Re: Chroma Subsampling [Was: JPEG decoding]

Pete Fraser <pfraser@xxxxxxxxx> wrote:
>
>> I would guess digital camera vendors are using 2x1 1x1 1x1 ... because
>> it creates much smaller files (often 2/3 at Q99) without creating as many
>> blocking artifacts as 2x2, which is not all that much more compact.
>
> I'd be surprised if a reduction to 2/3 was seen in anything other than
> the most extraordinary of circumstances.

I tried it before posting with one image (full of vegetative detail)
and will try it again with another image (portrait with less detail).
[wait]
Similar results; the 4:2:2 encoding is 68% the size of the 4:4:4 image.
Perhaps reducing IJG Q99 would produce different results, but Q99 is
about what most DSLR vendors use for ExtraFine JPEG.

> Sure, going from a 4:4:4 to a 4:2:2 color space reduces the raw data
> by 1/3, but that ignores the decorelative effect of the color space
> transform as well as the fact that most DSC images use a more severe
> quantizing table for chroma.

Just goes to show: theory sometimes fails to predict practice.

.