[Digikam-users] What am I doing wrong?

Stuart T Rogers stuart at stella-maris.org.uk
Sat Aug 30 20:38:51 BST 2014

On 30/08/14 18:23, Remco Viëtor wrote:
> 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
> 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. :-)
>>    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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Well I did just that (connvert to jpeg first and watermark second) and 
guess what .... the file ended up at 1.7MB with 100% set for the jpeg 

However I tried it again doing the watermark FIRST and the convert to 
jpeg second and this time I get a file of 6.5MB the same as GIMP.

So it looks like the batch watermark option seems to be at fault. 
Equally it makes no sense to convert a jpeg to a jpeg.

There HAS to be a bug here somewhere....

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