JPEG Thumbnailer

Andre Gemünd scroogie at
Tue Aug 12 15:12:27 BST 2008

Hi List,
I'm a fresh subscriber so forgive me if I don't follow the right rules 
for posting. I registered because Peter Penz was so nice as to introduce 
me to the approach of eventual contribution.
I found that the JPEG previews in Dolphin were quite slow so I had a 
look at the corresponding code. The Thumbnail KIO Slave loads the whole 
JPEG and scales it afterwards. So I tried out to load a scaled down 
version with libjpeg and here is the code attached as an additional 
thumbnail implementation for the KIOSlave. It's hard to measure the 
actual difference, but I think it's a significant speedup. I tried it 
out with a test program that uses the QImage way like the 
ImageThumbnailer and the former libjpeg method. If theres interest i can 
post it as well.
Of course it's a tradeoff between quality and speed, so it might need 
some discussion. I tried it out on a local build and attach a patch 
against 4.1.0.
To try it out Caulier Gilles has some nice test images of different 
sizes on
The method is well known and while browsing the websvn I found out that 
it's also used in Digikam and Gwenview for JPEG thumbnails. I've never 
contributed to KDE before, so I'm not familiar with logging or error 
recovery methods used, so I'd like to get some comments. I hope I can 
hop on the KDE train with that first patch and am looking forward to 
your answers!

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