decode_gif: add transparency support and image offsets

[NicolasHug]

This PR now properly handles transparency within the GIF, and also allows each individual image to start at its own Top and Left offset. For docs on how transparency is encoded in GIFs, see https://giflib.sourceforge.net/whatsinagif/animation_and_transparency.html

Turns out the decoder was unable to properly decode https://en.wikipedia.org/wiki/GIF#/media/File:Rotating_earth_(large).gif, but now it's fine.

Benchmark. Slighly slower than before but the relative speed compared with PIL is similar to before.

fire.gif                   size=[33, 3, 60, 30]
tv : med = 0.87ms +- 0.07
pil: med = 1.30ms +- 0.15
tv is 1.50x faster

gifgrid.gif                size=[3, 100, 100]
tv : med = 0.10ms +- 0.02
pil: med = 0.10ms +- 0.02
tv is 0.94x faster

porsche.gif                size=[3, 200, 320]
tv : med = 0.68ms +- 0.08
pil: med = 0.29ms +- 0.03
tv is 0.43x faster

solid2.gif                 size=[3, 400, 640]
tv : med = 2.46ms +- 0.15
pil: med = 0.90ms +- 0.12
tv is 0.37x faster

treescap-interlaced.gif    size=[3, 40, 40]
tv : med = 0.03ms +- 0.01
pil: med = 0.07ms +- 0.01
tv is 1.95x faster

treescap.gif               size=[3, 40, 40]
tv : med = 0.03ms +- 0.00
pil: med = 0.07ms +- 0.01
tv is 2.19x faster

welcome2.gif               size=[6, 3, 48, 290]
tv : med = 0.90ms +- 0.06
pil: med = 1.06ms +- 0.09
tv is 1.19x faster

x-trans.gif                size=[3, 100, 100]
tv : med = 0.09ms +- 0.01
pil: med = 0.11ms +- 0.02
tv is 1.23x faster

grace.gif                  size=[3, 606, 517]
tv : med = 4.31ms +- 0.23
pil: med = 2.70ms +- 0.12
tv is 0.63x faster

import torch
from time import perf_counter_ns


def bench(f, *args, num_exp=100, warmup=0, **kwargs):

    for _ in range(warmup):
        f(*args, **kwargs)

    times = []
    for _ in range(num_exp):
        start = perf_counter_ns()
        f(*args, **kwargs)
        end = perf_counter_ns()
        times.append(end - start)
    return torch.tensor(times).float()

def report_stats(times, unit="ms"):
    mul = {
        "ns": 1,
        "µs": 1e-3,
        "ms": 1e-6,
        "s": 1e-9,
    }[unit]
    times = times * mul
    std = times.std().item()
    med = times.median().item()
    print(f"{med = :.2f}{unit} +- {std:.2f}")
    return med


from PIL import Image, ImageSequence
from torchvision import io
from pathlib import Path

# files from https://sourceforge.net/p/giflib/code/ci/master/tree/pic/
paths = list(Path("/home/nicolashug/dev/giflib-code/pic/").glob("*.gif"))
paths += [Path("/home/nicolashug/grace.gif")]  # grace_hopper from torchvision/test/assets

def read_image_pil(path):
    for img in ImageSequence.Iterator(Image.open(path)):
        img.convert("RGB")

for path in paths:
    print()
    print(f"{path.name:<26} size={list(io.read_image(path).shape)}")
    print("tv : ", end="")
    times = bench(io.read_image, path, warmup=10)
    tv_med = report_stats(times)
    print("pil: ", end="")
    times = bench(read_image_pil, path, warmup=10)
    pil_med = report_stats(times)
    print(f"tv is {pil_med / tv_med:.2f}x faster")

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8419

• 📄 Preview Python docs built from this PR

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❌ 8 New Failures

As of commit 8c228f5 with merge base 51429c2 ():

NEW FAILURES - The following jobs have failed:

• Tests / unittests-extended / linux-job (gh)
  RuntimeError: Command docker exec -t 099777c10cf3470159497e0288bdaf430fd8e13c372d1f895ed3c541e6e5372e /exec failed with exit code 1
• Tests / unittests-linux (3.9, linux.12xlarge, cpu) / linux-job (gh)
  RuntimeError: Command docker exec -t 177c4df1f02fd100fc4097bbcda80609426b258a17bfa19b7ab72e1f580b2ff9 /exec failed with exit code 1
• Tests / unittests-macos (3.10, macos-12) / macos-job (gh)
  RuntimeError: MPS backend out of memory (MPS allocated: 0 bytes, other allocations: 0 bytes, max allowed: 1.70 GB). Tried to allocate 0 bytes on private pool. Use PYTORCH_MPS_HIGH_WATERMARK_RATIO=0.0 to disable upper limit for memory allocations (may cause system failure).
• Tests / unittests-macos (3.11, macos-12) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Tests / unittests-macos (3.12, macos-12) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Tests / unittests-macos (3.8, macos-12) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Tests / unittests-macos (3.9, macos-12) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 1
• Tests / unittests-windows (3.9, windows.4xlarge, cpu) / windows-job (gh)
  The process 'C:\Program Files\Git\cmd\git.exe' failed with exit code 128

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[vfdev-5]

LGTM, thanks @NicolasHug !

[github-actions]

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