[Digikam-users] What am I doing wrong?

Stuart T Rogers stuart at stella-maris.org.uk
Sun Aug 31 10:53:48 BST 2014

On 30/08/14 20:38, Stuart T Rogers wrote:
> 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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>> Digikam-users at kde.org
>> https://mail.kde.org/mailman/listinfo/digikam-users
> 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

Just to confirm after some more testing that everything I have tried 
(including now imagemagick) correctly saves a watermarked image with 
only slightly better compression than the original from my camera.

The only thing which fails to correctly save the image is a digikam 
watermark using the batch queue manager.

This is a real PITA as I was hoping to use this digikam facility to 
replace a windows program I was using, so until this bug is fixed I will 
have to continue booting a W7 VM to do my watermarking.

Please someone fix this.....


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