[Digikam-devel] Thumb database image format... More investiguation

[Mikolaj Machowski]

On Tuesday 09 June 2009 14:51:55 Gilles Caulier wrote:
> From speed viewpoint, it's really difficult to measure because a
> program need to be written instead to use command line.
>
> To check, i have used a slow computer (my old laptop PIII-600Mhz) and
> see from command line which conversion generate target files speedly.
> I can see JPG very fast and PGF is a little bit faster than JPEG2000.
>
> All test files are available here, if you want to check visual image
> quality : http://digikam3rdparty.free.fr/misc.tarballs/testdbthumbs.tar.bz2
>
> So, JPG sound like the best candidate...

When scaling image subsampling factor is very important. With default 2x1 
(using IM terms) you will get significantly worse results for some images than 
with 1x1. Images with 1x1 are slightly bigger but definitely worth it.

You can even lower quality option, combo of q=80 and sf=1x1 will be better 
than q=85 sf=2x1.

Another thing worth to think about are thumbnails dimensions. digiKam was tied 
with 256 due to .thumbnails standard. On one hand 256 is becoming really small 
with new big displays but on the other hand medium sized thumbs are most often 
used (also Qt scaling works really good up to 150% of original size of image).

IMatch leaves decision about thumbnails size to user with 200x200 as default.

m.