Average H of hsv

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.

[Jan]

Please post the code. It is impossible to debug "I tried mean function".

Answer 1

[Rene]

   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

Answer 2

[Jan]

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;

[Xiaochao]

Thank you very much. The the code does work.I'm a new user and still learnt the functions

Answer 3

[Sean de Wolski]

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(:)));

[Jan]

The Reshape/Sum method is faster than Blockproc. I think, this is a reason.

[Sean de Wolski]

Of course it's faster - but it still is only taking 7/100ths of a second for the 480x640 image above...

[Sean de Wolski]

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]

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...