Several filters silently no-op on animated GIF input (v1.8.6)

[nlemoine]

Several filters return the source unchanged (byte-identical to the unfiltered passthrough) when the input is an animated GIF. No error, no warning, just the original bytes back with Content-Type: image/gif.

Tested on v1.8.6. Same animated GIF source for every row, only the filter changes:

Filter	Result
(no filter)	passthrough (baseline)
blur(8)	no-op (byte-identical to baseline)
sharpen(50)	no-op
pixelate(8)	no-op
saturation(-50)	no-op
hue(60)	no-op
negative()	no-op
brightness(30)	applied (different bytes)
contrast(30)	applied
grayscale()	applied
rotate(90)	applied
round_corner(20,20,255,255,255)	applied
rgb(50,0,0)	applied
format(webp)	applied

So the no-ops are blur, sharpen, pixelate, saturation, hue, negative. The other filters work fine on the same input. The silent passthrough is hard to spot in client code: caller gets a 200 with valid GIF bytes and assumes it worked.

Repro:

/unsafe/filters:blur(8)/<any animated GIF URL>

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove the stale label or add a comment, or this issue will be closed in 15 days. You can always re-open if you still feel this is still an issue. Tag @cshum for more information.

[nlemoine]

Hey @cshum! Did you get the chance to take a look at it?

[cshum]

Hi @nlemoine I will be taking a temporary break to focus on personal matters. I will be completely offline and won't be reviewing issues or pull requests until July. Thank you for your patience and understanding.

[nlemoine]

No problem, take care!

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove the stale label or add a comment, or this issue will be closed in 15 days. You can always re-open if you still feel this is still an issue. Tag @cshum for more information.

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove the stale label or add a comment, or this issue will be closed in 15 days. You can always re-open if you still feel this is still an issue. Tag @cshum for more information.

[readtedium]

Was wondering about this too—and I should note that the silent fail also happens on static GIFs as well. I think the quality filter is the big one, as quality degrades differently for GIFs. (I currently have to use a different process to build the degraded GIF look I use on my site.) It may also help to have some sort of dithering or noise effect perhaps? @cshum happy to offer some feedback if you end up testing this in the imagor editor.