RGB to YIQ conversion - matlab

Question

I wrote code for rgb to yiq conversion.I get results but i don't know if this is correct.
%extract the red green blue elements
ImageGridRed = double(ImageRGB(:,:,1))';
ImageGridGreen = double(ImageRGB(:,:,2))';
ImageGridBlue = double(ImageRGB(:,:,3))';
%make the 300x300 matrices into 1x90000 matrices
flag = 1;
for i =1:1:300
for j = 1:1:300
imageGR(flag) = ImageGridRed(j,i);
imageGG(flag) = ImageGridGreen(j,i);
imageGB(flag) = ImageGridBlue(j,i);
flag = flag+1;
end
end
%put the 3 matrices into 1 matrix 90000x3
for j=1:1:300*300
colorRGB(j,1) = imageGR(j);
colorRGB(j,2) = imageGG(j);
colorRGB(j,3) = imageGB(j);
end
YIQ = rgb2ntsc([colorRGB(:,1) colorRGB(:,2) colorRGB(:,3)]);
I wrote this because the rgb2ntsc function needs mx3 matrix for input.I use the number 300 beacuse the picture is 300x300 pixels.I am going to seperate the picture in blocks in my project so dont give attention to the 300 number because i am going to change that, i put it just as an example.
thank you.

Answer 1

What you're doing is completely unnecessary. If you consult the documentation on rgb2ntsc, it also accepts a RGB image. Therefore, when you put in a RGB image, the output will be a 3 channel image, where the first channel is the luminance, or Y component and the second and third channels are the hue and saturation information (I and Q respectively). You don't need to decompose the image into a M x 3 matrix.
Therefore, simply do:
YIQ = rgb2ntsc(ImageRGB);
Make sure that ImageRGB is a RGB image where the first channel is red, second is green and third is blue.
Edit
With your comments, you want to take all of the pixels and place it into a M x 3 matrix where M is the total number of pixels. You would use this as input into rgb2ntsc. The function accepts a M x 3 matrix of RGB values where each row is a RGB tuple. The output in this case will be another M x 3 matrix where each row is its YIQ counterpart. Your code does do what you want it to do, but I would recommend that you do away with the for loops and replace it with:
colorRGB = reshape(permute(ImageRGB, [3 1 2]), 3, []).';,
After, do YIQ = rgb2ntsc(colorRGB);. colorRGB will already be a M x 3 matrix, so that column indexing you're doing is superfluous.
With the above using reshape and permute, it's very unnecessary to use the loops. In fact, I would argue that the for loop code is slower. Stick with the above code to get this done fast. Once you have your matrix in this fashion, then I suppose the code is doing what you want it to do.... however, I would personally just do a conversion on the image itself, then split it up into blocks or whatever you want to do after the fact.