Re: Speeding up thumbnail generation (like multi threaded). Thoughts please.

["Dr. Michael J. Chudobiak" <mjc avtechpulse com>]

On 08/30/2009 09:51 AM, jcupitt gmail com wrote:
> able to supply pixels at a certain size. In particular, libjpeg can do
> a very quick load-at-1/8th-size read where it just decompresses enough
> to be able to get the DC component of each 8x8 block. If you use
> libjpeg like this you can expect around a 100x speedup of the
> decompress step.

The gdk-pixbuf jpeg loader does this already.

The key is to improve the generic down-scaling routines in gdk-pixbuf, 
as noted here:

http://bugzilla.gnome.org/show_bug.cgi?id=80925

The scaling routines are currently quite bad at downscaling by large 
factors.

For example, last year I used a test folder that had four 15000x400 tif 
images and four 15000x400 png images (solid colors). Nautilus took 4 
minutes and 30 seconds to thumbnail the folder.

I then patched gdk-pixbuf to do scaling in two steps (scale by root-N 
twice, instead of scale by N). With this approach, Nautilus took 8 
seconds. See http://bugzilla.gnome.org/show_bug.cgi?id=522803 for details.

I'm not saying that is a good approach (the quality would be poor for 
line drawings, for example, but OK for photos). But it shows that there 
is lots of room for improvement!

- Mike