3 ms·

Your idea is more or less right, but the problem here is that Nyquist only applies to continuous functions, which a GIF is not. At frame boundaries the frequenc

by kaoD 4y ago

Your idea is more or less right, but the problem here is that Nyquist only applies to continuous functions, which a GIF is not. At frame boundaries the frequency is potentially infinite, like an ideal square wave. A GIF is already a sampled signal. Yes, you can LP-filter the GIF but that's no longer reproducing the original signal, which wasn't band-limited to begin with. You're right that, if the original signal that the GIF samples is band-limited to 1/2 the Nyquist rate (which is a big "if") it can be accurately reproduced without aliasing by passing it through a reconstruction filter.

moring 4y ago
One could argue that the same applies to old pixelated graphics, such as the original Super Mario sprite. It has to be pixelated, if you lowpass-filter it it looks ugly. So that one is non-continuous / not band-limited in the spatial direction. This brings me back to this question: > So the first question is whether these "jumpiness" is an inherent, desired property of GIFs or an artifact of bad renderers. Like for pixel-y displays, I assume that this was never well-defined for the GIF format, so what is basically a property of the first GIF renderers became a de-facto property of the format. I'd argue that GIFs that exploit this property and rely on the "jumpiness" to look good are not band-limited, require a "jumpy" renderer and will suffer from the 50Hz/60Hz problem. On the other hand, GIFs that don't exploit this property are meant to be continuous and are just sampled at a low frequency, and for those re-sampling in time will allow to fix the 50Hz/60Hz problem. Though another problem will turn up then: when the latter were sampled at a low frequency, they were likely not lowpass-filtered beforehand, so they already contain aliasing artifacts you cannot get rid of anymore.

3 ms·

Your idea is more or less right, but the problem here is that Nyquist only applies to continuous functions, which a GIF is not. At frame boundaries the frequenc

by kaoD 4y ago

moring 4y ago
One could argue that the same applies to old pixelated graphics, such as the original Super Mario sprite. It has to be pixelated, if you lowpass-filter it it looks ugly. So that one is non-continuous / not band-limited in the spatial direction. This brings me back to this question: > So the first question is whether these "jumpiness" is an inherent, desired property of GIFs or an artifact of bad renderers. Like for pixel-y displays, I assume that this was never well-defined for the GIF format, so what is basically a property of the first GIF renderers became a de-facto property of the format. I'd argue that GIFs that exploit this property and rely on the "jumpiness" to look good are not band-limited, require a "jumpy" renderer and will suffer from the 50Hz/60Hz problem. On the other hand, GIFs that don't exploit this property are meant to be continuous and are just sampled at a low frequency, and for those re-sampling in time will allow to fix the 50Hz/60Hz problem. Though another problem will turn up then: when the latter were sampled at a low frequency, they were likely not lowpass-filtered beforehand, so they already contain aliasing artifacts you cannot get rid of anymore.