[Digikam-users] What am I doing wrong?

Remco Viëtor remco.vietor at wanadoo.fr
Sat Aug 30 18:23:05 BST 2014

On Saturday 30 August 2014 13:04:29 George Avrunin wrote:
> On Sat, 30 Aug 2014 13:40:03 +0200, Remco Viëtor wrote:
> > 
> > As you manipulate the image by adding the watermark, the image will 
> > 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 
> 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 
> 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. :-)
>   George

True, so that's not the cause. 

What Stuart might try is add a 'convert to jpeg' step in tgeh batch queue 
and see if that changes things (this will also allow him to play with the 
jpeg compression paramters).

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