Enhancement: non-animated GIF output should default to no-loop

[thatsmydoing]

Question about an existing feature

What are you trying to achieve?

When naively converting non-GIF to GIF, the output GIF is set to loop forever. It might make more sense to default to 1 instead.

When you searched for similar issues, what did you find that might be related?

I did not find anything related to loop forever.

Please provide a minimal, standalone code sample, without other dependencies, that demonstrates this question

const sharp = require('sharp');

async function main() {
  await sharp(process.argv[2]).gif().toFile('out.gif');
  console.log(await sharp('out.gif').metadata());
}

main();

will have metadata loop: 0 when passed in a non-GIF but loop: 1 when passed in a non-animated GIF. In practice, they probably display the same but the output is technically an infinitely looping GIF with 1 frame. Tools like gifsicle also indicate "loop forever" on the output image.

Please provide sample image(s) that help explain this question

PNG input -> "animated" GIF output

non-animated GIF input -> non-animated GIF output

[lovell]

As you've seen, loop is not set for non-animated images, so the output will either use the loop value from the input or default to -1, which is "loop forever".

We'll need to copy/set loop to 1 when the page count is also 1, somewhere around here:

sharp/src/common.cc

Lines 570 to 571 in eacb833

	// Avoid a copy if none of the animation properties are needed.
	if (nPages == 1 && !hasDelay && loop == -1) return image;

[lovell]

v0.34.0 now available with this improvement, thanks for the original report.

[bra1n]

This seems to have broken the (animated) GIF -> WEBP pipeline, because previously it would default to loop: 0, as the docs suggest, and now no longer does. If I load a GIF with several frames and animated:true option, then convert it to WEBP without any other options, the default should still be loop: 0, or the doc needs to be updated.

Example code:

await sharp(req.file.buffer, { animated: true })
          .resize({
            width: 600,
            height: 600,
            fit: "cover",
            withoutEnlargement: true,
          })
          .rotate()
          .webp({ quality: 80 })

This will output a .webp file that loops infinitely with Sharp <3.4 and a .webp file that loops only once with Sharp >=3.4

[lovell]

@bra1n Thanks for reporting, I can reproduce. This should be fixed by commit 701143a, which also adds a unit test to help prevent regression.