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 -- [EMAIL PROTECTED]
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
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