Either way, wouldn't you need some kind of memory to deal with 2D blocks? Averaging luminance over a certain period on a given scan line (thus over a certain width) is easy, but averaging it over a certain square area? Can you do it without memory?
You misunderstand, there is no averaging involved. It is just reducing levels of quantization to some small number of levels. It's something like having a 1-digit voltmeter that can only display "0" through "9" instead of "0.000" through "9.999".
Well, I was actually thinking about a bit fancier "posterization" effect (I happen to have worked on one a long time ago). Just quantizing the signal "pixel" by "pixel" doesn't look very good. By "posterization", I was understanding any effect that would quantize luminance (or colors, depends on the effect you're after) to a limited number of levels, which is exactly what you said above. But there are various ways of doing it, some that will look much "nicer", and involving averaging over a certain area around every pixel (or otherwise some other kind of filtering), thus my question. You can discretize luminance with a much better looking effect.
Now if we are talking about basic quantization pixel by pixel without anything fancier, obviously I agree, you don't need any memory or any kind of filtering.
And for this, yes, once you have a sync detection circuit, that can be done with a relatively simple analog front-end and a flash ADC (+DAC, the DAC can be just a few resistors though, not that accuracy is probably a concern here.)
Obviously discrete comparators can also be used; their number will quickly add up though unless you're after a very small number of levels (admittedly, 16 levels for instance, would "only" require 4 ICs using quad comparators). Flash ADCs are actually made of fast comparators (plus some logic).
https://en.wikipedia.org/wiki/Flash_ADCI'm trying to think of a purely analog way of doing this. Sure you can argue that using comparators would be purely analog. I wonder if we can find a way of getting a similar "posterization" effect without comparators, with a reduced part count, using some clever trick.