Re: OT: Why is usb data many times the cpu hog that firewire is?

Geert Uytterhoeven
2005-02-23T17:18:16Z

On Wed, 23 Feb 2005, Paulo Marques wrote:
> Matt Mackall wrote:
> > [...]
> > JPEG data is DCT of 8x8 pixel chunks. If you can get at that, you can
> > compare the DC terms of each chunk with minimal decoding. Various
> > thumbnailers do this for speed already.
>
> I really doubt that this would work. It seems to me that you can have very
> different DC terms with very similar results. In other words, even a little
> noise in the picture might produce very different DC terms.

It will cause only a slight difference, noise is mainly visible in the
high-frequence parts.

> Instead of comparing the DC terms, you could compare just the luminance. You
> would have to decompress just half the data for that and you wouldn't need to
> make the YUV->RGB conversion. That would probably save a few cycles.

And since you cannot do `exact' comparisons of the luminance due to noise, but
have to do some averaging, you're back to the previous solution: Hence compare
the DC terms of the luminance. They're the average luminance of the 8x8 chunks.

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds