MATLAB: Bitmap to RGB array

Question

I have written the following code to input a bitmap image and output a '.ppm' file. For this I first read the rgb values and then output them to a ppm file. I manually add P3 133 100 %dimensions of the image 255 to the top of the .ppm file. Still, .ppm file is incorrect. Do, I need to do anything else to extract rgb values in an array from bitmap image.

I=imread('butterfly.bmp','bmp');
[n,m]=size(I);   %n=100; m=133
fName = 'test.ppm';
fid = fopen(fName,'w'); 
for i=1:1:n
    for j=1:1:m
        fprintf(fid,'%3d ',I(i,j));
        if(mod(j,3) == 0)
            fprintf(fid,' ');
        end
    end  if true
    % code
  end
    fprintf(fid,'\n');
end

Answer 1

I don't know why you want ppm in the first place. What's wrong with the original bmp or an increasingly more common PNG? Anyway, if you need ppm, why not just use imwrite, which can write that format?

imwrite(I, 'butterfly.ppm');