Working with torch.Tensor

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pytorch, tensors, and torch.Tensor

All the core number crunching in Comfy is done by pytorch. If your custom nodes are going to get into the guts of stable diffusion you will need to become familiar with this library, which is way beyond the scope of this introduction.

However, many custom nodes will need to manipulate images, latents and masks, each of which are represented internally as torch.Tensor, so you’ll want to bookmark the documentation for torch.Tensor.

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What is a Tensor?

torch.Tensor represents a tensor, which is the mathematical generalization of a vector or matrix to any number of dimensions. A tensor’s rank is the number of dimensions it has (so a vector has rank 1, a matrix rank 2); its shape describes the size of each dimension.

So an RGB image (of height H and width W) might be thought of as three arrays (one for each color channel), each measuring H x W, which could be represented as a tensor with shape [H,W,3]. In Comfy images almost always come in a batch (even if the batch only contains a single image). torch always places the batch dimension first, so Comfy images have shape [B,H,W,3], generally written as [B,H,W,C] where C stands for Channels.

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squeeze, unsqueeze, and reshape

If a tensor has a dimension of size 1 (known as a collapsed dimension), it is equivalent to the same tensor with that dimension removed (a batch with 1 image is just an image). Removing such a collapsed dimension is referred to as squeezing, and inserting one is known as unsqueezing.

To represent the same data in a different shape is referred to as reshaping. This often requires you to know the underlying data structure, so handle with care!

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Important notation

torch.Tensor supports most Python slice notation, iteration, and other common list-like operations. A tensor also has a .shape attribute which returns its size as a torch.Size (which is a subclass of tuple and can be treated as such).

There are some other important bits of notation you’ll often see (several of these are less common standard Python notation, seen much more frequently when dealing with tensors)

• torch.Tensor supports the use of None in slice notation to indicate the insertion of a dimension of size 1.

• : is frequently used when slicing a tensor; this simply means ‘keep the whole dimension’. It’s like using a[start:end] in Python, but omitting the start point and end point.

• ... represents ‘the whole of an unspecified number of dimensions’. So a[0, ...] would extract the first item from a batch regardless of the number of dimensions.

• in methods which require a shape to be passed, it is often passed as a tuple of the dimensions, in which a single dimension can be given the size -1, indicating that the size of this dimension should be calculated based on the total size of the data.

>>> a = torch.Tensor((1,2))
>>> a.shape
torch.Size([2])
>>> a[:,None].shape 
torch.Size([2, 1])
>>> a.reshape((1,-1)).shape
torch.Size([1, 2])

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Elementwise operations

Many binary on torch.Tensor (including ’+’, ’-’, ’*’, ’/’ and ’==’) are applied elementwise (independantly applied to each element). The operands must be either two tensors of the same shape, or a tensor and a scalar. So:

>>> import torch
>>> a = torch.Tensor((1,2))
>>> b = torch.Tensor((3,2))
>>> a*b
tensor([3., 4.])
>>> a/b
tensor([0.3333, 1.0000])
>>> a==b
tensor([False,  True])
>>> a==1
tensor([ True, False])
>>> c = torch.Tensor((3,2,1)) 
>>> a==c
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: The size of tensor a (2) must match the size of tensor b (3) at non-singleton dimension 0

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Tensor truthiness

You may be familiar with the truthy value of a Python list as True for any non-empty list, and False for None or []. By contrast A torch.Tensor (with more than one elements) does not have a defined truthy value. Instead you need to use .all() or .any() to combine the elementwise truthiness:

>>> a = torch.Tensor((1,2))
>>> print("yes" if a else "no")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: Boolean value of Tensor with more than one value is ambiguous
>>> a.all()
tensor(False)
>>> a.any()
tensor(True)

This also means that you need to use if a is not None: not if a: to determine if a tensor variable has been set.