Question: NumPy/PyTorch-style “coordinate list” integer indexing for Tensor type?

[zihay]

Hi, I’m experimenting with some basic tensor indexing operations (e.g., slicing) in Dr.Jit and noticed that integer indexing behaves a bit differently from NumPy/PyTorch.

In NumPy/PyTorch, a pair of integer index tensors is interpreted as a coordinate list (with broadcasting if needed). In Dr.Jit, the same code seems to broadcast the index tensors in a way that gives a different result shape.

Here is a minimal example:

import drjit as dr
from drjit.cuda.ad import TensorXf, TensorXi

t = dr.zeros(TensorXf, shape=(100, 200))
b = dr.arange(TensorXi, 100)
d = dr.arange(TensorXi, 100)

print(t[b, d].shape)  # Dr.Jit: (100, 100)

# PyTorch equivalents
print(t.torch()[b.torch(), d.torch()].shape)           # torch.Size([100])
print(t.torch()[b.torch()[:, None], d.torch()].shape)  # torch.Size([100, 100])

So in this example:

t[b, d] in Dr.Jit produces a (100, 100) tensor.

In PyTorch, the grid-like behavior corresponds to t[b[:, None], d], while t[b, d] itself uses the “coordinate list” interpretation and returns a (100,) tensor.

Right now, to mimic the NumPy/PyTorch “coordinate list” behavior in Dr.Jit, I’m flattening the tensor and manually computing a 1D index. Is there a recommended way in Dr.Jit to perform NumPy/PyTorch-style “coordinate list” integer indexing without manually flattening and computing the indices?

Thanks!

[merlinND]

FYI: this PR changes tensor indexing to be closer to PyTorch: https://github.com/mitsuba-renderer/drjit/pull/460