Overcome for loops for fast processing

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I have the following part of MATLAB code
skin_rgb = zeros(256.^3,1);
for i = [2000 : 2003,2008,2010:2011,2013:2015,2017:2018,2021,2024,2027:2041,2043,2045:2059,2061:2063]
[x,y,~]= size(I);
for l = 1:x
for j = 1:y
if (I(l,j,1) ~= 255 && I(l,j,2) ~= 255 && I(l,j,3) ~= 255)
aa = double([I(l,j,1) , I(l,j,2) , I(l,j,3)]);
[~,ee1] = ismember(aa,rgb_hist,'rows');
skin_rgb(ee1) = skin_rgb(ee1) + 1;
b1 = b1 + 1;
end
end
end
end
The output of the code is perfect. The issue is it is very slow. Inside the if loop the size of rgb_hist variable is 255.^3 x 3. Is there any way to get rid of the two nested for loops? As I am doing Image processing on high resolution image the variable x and y (different for each "i" iteration) have a big number in it.
Thank you

Walter Roberson on 11 Nov 2018
Is every possible combination in rgbhist?
Muhammad Farhan Mughal on 11 Nov 2018
Yes, every possible combination is in rgb_hist.

Bruno Luong on 11 Nov 2018
Edited: Bruno Luong on 14 Nov 2018
skin_rgb = 0;
for i = [2000 : 2003,2008,2010:2011,2013:2015,2017:2018,2021,2024,2027:2041,2043,2045:2059,2061:2063]
RGB = reshape(double(I),[],3)+1;
skin_rgb = skin_rgb + accumarray(RGB,1,256+[0 0 0]);
end
skin_rgb = permute(skin_rgb,[3 2 1]);
skin_rgb = skin_rgb(:);

1 Comment

Muhammad Farhan Mughal on 12 Nov 2018
Thanks a lot.

Walter Roberson on 11 Nov 2018
skin_rgb = zeros(256.^3,1);
for i = [2000 : 2003, 2008, 2010:2011, 2013:2015, 2017:2018, 2021, 2024, 2027:2041, 2043, 2045:2059, 2061:2063]
r = 1 + double(I(:,:,1)); g = 1 + double(I(:,:,2)); b = 1 + double(I(:,:,3));
skin_rgb = skin_rgb + accumarray([r(:), g(:), b(:)], 1, [256.^3, 1]);
end

Muhammad Farhan Mughal on 14 Nov 2018
Yes, I understad that, but in the folowing code I have already added 1 in variable "aa".
>> aa = 1 + [I(500,500,1),I(500,500,2),I(500,500,3)];
>> [a1,a2] = ismember(aa,rgb_hist,'rows');
>> skin_rgb(a2)
ans =
0
Bruno Luong on 14 Nov 2018
This test code returns 1 everytime
% Such simple generate thing that you never bother to post
[B,G,R] = ndgrid(1:256);
rgb_hist = [R(:),G(:),B(:)];
% Quick check for order
rgb_hist(1:10,:)
rgb_hist(end+(-9:0),:)
% Random data
I = floor(256*rand(100,100,3));
aa = 1 + [I(100,100,1),I(100,100,2),I(100,100,3)]
% Counting algo
skin_rgb = accumarray(aa,1,256+[0 0 0]);
skin_rgb = permute(skin_rgb,[3 2 1]);
skin_rgb = skin_rgb(:);
% Check
[a1,a2] = ismember(aa,rgb_hist,'rows');
skin_rgb(a2) % <- This returns correctly 1
Muhammad Farhan Mughal on 14 Nov 2018
Thanks a lot