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]

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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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