Average H of hsv

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Xiaochao
Xiaochao 2012-9-11
I want to divide a picture of 640*480 pixels to 16*16 blocks and get average value H of every block. The result shoud be a 40*30 array. But I tried mean function, the result always is a 1*16 array.
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Jan
Jan 2012-9-11
Please post the code. It is impossible to debug "I tried mean function".

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回答（3 个）

Rene
Rene 2012-9-11
A = rand(480,640);
B=[];
for i=1:16:480
for j=1:16:640
B = [B,mean(mean(A(i:i+15,j:j+15)))];
end
end
B = reshape(B,30,40);
should do the trick
note: inefficient for large matrices
Jan
Jan 2012-9-11
Exactly as I have posted in your other thread, which contains almost the same question:
H = rand(640, 480);
H = reshape(H, 16, 40, 16, 30);
blockMean = reshape(sum(sum(H, 1), 3), 40, 30) / 256;
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Xiaochao
Xiaochao 2012-9-11
Thank you very much. The the code does work.I'm a new user and still learnt the functions

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Sean de Wolski
Sean de Wolski 2012-9-11
编辑：Sean de Wolski 2012-9-11
Why not use blockproc() like I suggested in the other thread? It will do this for you completely automagically - there is no reason to reinvent the wheel.
bm = blockproc(H,[16 16],@(blk)mean(blk.data(:)));
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Sean de Wolski
Sean de Wolski 2012-9-11
0.06 seconds with a little more intelligence in anonymous function creation:
blksz = [16 16];
n = 16^2;
bm = blockproc(H,blksz,@(blk)sum(sum(blk.data))./n);
Jan
Jan 2012-9-11
Ok, Sean. I admit one 640x480 picture is tiny. Then I claim that the Resahpe/Sum approach is simpler that Blockproc with an anonymous function. But this is not a convincing argument also: Both solutions can be used by copy&paste now, such that the "programming" will need a few seconds only.
I avoid to suggest the faster C-Mex function http://www.mathworks.com/matlabcentral/fileexchange/24812-blockmean, which I have published after the "mean over blocks"-questions have been written repeatedly. Here the speed advantage will be eaten up by the time reuquired to install the C-compiler...

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