, num_channels]``` tensor.
* indices: ```[N, (batch_idx + x + y + z)]``` coordinate tensor with batch axis. note that the coordinates xyz order MUST match spatial shape and conv params such as kernel size
```Python
features = # your features with shape [N, num_channels]
indices = # your indices/coordinates with shape [N, ndim + 1], batch index must be put in indices[:, 0]
spatial_shape = # spatial shape of your sparse tensor, spatial_shape[i] is shape of indices[:, 1 + i].
batch_size = # batch size of your sparse tensor.
x = spconv.SparseConvTensor(features, indices, spatial_shape, batch_size)