Understanding how image size is handled in fast.ai


(Michael Slater) #1

I’m playing with the dog-breed data. I am not clear how size of images is handled by the fast.ai/pytorch library.

The images are generally rectangular 443 wide X 387 high.
2018-02-26_21-22-20

The lecture said that ImageNet’s network was trained on pictures either 224x224 or 299x299.
So the average dog-breed picture is bigger than that, and not square.

Stealing boilerplate code from the teacher, I have:

sz=224

arch=resnext101_64  #epoch[1] accuracy = 0.9257   (his first run was 83.6%)

tfms = tfms_from_model(arch, sz, aug_tfms=transforms_side_on, max_zoom=1.1)

n = len(list(open(f'{PATH}labels.csv')))-1
val_idxs= get_cv_idxs(n)  #get cross validation indices  [should happen by default according to source]

data = ImageClassifierData.from_csv(PATH, folder='train', csv_fname=f'{PATH}labels.csv', tfms=tfms,
        suffix='.jpg', val_idxs=val_idxs, bs=32)  #reduced bs from 64

So what is going to happen? is the tfms_from_model going to take every image, chop it square, and resize (up if it’s tiny, or down if it’s big) to sz=224 pixels square? Then data object will be entirely full of 224x224 pictures?

But I’m further-confused by the get_data(sz, bs) function the professor showed during his second lecture:

I call get_data() with the size and batch-size I need. So I cannot understand what the conditional return does: If the sz parameter >= 300, then leave the data object (which was set up with a tfms of sz > 300) alone and return it. But if, for some reason, I specifed a sz less than 300, apply .resize() directly to the data object and make it bigger ?340x340?

Init signature: ImageClassifierData(path, datasets, bs, num_workers, classes)
Docstring:      <no docstring>
File:           ~/fastai/courses/dl1/fastai/dataset.py
Type:           type

Unfortunately there is no docstring for ImageClassifierData, so I don’t know what that function is trying to do, or how it is different/overrides whatever tfms is doing.

I would appreciate if someone can explain how image size flows through the system and is handled.


Lesson 3: Couldn't understand data.resize() for multi label classification for planet data set?
(Vadim K) #2

@karavshin I got exactly the same question. Did you succeed to understand that?


(Vadim K) #3

OK, I think I figured this out.
Actually when the images are loaded to the data object - they are cropped to square (side of the square = smaller dimension of original image) and they are resized till the value of sz (i.e. 64x64 if sz=64).

To make it visual - here is the result of my experiments with couple of them:
Original images
image image

sz = 64
image image

sz=128
image image

sz=32
image image

You can check out this small notebook for more details.


(Ravi) #4

@t0tem As rectangular image gets cropped, do you think squaring image (without cropping, maybe stretching along smaller side) will improve accuracy. As squaring doesnt lose any data


(why) #5

it will help.My suggestion is to try out each augmentation on mini batch and then combine all and then see the results


(Manu) #6

Still don’t get the magic of numbers 300 and 340 (if the image is less than 300, then resize to 340)