[Digikam-users] What am I doing wrong?

cerp cerp at eeos.biz
Sat Aug 30 20:50:09 BST 2014

I agree Stuart .... I am experiencing the same problem, and I had  
already posted the same issue in the last week.


Quoting Stuart T Rogers <stuart at stella-maris.org.uk>:

> 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 option.
> 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....
> Stuart
> -- 
> Website: http://www.stella-maris.org.uk
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