[Digikam-users] What am I doing wrong?

George Avrunin avrunin at comcast.net
Sat Aug 30 18:04:29 BST 2014

On Sat, 30 Aug 2014 13:40:03 +0200, Remco Viƫtor wrote:

> As you manipulate the image by adding the watermark, the image will have
> to be recompressed on saving. And like was said in a similar thread, a
> PC has more power than a camera, so it might do a better job at
> compressing w/o degradation.

I don't think this is true, given how the JPEG algorithm does
compression.  It's been a long time since I looked carefully at this, but
my recollection is that the "compression ratio" you set directly affects
only a single step where you divide the discrete cosine transform
coefficients (for the standard 8x8 block that JPEG operates on) by the
corresponding coefficients in a quantization matrix that is determined by
the percentage compression you specify.  It's conceivable that there's
some difference in numerical precision between what's done on a camera and
what's done on various computers, which would affect other parts of the
JPEG process, and that this would affect the reduction achieved by
the lossless run-length compression that's done afterwards, but I don't
think that having additional computing power means that compression with
specified percentage will typically yield a smaller file.  As I understand
it, higher numerical precision might make the run-length compression less
effective, depending on the inputs.

If I'm wrong about that, I'd be grateful if someone would straighten me
out. :-)


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