[glob2-devel] new gradient system

[Nuage]

Summary: improved gradient speed by 97%, which improves overall speed by 5%.

Hello, I discovered a new way to compute the gradient. It has the advantage
to afford *any* kind of obstacle until the depth is <255 of distance. The old
system could only afford a limited complexity of obstacle. I tryed alot of
ways to implement it, and tryed to tune it. So we have a nice feature
improvement but it's finally hard to say if there is any speed improvements.

All test are based on a build with optimisation flags:
-O3 -march=athlon-xp -funroll-loops -fprefetch-loop-arrays -fomit-frame-pointer 
-Wall -g


If we look only at the gradients, there is a nice speed improvement:
* Test 1 (128x128):

Old:

gradientAverageTicks=0.655172
gradientAverageTicks=0.962457
gradientAverageTicks=0.926871
gradientAverageTicks=0.754266
average=0.8246915

New:

gradientAverageTicks=0.149228
gradientAverageTicks=0.697074
gradientAverageTicks=0.154762
gradientAverageTicks=0.676320

average=0.419346

gradient speedup=1.97


But if we look at the globat improvement, we only get a 5% real improvement:
* Test 2 (256x256):

Old:

cpu usage graph:
100.0 % | **
 98.5 % |
 95.0 % |
 93.5 % |
 90.0 % |
 88.5 % | *
 85.0 % |
 83.5 % | *
 80.0 % |
 78.5 % | *
 75.0 % |
 73.5 % | *
 70.0 % | *
 68.5 % | *
 65.0 % | *
 63.5 % | **
 60.0 % | *
 58.5 % | ****
 55.0 % | *****
 53.5 % | ****************
 50.0 % | ********
 48.5 % | ***********
 45.0 % | **
 43.5 % | **
 40.0 % | **
 38.5 % | ****
 35.0 % | *************
 33.5 % | ***************
 30.0 % | ******
 28.5 % | *
 25.0 % |
 23.5 % |
 20.0 % |
 18.5 % |
 15.0 % |
 13.5 % |
 10.0 % |
  8.5 % |
  5.0 % |
  3.5 % |
  0.0 % |

New:

cpu usage graph:
100.0 % | *
 98.5 % |
 95.0 % |
 93.5 % |
 90.0 % |
 88.5 % |
 85.0 % |
 83.5 % |
 80.0 % |
 78.5 % |
 75.0 % |
 73.5 % | *
 70.0 % | *
 68.5 % | *
 65.0 % | *
 63.5 % | *
 60.0 % | *
 58.5 % | **
 55.0 % | ******
 53.5 % | ******
 50.0 % | *******
 48.5 % | *************
 45.0 % | ***********
 43.5 % | ******
 40.0 % | *******
 38.5 % | ******
 35.0 % | ****
 33.5 % | *****
 30.0 % | *****
 28.5 % | ********
 25.0 % | ********
 23.5 % |
 20.0 % |
 18.5 % |
 15.0 % |
 13.5 % |
 10.0 % |
  8.5 % |
  5.0 % |
  3.5 % |
  0.0 % |