RandomZoom on non-zero areas

[maxencewynen]

Hi!

I am developing an object detection network in the MONAI framework and liking it a lot so far.

I would like to use a random zoom augmentation (Random Crop + Keep original size by interpolating the result) but make sure the cropped ROI includes at least one of the objects the network is supposed to detect.

Is there an easy way to do this? I could not find any argument in the Zoom transforms, and the Crop transforms do not seem to integrate the keeping the original size and interpolation options either.

Thanks for your help!

[KumoLiu]

Hi @maxencewynen,
I think maybe you can use RandCropByPosNegLabel with pos=1, neg=0 to ensure crop with foreground and Resize to ensure your spatial size.
Hope it will helps you!

[maxencewynen]

Hey @KumoLiu,
I think this could indeed solve my problem, thanks for your answer!
I am wondering how I could handle the randomness of the zoom factor using this strategy, any idea? By that I mean that I always will have to specify the spatial_size parameter manually
A RandZoomByPosNegLabel would be really useful but I don't think it exists
Thanks again!

[KumoLiu]

Hi, I'm not really understand you, I think you only need to set the spatial size once in the instantiated. Like this

RandCropByPosNegLabeld(keys=["image", "label"], label_key="seg", spatial_size=(xx, xx), pos=1, neg=0),
Resized(keys=["image", "label"], spatial_size=(xx,xx))

Besides, RandZoomByPosNegLabel you suggest would perhaps need to consider how to crop and resize, so I guess that's enough for now.
Thanks for your advice!

[maxencewynen]

Hi, I'm sorry I've not been clear enough.
My problem is specifically with the spatial_size that I have to specify .
To better explain this, the RandomZoomd transform has 2 parameters called min_zoom and max_zoom, meaning the spatial size in that case is actually randomly taken between image_size * min_zoom and image_size * max_zoom.
If I understood correctly, the spatial_size parameter of RandCropByPosNegLabeld cannot be randomized, which I specifically need in order to perform a random zoom operation.
Hope this is clear now!

[KumoLiu]

Hi, I think your random zoom operation ia aim to keep original size. Am I right?
If like that, RandCropByPosNegLabeld can help you to random crop a patch and use the Resize with spatial_size as your original_size would help you keep your original size. Additional, you can sepcify the spatial_size in RandCropByPosNegLabeld with original_size/n which just mean your want to upsample your patches n times to your original shape.