nested forloops for running sliding window function for MATLAB - matlab

Question

I'm trying to write a simple code that will generate the sum of a large window and divide by the sum of the small running window to get the energy ratio.
my code looks like this in MATLAB
S = data1;
[nt,ntraces] = size(S);
!Create sliding windows for First Break Picking:
!define a window length
!for large Window
nl = 300
!for small running Window
ns = 50
! tolerance/Fudge Factor
beta = 0.0000
for i_slide = 1:nt-nl
for i_large = i_slide:(i_slide+nl)
large_window(i_large) = sum(S(i_large).^2)';
for i_small = i_slide+ns:i_slide+nl
small_window(i_small) = sum(S(i_small).^2)';
end
end
ER(i_slide) = small_window/(large_window + beta);
end
The problem i am having is that my small running window is not indexing correctly nor is it running the sum along the whole large window length at the maximum slide.
any ideas how i can overcome this problem?

Answer 1

In general, the problem you're really trying to solve seems to be general 2-D (or 1-D?) convolution. You can use MATLAB's conv or conv2 function (or filter or imfilter, if you have image processing toolbox) to do this. If you need to write a 2-D convolution function, I wouldn't try and write one that does two convolutions and takes the ratio. Instead write a simple convolution function: my_conv and run it twice, and take the ratio. e.g., you're trying to write:
output = my_double_conv(data,smallFilt,bigFilt); %this does ratios
I don't think that's a good idea in general. Don't do that. Do
output = my_conv(data,smallFilt) ./ my_conv(data,bigFilt);
You might see some speed benefits from not having to index everything twice in my_double_conv, but if computational concerns are your issue, you shouldn't be writing your own convolution in the first place; instead you should be using FFT convolutions, or integral-image convolutions (e.g., http://hebb.mit.edu/courses/9.29/2004/readings/c13-1.pdf or http://en.wikipedia.org/wiki/Summed_area_table )
That said, your code has several problems. Have you tried debugging with the MATLAB debugger?
For example, this is clearly wrong, since i_small is a scalar index:
for i_small = i_slide+ns:i_slide+nl
small_window(i_small) = sum(S(i_small).^2)';
end
That sum is not going to "sum" over anything, since i_small will be a scalar...
Do you want:
small_window= S(i_slide+ns:i_slide+nl);
small_window_sum = sum(small_window.^2);
Also note that for element-wise matrix operations, like:
small_window/(large_window + beta);
Where small_window and large_window are scalars, you want:
small_window./(large_window + beta); %note the "."