Scipy rotate and zoom an image without changing its dimensions

Question

For my neural network I want to augment my training data by adding small random rotations and zooms to my images. The issue I am having is that scipy is changing the size of my

Answer 1

I recommend using cv2.resize because it is way faster than scipy.ndimage.zoom, probably due to support for simpler interpolation methods.

For a 480x640 image :

• cv2.resize takes ~2 ms
• scipy.ndimage.zoom takes ~500 ms
• scipy.ndimage.zoom(...,order=0) takes ~175ms

If you are doing the data augmentation on the fly, this amount of speedup is invaluable because it means more experiments in less time.

Here is a version of clipped_zoom using cv2.resize

def cv2_clipped_zoom(img, zoom_factor):
    """
    Center zoom in/out of the given image and returning an enlarged/shrinked view of 
    the image without changing dimensions
    Args:
        img : Image array
        zoom_factor : amount of zoom as a ratio (0 to Inf)
    """
    height, width = img.shape[:2] # It's also the final desired shape
    new_height, new_width = int(height * zoom_factor), int(width * zoom_factor)

    ### Crop only the part that will remain in the result (more efficient)
    # Centered bbox of the final desired size in resized (larger/smaller) image coordinates
    y1, x1 = max(0, new_height - height) // 2, max(0, new_width - width) // 2
    y2, x2 = y1 + height, x1 + width
    bbox = np.array([y1,x1,y2,x2])
    # Map back to original image coordinates
    bbox = (bbox / zoom_factor).astype(np.int)
    y1, x1, y2, x2 = bbox
    cropped_img = img[y1:y2, x1:x2]

    # Handle padding when downscaling
    resize_height, resize_width = min(new_height, height), min(new_width, width)
    pad_height1, pad_width1 = (height - resize_height) // 2, (width - resize_width) //2
    pad_height2, pad_width2 = (height - resize_height) - pad_height1, (width - resize_width) - pad_width1
    pad_spec = [(pad_height1, pad_height2), (pad_width1, pad_width2)] + [(0,0)] * (img.ndim - 2)

    result = cv2.resize(cropped_img, (resize_width, resize_height))
    result = np.pad(result, pad_spec, mode='constant')
    assert result.shape[0] == height and result.shape[1] == width
    return result