Sliding neighborhood - how to vectorize?

2015 9 25
2 个回答
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更新时间：2015 10 2
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0 个投票

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
Joseph Cheng 2015-9-25

0 个投票

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

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Alex Kurek
Alex Kurek 2015-9-29
I just realized that this is computed in loop 30x in average:
function signalInBlock = squaredFrameProcess(noised, signalInBlock)
arrayOnes = ones(31,31);
temp = conv2(noised, arrayOnes, 'valid');
signalInBlock(16:214, 16:214) = temp;
end
And i refere to it like this:
...
signalInBlock = squaredFrameProcess(noiseFrame, signalInBlock);
blurred = imfilter(signalInBlock, GaussianFilter);
...
So is there a way to make it faster? Can I somehow put it to one datacube and execute (on GPU?) in vectorized format?
Alex Kurek
Alex Kurek 2015-9-30
编辑：Alex Kurek 2015-9-30
I tried this:
noiseFrameCollector = zeros(230, 230, 30);
signalInBlock = noiseFrameCollector;
zzz = 1:1:30;
tic
signalInBlock(:,:,zzz) = squaredFrameProcess(noiseFrameCollector(:,:,zzz), signalInBlock);
toc
But got the following error:
Undefined function 'conv2' for input arguments of type 'double' and attributes 'full 3d real'. Error in squaredFrameProcess (line 3) temp = conv2(noised, arrayOnes, 'valid');
Is there any other possibility?
Joseph Cheng
Joseph Cheng 2015-10-2
编辑：Joseph Cheng 2015-10-2
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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更多回答（1 个）

Alex Kurek
Alex Kurek 2015-9-25

0 个投票

Thank you,
I implemented it like this (toc after figure, preallocation):
signalInBlock2 = zeros(230, 230);
tic
arrayOnes = ones(32,32);
temp = conv2(noised, arrayOnes, 'valid');
signalInBlock2(16:214, 16:214) = temp;
toc
figure, imshow(signalInBlock2, []);
And it takes 0.005525 seconds with is 34x faster.
Now I wonder if there is something faster than conv2

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Joseph Cheng
Joseph Cheng 2015-9-25
编辑：Joseph Cheng 2015-9-25
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
Image Analyst 2015-9-25
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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