Pillow - Resizing a GIF

问题:

I have a gif that I would like to resize with pillow so that its size decreases. The current size of the gif is 2MB.

I am trying to

1. resize it so its height / width is smaller

2. decrease its quality.

With JPEG, the following piece of code is usually enough so that large image drastically decrease in size.

from PIL import Image

im = Image.open("my_picture.jpg")
im = im.resize((im.size[0] // 2, im.size[1] // 2), Image.ANTIALIAS)  # decreases width and height of the image
im.save("out.jpg", optimize=True, quality=85)  # decreases its quality

With a GIF, though, it does not seem to work. The following piece of code even makes the out.gif bigger than the initial gif:

im = Image.open("my_gif.gif")
im.seek(im.tell() + 1)  # loads all frames
im.save("out.gif", save_all=True, optimize=True, quality=10)  # should decrease its quality

print(os.stat("my_gif.gif").st_size)  # 2096558 bytes / roughly 2MB
print(os.stat("out.gif").st_size)  # 7536404 bytes / roughly 7.5MB

If I add the following line, then only the first frame of the GIF is saved, instead of all of its frame.

im = im.resize((im.size[0] // 2, im.size[1] // 2), Image.ANTIALIAS)  # should decrease its size

I've been thinking about calling resize() on im.seek() or im.tell() but neither of these methods return an Image object, and therefore I cannot call resize() on their output.

Would you know how I can use Pillow to decrease the size of my GIF while keeping all of its frames?

[edit] Partial solution:

Following Old Bear's response, I have done the following changes:

• I am using BigglesZX's script to extract all frames. It is useful to note that this is a Python 2 script, and my project is written in Python 3 (I did mention that detail initially, but it was edited out by the Stack Overflow Community). Running 2to3 -w gifextract.py makes that script compatible with Python 3.

• I have been resicing each frame individually: frame.resize((frame.size[0] // 2, frame.size[1] // 2), Image.ANTIALIAS)

• I've been saving all the frames together: img.save("out.gif", save_all=True, optimize=True).

The new gif is now saved and works, but there is 2 main problems :

• I am not sure that the resize method works, as out.gif is still 7.5MB. The initial gif was 2MB.

• The gif speed is increased and the gif does not loop. It stops after its first run.

Example:

original gif my_gif.gif:

Gif after processing (out.gif) https://i.imgur.com/zDO4cE4.mp4 (I could not add it to Stack Overflow ). Imgur made it slower (and converted it to mp4). When I open the gif file from my computer, the entire gif lasts about 1.5 seconds.

回答1:

According to Pillow 4.0x, the Image.resize function only works on a single image/frame.

To achieve what you want, I believe you have to first extract every frame from the .gif file, resize each frame one at a time and then reassemble them up again.

To do the first step, there appears to be some detail that needs to be attended to. E.g. whether each gif frame uses a local palette or a global palette is applied over all frames, and whether gif replace each image using a full or partial frame. BigglesZX has developed a script to address these issues while extracting every frame from a gif file so leverage on that.

Next, you have to write the scripts to resize each of the extracted frame and assemble them all as a new .gif using the PIL.Image.resize() and PIL.Image.save().

I noticed you wrote "im.seek(im.tell() + 1) # load all frames". I think this is incorrect. Rather it is use to increment between frames of a .gif file. I noticed you used quality=10 in your save function for your .gif file. I did not find this as provided in the PIL documentation. You can learn more about the tile attribute mentioned in BiggleZX's script by reading this link

回答2:

Using BigglesZX's script, I have created a new script which resizes a GIF using Pillow.

Original GIF (2.1 MB):

Output GIF after resizing (1.7 MB):

I have saved the script here. It is using the thumbnail method of Pillow rather than the resize method as I found the resize method did not work.

The is not perfect so feel free to fork and improve it. Here are a few unresolved issues:

• While the GIF displays just fine when hosted by imgur, there is a speed issue when I open it from my computer where the entire GIF only take 1.5 seconds.
• Likewise, while imgur seems to make up for the speed problem, the GIF wouldn't display correctly when I tried to upload it to stack.imgur. Only the first frame was displayed (you can see it here).

