Sliding neighborhood - how to vectorize?
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Alex Kurek
le 25 Sep 2015
Modifié(e) : Joseph Cheng
le 2 Oct 2015
Dear all,
Can you please help me to vectorize (or speed-up somehow else) this code? Below is the original (parfor) version and the vectorized one, but its not working (the image is different). How to vectorize this, where is the error? The inner loop (two lines) is executed 47 bln times in my code, so any speed up is a good thing.
noised = imnoise(zeros(230,230), 'salt & pepper', 0.2);
imshow(noised, []); impixelinfo
%%Oryginal
myTempModel = zeros(1, 230);
signalInBlock = zeros(230, 230);
tic
parfor i = 1 : 199
myTemp = myTempModel;
ii=i+31;
for j = 1 : 199
block = noised( i:ii, j:j+31);
myTemp(j+15) = sum(block(:));
end
signalInBlock(i+15, :) = myTemp;
end
toc
imshow(signalInBlock,[]); impixelinfo
%%Vectorized, but not working
signalInBlock = zeros(230, 230);
tic
i = 1:1:199;
j = 1:1:199;
signalInBlock(i+15, j+15) = sum(sum(noised(i:i+31, j:j+31)));
toc
imshow(signalInBlock,[]); impixelinfo
Best regards, Alex
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Joseph Cheng
le 25 Sep 2015
why not use conv2?
signalInBlock2 = zeros(230, 230);
tic
temp = conv2(noised,ones(32,32),'valid');
signalInBlock2(16:214,16:214)=temp;
figure,imshow(signalInBlock2,[]);
toc
when running your code the parfor took 0.327807 seconds, the conv2 took 0.131374 seconds
3 commentaires
Joseph Cheng
le 2 Oct 2015
Modifié(e) : Joseph Cheng
le 2 Oct 2015
for that conv2 is for a 2D matrix if my memory of the documentation is correct. you can write a for loop to go through each "layer" of signalblock. which if large the parallel tool box can make if faster if it is really slow since each "layer" is not dependent on each other. As for GPU processing, i'm still dabbling in using the GPU so i'm not sure.
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Alex Kurek
le 25 Sep 2015
2 commentaires
Joseph Cheng
le 25 Sep 2015
Modifié(e) : Joseph Cheng
le 25 Sep 2015
good catch, I stuck the figure portion towards the end to visually compare the parfor output and the conv2 output. forgot to copy the timing results without the figure when replying to you
Image Analyst
le 25 Sep 2015
conv2() is highly optimized, especially for separable kernels like you're using (just a flat box filter). You won't find anything faster. You could compare it with imfilter() if you want - it's similar.
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