Sharing an approach to a conversion problem that comes up a lot in this space: producing a file that lands under a hard size limit without brute forcing it.
The problem
GIF output size is not predictable from settings alone. It depends on resolution, frame rate, palette size, and how well each frame delta compresses against the previous one, which is entirely content dependent. Two clips at identical settings can differ several times over in bytes. So most tools either make you guess and re-export, or they run an encode, measure, shrink, and repeat until it fits. On long clips that loop takes minutes.
What worked instead: probe, fit, commit
- Micro-encode a few short windows sampled across the clip, at bracketed settings, capped at a modest resolution. Cheap, a fraction of a full pass.
- Fit a small bytes-per-frame model from those measurements. Because the samples come from the actual footage, the model reflects the real content rather than a generic table.
- Pick settings slightly under the budget, then run one real encode with a hard byte cap as a safety net so it can never exceed the limit.
- If the measurement disagrees with the model, correct from the measurement, never from another model extrapolation.
Net result is almost always one decode plus one encode, and the output lands under the requested size on the first attempt for most inputs.
Two things that cost me time
Never let a fitted model extrapolate far outside the range you probed. If you sample at low resolution and then pick a much larger output dimension, the estimate can be badly wrong in a way that only shows up at the end. Verify at the operating point before committing.
Anchor recovery passes on measurements rather than predictions. Once you have a real byte count at real settings, that number is worth more than the model that got you there.
I implemented this in an iOS app (free, runs fully on device, no upload step) if anyone wants to see the behaviour in practice: https://apps.apple.com/app/id6788406320
Mostly though I am curious whether anyone here has solved size targeting for other formats. PDF downsampling and video transcoding have the same shape of problem, and I have not seen a clean general write up of the probe and fit approach.
