Manually changing RGB to HSI matlab - matlab

Question

I would like to preface this by saying, I know some functions, including RGB2HSI could do this for me, but I would like to do it manually for a deeper understanding.
So my goal here is to change my RGB image to HSI color scheme. The image is in .raw format, and i am using the following formulas on the binary code to try and convert it.
theta = arccos((.5*(R-G) + (R-B))/((R-G).^2 + (R-B).*(G-B)).^.5);
S = 1 - 3./(R + G + B)
I = 1/3 * (R + G + B)
if B <= G H = theta if B > G H = 360 - theta
So far I have tried two different things, that have resulted in two different errors. The first attempted was the following,
for iii = 1:196608
C(iii) = acosd((.5*(R-G) + (R-B))/((R-G).^2 + (R-B).*(G-B)).^.5);
S(iii) = 1 - 3./(R + G + B);
I(iii) = 1/3 * (R + G + B);
end
Now in attempting this I knew it was grossly inefficent, but I wanted to see if it was a viable option. It was not, and the computer ran out of memory and refused to even run it.
My second attempt was this
fid = fopen('color.raw');
R = fread(fid,512*384*3,'uint8', 2);
fseek(fid, 1, 'bof');
G = fread(fid, 512*384*3, 'uint8', 2);
fseek(fid, 2, 'bof');
B = fread(fid, 512*384*3, 'uint8', 2);
fclose(fid);
R = reshape(R, [512 384]);
G = reshape(G, [512 384]);
B = reshape(B, [512 384]);
C = acosd((.5*(R-G) + (R-B))/((R-G).^2 + (R-B).*(G-B)).^.5);
S = 1 - 3./(R + G + B);
I = 1/3 * (R + G + B);
if B <= G
H = B;
if B > G
H = 360 - B;
end
end
H = H/360;
figure(1);
imagesc(H * S * I)
There were several issues with this that I need help with. First of all, the matrix 'C' has different dimensions than S and I so multiplication is impossible, so my first question is, how would I call up each pixel so I could perform the operations on them individually to avoid this dilemma.
Secondly the if loops refused to work, if I put them after "imagesc" nothing would happen, and if i put them before "imagesc" then the computer would not recognize what variable H was. Where is the correct placement of the ends?

Answer 1

Normally, the matrix 'C' have same dimensions as S and I because:
C = acosd((.5*(R-G) + (R-B))/((R-G).^2 + (R-B).*(G-B)).^.5);
should be
C = acosd((.5*(R-G) + (R-B))./((R-G).^2 + (R-B).*(G-B)).^.5);
elementwise division in the middle was missing . Another point is:
if B <= G
H = B;
if B > G
H = 360 - B;
end
end
should be
H = zeros(size(B));
H(find(B <= G)) = B(find(B <= G));
H(find(B > G)) = 360 - B(find(B > G));