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Subject: Re: 2D convolution performance expectation



François wrote:
> Dear all,
>
>
> I just begin to implment a 2d spatial convolution of an image with 7*7
> float kernel using C++.
> I was wondering what kind of performance I should expect when processing
> 1000*1000 images
>
> For the moment, in release mode, it takes around 1.5sec for the whole
> process. Does it sound ok or some optimization still have to be taken?
>
> thanks in advance
>
>
> François
Why are you using floating point ?
You also did not say wether the image is color or Grayscale
the usual manner for a kernel convolution is to use integers
For Instance with a 7 x 7 kernel there would 49 Multiplies and additions
and 1 divide for nearly every point in the 1000 x 1000 image array
If you neglect the edges then the image convolution takes place over
951 x 951
this is 43277 multiples and adds
On a modern Pentium 4 with 4 MB LII cache this can all take place in the
cache for a grayscale image which can take 2 cycles for each multiply
and 1 cycle for each add and I think it is 10 cycles for a divide

which works out to 692432 clock cycles which on a 2 GHz machine should
only take about 339 Milliseconds for color the math is very similar

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