[FFmpeg-user] Glossary: Nyquist

[Mick Finn]

Most relevant to input sampling and not so relevant for displays unless higher spatial frequencies are actually present in data
Not to mention most cameras have optical low pass filters
In other words just beachside a 4k camera was used does not mean it can display alias free in 4k display
So please keep bad info off the list

Sent from my iPhone

> On Oct 1, 2020, at 9:18 PM, Mark Filipak (ffmpeg) <markfilipak at bog.us> wrote:
> 
> On 10/01/2020 01:20 AM, Eduardo Alarcón wrote:
>> Nyquist is a noun, not an adjective, for Harry Nyquist. ...
> 
> Hi Eduardo, Thanks.
> 
> Well, technically, a noun used as an adjective: "Nyquist sampling", makes it an adjective, but no matter.
> 
> What if I narrow the scope to solely the sampling theory? thusly:
> 
> Nyquist sampling: The principle [1] that, to most faithfully reproduce
>  an image at a given digital display's resolution, samples must be
>  made at or above twice the display's resolution, both horizontally &
>  vertically [2].
>  [1] The Nyquist principle applies to film sampling and to digital
>      cameras, but, provided that resolution is unchanged, not to
>      transcoding (because the transcoder inputs are already digital).
>      As proved by the improved appearance of SD media made from 2K
>      samples, SD mastering prior to the advent of 2K sampling (e.g.
>      DVDs mastered before the advent of HD) generally ignored the
>      Nyquist principle and were undersampled. HDs sampled at 2K and
>      UHDs sampled at 4K are likewise undersampled.
>  [2] As a convenience, the Nyquist threshold is currently (in 2020)
>      specified solely by horizontal sample count rounded up to whole
>      kilo-samples (2K, 4K, 8K).
>                        display    Nyquist threshold
>      UHD 16:9-2160:  3840 x 2160         8K
>           4:3-2160:  2880 x 2160         8K
>       HD 16:9-1080:  1920 x 1080         4K
>           4:3-1080:  1440 x 1080         4K
>       SD  16:9-576:  1024 x 576          4K
>            4:3-576:   768 x 576          2K
>           16:9-480:   853 x 480          2K
>            4:3-480:   640 x 480          2K
> 
>> ... The Nyquist–Shannon
>> sampling theorem is applicable to analog to digital conversion of signals
>> (continuous to discrete), images are a type of signal.
> 
> Well, I thought that was what I wrote. What doesn't work for you?
> 
> The reason I wrote "both horizontally & vertically" was to resolve that, unlike sampling a 1-dimensional (serial) signal, 2-dimensional sampling (e.g. from film) or within a camera, requires the Nyquist principle be applied in both dimensions. But perhaps that's not what you find lacking. Could you suggest different wording maybe?
> 
> -- 
> What if you woke up and found yourself in a police state?
> African-Americans wake up in a police state every day.
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