Downsample thumbnail by powers of two before doing lanczos

Downsample thumbnail by powers of two before doing lanczos

This speeds up the resizing portion of thumbnailing from 40ms->15ms on my linux z600 at nearly fullscreen resolution.  Thumbnail resizing times on weaker hardware (like netbooks) has been observed in the 150ms+ range, so hopefully this is a proportional speedup there as well.

TEST=none
BUG=62820
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=65982

[jamesr, 2010-11-12 04:39:34]

Patch to mitigate thumbnail lockups.  I don't know how to test that this is
correct and/or that the thumbnail still looks 'good enough' (hoping that there's
some test that the trybots will run).

[brettw, 2010-11-12 07:05:57]

What timings are you getting on the old code versus the new? There is already a
histogram for this so it should be easy to check.

http://codereview.chromium.org/4846002/diff/1/chrome/renderer/render_view.cc
File chrome/renderer/render_view.cc (right):

http://codereview.chromium.org/4846002/diff/1/chrome/renderer/render_view.cc#...
chrome/renderer/render_view.cc:1110: SkBitmap downsampledSubset =
Le style Goog: downsampled_subset

[jamesr, 2010-11-12 19:53:17]

On 2010/11/12 07:05:57, brettw wrote:
> What timings are you getting on the old code versus the new? There is already
a
> histogram for this so it should be easy to check.

It cuts the time spent in the resize portion of this function from around 40ms
to 12-15ms on my linux z600 at large window size.

> 
> http://codereview.chromium.org/4846002/diff/1/chrome/renderer/render_view.cc
> File chrome/renderer/render_view.cc (right):
> 
>
http://codereview.chromium.org/4846002/diff/1/chrome/renderer/render_view.cc#...
> chrome/renderer/render_view.cc:1110: SkBitmap downsampledSubset =
> Le style Goog: downsampled_subset

[jamesr, 2010-11-12 19:55:05]

The thumbnails look OK in the NTP.  I'm not sure how else to verify that this is
correct :)

[brettw, 2010-11-12 20:05:53]

Sounds good, LGTM

[Stephen White, 2010-11-12 20:34:22]

LGTM too.  Nice optimization!