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

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:

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 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

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