Full code (should the above gist be deleted):

def resize_gif(path, save_as=None, resize_to=None):
    """
    Resizes the GIF to a given length:

    Args:
        path: the path to the GIF file
        save_as (optional): Path of the resized gif. If not set, the original gif will be overwritten.
        resize_to (optional): new size of the gif. Format: (int, int). If not set, the original GIF will be resized to
                              half of its size.
    """
    all_frames = extract_and_resize_frames(path, resize_to)

    if not save_as:
        save_as = path

    if len(all_frames) == 1:
        print("Warning: only 1 frame found")
        all_frames[0].save(save_as, optimize=True)
    else:
        all_frames[0].save(save_as, optimize=True, save_all=True, append_images=all_frames[1:], loop=1000)


def analyseImage(path):
    """
    Pre-process pass over the image to determine the mode (full or additive).
    Necessary as assessing single frames isn't reliable. Need to know the mode
    before processing all frames.
    """
    im = Image.open(path)
    results = {
        'size': im.size,
        'mode': 'full',
    }
    try:
        while True:
            if im.tile:
                tile = im.tile[0]
                update_region = tile[1]
                update_region_dimensions = update_region[2:]
                if update_region_dimensions != im.size:
                    results['mode'] = 'partial'
                    break
            im.seek(im.tell() + 1)
    except EOFError:
        pass
    return results


def extract_and_resize_frames(path, resize_to=None):
    """
    Iterate the GIF, extracting each frame and resizing them

    Returns:
        An array of all frames
    """
    mode = analyseImage(path)['mode']

    im = Image.open(path)

    if not resize_to:
        resize_to = (im.size[0] // 2, im.size[1] // 2)

    i = 0
    p = im.getpalette()
    last_frame = im.convert('RGBA')

    all_frames = []

    try:
        while True:
            # print("saving %s (%s) frame %d, %s %s" % (path, mode, i, im.size, im.tile))

            '''
            If the GIF uses local colour tables, each frame will have its own palette.
            If not, we need to apply the global palette to the new frame.
            '''
            if not im.getpalette():
                im.putpalette(p)

            new_frame = Image.new('RGBA', im.size)

            '''
            Is this file a "partial"-mode GIF where frames update a region of a different size to the entire image?
            If so, we need to construct the new frame by pasting it on top of the preceding frames.
            '''
            if mode == 'partial':
                new_frame.paste(last_frame)

            new_frame.paste(im, (0, 0), im.convert('RGBA'))

            new_frame.thumbnail(resize_to, Image.ANTIALIAS)
            all_frames.append(new_frame)

            i += 1
            last_frame = new_frame
            im.seek(im.tell() + 1)
    except EOFError:
        pass

    return all_frames

回答3:

I am using the function below to resize and crop images including animated ones (GIF, WEBP) Simply, we need to iterate each frame in the gif or webp.

from math import floor, fabs
from PIL import Image, ImageSequence

def transform_image(original_img, crop_w, crop_h):
  """
  Resizes and crops the image to the specified crop_w and crop_h if necessary.
  Works with multi frame gif and webp images also.

  args:
  original_img is the image instance created by pillow ( Image.open(filepath) )
  crop_w is the width in pixels for the image that will be resized and cropped
  crop_h is the height in pixels for the image that will be resized and cropped

  returns:
  Instance of an Image or list of frames which they are instances of an Image individually
  """
  img_w, img_h = (original_img.size[0], original_img.size[1])
  n_frames = getattr(original_img, 'n_frames', 1)

  def transform_frame(frame):
    """
    Resizes and crops the individual frame in the image.
    """
    # resize the image to the specified height if crop_w is null in the recipe
    if crop_w is None:
      if crop_h == img_h:
        return frame
      new_w = floor(img_w * crop_h / img_h)
      new_h = crop_h
      return frame.resize((new_w, new_h))

    # return the original image if crop size is equal to img size
    if crop_w == img_w and crop_h == img_h:
      return frame

    # first resize to get most visible area of the image and then crop
    w_diff = fabs(crop_w - img_w)
    h_diff = fabs(crop_h - img_h)
    enlarge_image = True if crop_w > img_w or crop_h > img_h else False
    shrink_image = True if crop_w < img_w or crop_h < img_h else False

    if enlarge_image is True:
      new_w = floor(crop_h * img_w / img_h) if h_diff > w_diff else crop_w
      new_h = floor(crop_w * img_h / img_w) if h_diff < w_diff else crop_h

    if shrink_image is True:
      new_w = crop_w if h_diff > w_diff else floor(crop_h * img_w / img_h)
      new_h = crop_h if h_diff < w_diff else floor(crop_w * img_h / img_w)

    left = (new_w - crop_w) // 2
    right = left + crop_w
    top = (new_h - crop_h) // 2
    bottom = top + crop_h

    return frame.resize((new_w, new_h)).crop((left, top, right, bottom))

  # single frame image
  if n_frames == 1:
    return transform_frame(original_img)
  # in the case of a multiframe image
  else:
    frames = []
    for frame in ImageSequence.Iterator(original_img):
      frames.append( transform_frame(frame) )
    return frames