Strange 3x3 Lowpass filter

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Fabio Corres
Fabio Corres 2017-3-11
编辑： Fabio Corres 2017-3-12
I need to perform an article about hdr imagining and i see a formula that smooth the pixel.But it is different formulla. Can someone help me to code it on matlab
but i think first iteration will be problem. when i and j start with 1 and k is -1 first iteration will be ymax(0,0) but there is no such a thing like that in MATLAB.I think we should add +1 near i+k and j+l.
Thank you

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Chad Greene
Chad Greene 2017-3-11
It looks like a pretty straightforward 3x3 averaging filter. The catch is you have to start with (i=2,j=2) so i+(-1) will be 1 and same for j. You can do this pixel by pixel, or you can do it much faster with imfilter:
ylpf = imfilter(ymax,fspecial('average',[3 3]));
No loops necessary.
  1 个评论
Chad Greene
Chad Greene 2017-3-11
If you do not have the Image Processing toolbox, you can use conv2 instead:
P = peaks(50) + 2*rand(50);
% With image toolbox:
Pf = imfilter(P,fspecial('average',[3 3]));
% Without image toolbox:
Pf2 = conv2(P,ones(3)/9,'same');
figure
subplot(1,3,1)
imagesc(P)
title 'noisy data'
subplot(1,3,2)
imagesc(Pf)
title 'imfilter'
subplot(1,3,3)
imagesc(Pf2)
title 'conv2'

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更多回答（1 个）

Walter Roberson
Walter Roberson 2017-3-11
There is no problem if you restrict i to be in [2, (number of rows - 1)] and restrict j to be in [2, (number of columns - 1)]
For areas outside that, the formula is not well defined.
Note: consider using the single statement
y_lpf = conv2(y_max, ones(3,3)/9, 'same');
... except, that is, that the result is well defined at the edges.

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