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Squeezing image loading performance using libjpeg in Pytorch

Sometimes in your project you seek cheap thrills out of changing couple of lines of code. This is what happened when I came across the libjpeg-turbo project. The turbo version of libjpeg boasts to be 2-6x faster in performing image processing operations. Performance is what everyone wants, right?

I work with Pytorch and so I started looking for Python bindings for libjpeg-turbo. Fortunately, Github user @ajkxyz has provided cffi bindings for libjpeg-turbo. Great! As per the author, in single threaded mode, we should expect a 30% improvement in image loading operations using these bindings. I was all in. 👀

Using these Python bindings is pretty straight-forward, so I went ahead to implement an end-to-end Pytorch dataloader that uses libjpeg-turbo backend to load images. Further, I also numpy-fied the random horizontal and vertical flipping operations to avoid expensive PIL.Image calls. For the code below, I assume that the image folder is flat and contain images with same dimensions.

import os
import fire
import glob

import torch
import numpy as np
from PIL import Image
import jpeg4py as jpeg
from torch.utils import data

class MyDataSet(data.Dataset):
    A custom dataset to load images
    def __init__(self, *args, **kwargs):
        self.image_files = kwargs.get('image_files')
    def __len__(self):
        return len(self.image_files)
    def __getitem__(self, index):
        img = self.image_files[index]
        # -- PIL.Image loader -- #
        # return np.array(
        # -- libjpeg-turbo loader -- #
        return jpeg.JPEG(img).decode()

def fast_collate(images) -> torch.Tensor:
    Converts input batches from numpy arrays to Pytorch tensors
    :param images: batch consisting of images
    :return: images as tensors
    # -- implements random horizontal and vertical flipping -- #
    images = [np.flip(img, axis=1) if np.random.rand() > 0.5 else img for img in images]
    images = [np.flip(img, axis=0) if np.random.rand() > 0.5 else img for img in images]

    w, h = images[0].shape[:2]
    image_tensor = torch.zeros((len(images), 3, h, w), dtype=torch.uint8)
    for i, image_array in enumerate(images):
        if image_array.ndim < 3:
            image_array = np.expand_dims(image_array, axis=-1)
        image_array = np.rollaxis(image_array, 2)
        # -- np.ascontiguousarray is needed becase np.flip operation stores negative strides -- #
        image_tensor[i] += torch.from_numpy(np.ascontiguousarray(image_array))
    return image_tensor

def run(image_folder: str):
    Iterates over one single batch of images
    :param image_folder: flattened folder contained jpeg images
    assert os.path.exists(image_folder), f"{image_folder} does not exist"
    files = glob.glob(image_folder + "*.jpg")
    data_set = MyDataSet(image_files=files)
    dl = iter(data.DataLoader(data_set, batch_size=32, num_workers=4, 
                              pin_memory=True, collate_fn=fast_collate))

if __name__ == "__main__":

I use a custom collate function because the in-built toTensor is a pretty involved function which slows down the whole array to tensor creation part. This collate function has been borrowed from the Nvidia/apex which lets you do half-precision training. Let’s keep that discussion for another post.

Google Colab
MIT License

I was able to squeeze +20% performance. Not bad! Change the batch size for more juice maybe?

Stay classy.

  • libjpeg-turbo project
  • cffi Python bindings repo for libjpeg-turbo