GIF's n_frames is (about 60x) slower than it should be #6105
Description
While idling around on SO and answering a question I noticed that n_frames is surprisingly slow for GIF:
# setup code based on ImageIO
# because it's short and we can avoid measuring disk IO
import imageio.v3 as iio
from PIL import Image
import io
img = iio.imread("imageio:newtonscradle.gif", index=None)
img_bytes = iio.imwrite("<bytes>", img, format="GIF")The desired way to do things:
%%timeit
with Image.open(io.BytesIO(img_bytes)) as file:
n_frames = file.n_framesTiming for this approach:
13.2 ms ± 94.3 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
This is much more time than it should take to check a series of block headers. So I checked, and - indeed - n_frames parses pixel data, which (in pillow9) involves a conversion to RGB(A) which is what makes this slow. Here is an alternative approach that only reads headers and is - more or less - a drop-in replacement:
%%timeit
# based on SO: https://stackoverflow.com/a/7506880/6753182
with Image.open(io.BytesIO(img_bytes)) as file:
def skip_color_table(flags):
if flags & 0x80:
file.fp.seek(3 << ((flags & 7) + 1), 1)
current_fp = file.fp.tell()
total_frames = file.tell() # start counting from the current frame
# seek to beginning of next block
buffer_start = file.tile[0][2]
file.fp.seek(buffer_start)
while True:
size = file.fp.read(1)
if size and size[0]:
file.fp.seek(size[0], 1)
else:
break
# count the remaining blocks
while True:
block = file.fp.read(1)
if block == b';':
break
if block == b'!':
file.fp.seek(1, 1)
elif block == b',':
total_frames += 1
file.fp.seek(8, 1)
skip_color_table(ord(file.fp.read(1)))
file.fp.seek(1, 1)
else: raise RuntimeError("unknown block type")
# skip to next block instead of loading pixels
while True:
l = ord(file.fp.read(1))
if not l: break
file.fp.seek(l, 1)
file.fp.seek(current_fp)The timings:
211 µs ± 529 ns per loop (mean ± std. dev. of 7 runs, 1000 loops each)
So if we only check block headers we can be about 60x faster than we are now. If desired, and there is somebody willing to review, I can submit a PR that adds this some time next week :)
(tests were done on Windows 11 with a AMD Ryzen 7 5800X 8-Core Processor and 2x Kingston KHX3200C16D4/32GX 32GB)