Sample in pixel units, not CRS units

[RitwikGupta]

Related to #278, once our datasets are projected to UTM with fixed xres and yres, I want to sample a set amount of pixels, say 1024 x 1024. Current samplers sample in the unit of the CRS (in this case meters). Therefore, I have to set the sample size to desired_chip_size / xres_yres_of_imagery. While this works, a flag which lets the user choose which unit to sample in would be great.

[calebrob6]

I'm fine with adding this as a feature -- this is especially annoying with datasets in units of degrees (we do this here https://github.com/microsoft/torchgeo/blob/main/benchmark.py#L143)

[RitwikGupta]

Perhaps you can help me understand what the intended behavior is, based on that benchmark. When passing a size to the sampler, does it give you a patch that is (size, size) pixels large (so a Tensor that is [1, 1, size, size]), or is the intended behavior that it will give you a patch that is (size / res, size / res) pixels large?

Currently, I am getting the latter behavior (which seems to be the intended one.

from datasets import WatchRaster
import matplotlib.pyplot as plt
from torch.utils.data import DataLoader
from torchgeo.samplers import RandomGeoSampler
from torchgeo.datasets import stack_samples
import torch

ds = WatchRaster()
print(f'Dataset resolution: {ds.res}')
sampler = RandomGeoSampler(ds, size=512, length=5)
dl = DataLoader(ds, sampler=sampler, collate_fn=stack_samples)

for sample in dl:
    image = sample['image']
    print(image.shape)

Dataset resolution: 2.4
torch.Size([1, 1, 214, 214])
torch.Size([1, 1, 214, 214])
torch.Size([1, 1, 214, 214])
torch.Size([1, 1, 213, 213])
torch.Size([1, 1, 213, 213])

The proposed change could be to modify get_random_bounding_box in utils to add a pixelspace boolean flag. This would change maxx and maxy to be minx + t_size[1] * res.

[calebrob6]

Yes, latter behavior is correct, and proposed change is correct! We may want to think about how we name the args in the samplers, options:

• mutually exclusive size_in_pixels and size_in_crs_units args
• size arg and pixelspace flag

[RitwikGupta]

I like former better. Make it a required arg so that the user is forced to choose. What's the voting process for these changes? @adamjstewart

[RitwikGupta]

The former can be implemented as an enum to make it explicit in the code. sampler(sample_mode=utils.SIZE_IN_PIXELS)

[adamjstewart]

I feel like there was a reason I originally chose to do things in CRS units, but I can't remember what that reason was. I'm fine with whatever. We could just only support sampling in pixel units.