Is it possible to vectorize (perform on batch) random cropping per element?

Nevermind, solved.

EDIT: Got it working, there were two bugs. First, I had mistakenly initialized batch_size twice during all my editing, and so I was mismatching between batch_size depending on where in the code I was. The second bug, which I still haven’t entirely fixed, is that if the batch size does not evenly divide the input generator the code fails, even though I have it set with a take amount that IS divisible by the batch_size. I get the same error even if I set steps per epoch to 1 (so it should never be reaching the end of the batches). I can only assume it’s an error during graph generation where it’s trying to define that last partial batch even though it will never be trained over. Hmm.

EDIT EDIT: Carefully following the size of my dataset throughout my pipeline, I discovered the source of the second issue, which is actually just that I didn’t delete my cache files when I previously had a larger take. The last thing I would still like to do is fix the code such that it actually CAN handle variable length batches so I don’t have to worry about making sure I don’t have partial batches. However, from what I can see, tf.unstack along variable length dimensions is straight up not supported, so this will require refactoring my computation to use some other method, like loops maybe. To be honest, though, it’s not worth my time to do so right now when I can just use drop_remainder = True and drop the last incomplete batch. In my real application there will be a lots of data per epoch, so losing 16 or so random examples from each epoch is rather minor.

So, I am making a project where I randomly crop images. In my data pipeline, I was trying to write code such that I could crop batches of my data at once, as the docs suggested that vectorizing my operations would reduce scheduling overhead.

However, I have run into some issues. If I use tf.image.random_crop, the problem is that the same random crop will be used on every image in the batch. I, however, want different random crops for every image. Moreover, since where I randomly crop an image will affect my labels, I need to track every random crop performed per image and adjust the label for that image.

I was able to write code that seems like it would work by using unstack, doing my operation per element, then restacking, like so:

images = tf.unstack( img, num=None, axis=0, name='unstack' ) xshifts = [] yshifts = [] newimages =[] for image in images: if not is_valid: x = np.random.randint(0, width - dx + 1) y = np.random.randint(0, height - dy + 1) else: x = 0 y = 0 newimages.append(image[ y:(y+dy), x:(x+dx), :]) print(image[ y:(y+dy), x:(x+dx), :]) xshifts.append((float(x) / img.shape[2]) * img_real_size) yshifts.append((float(y) / img.shape[1]) * img_real_size) images = tf.stack(newimages,0) 

But oh no! Whenever I use this code in a map function, it doesn’t work because in unstack I set num=None, which requires it to infer how much to unstack from the actual batch size. But because tensorflow for reasons decided that batches should have size none when specifying things, the code fails because you can’t infer size from None. If I patch the code to put in num=batch_size, it changes my datasets output signature to be hard coded to batch_size, which seems like it shouldn’t be a problem, except this happens.

tensorflow.python.framework.errors_impl.InvalidArgumentError: Input shape axis 0 must equal 2, got shape [1,448,448,3] [[{{node unstack}}]] 

Which is to say, it’s failing because instead of receiving the expected batch input with the appropriate batch_size (2 for testing), it’s receiving a single input image, and 2 does not equal 1. The documentation strongly implied to me that if I batch my dataset before mapping (which I do), then the map function should be receiving the entire batch and should therefore be vectorized. But is this not the case? I double checked my batch and dataset sizes to make sure that it isn’t just an error arising due to some smaller final batch.

To sum up: I want to crop my images uniquely per image and alter labels as I do so. I also want to do this per batch, not just per image. The code I wrote that does this requires me to unstack my batch. But unstacking my batch can’t have num=None. But tensorflow batches have shape none, so the input of my method has shape none at specification. But if I change unstack’s num argument to anything but None, it changes my output specification to that number (which isn’t None), and the output signature of my method must ALSO have shape none at specification. How can I get around this?

Or, if someone can figure out why my batched dataset, batched before my map function, is apparently feeding in single samples instead of full batches, that would also solve the mystery.

submitted by /u/Drinniol
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