Here I wrote a function to save image/matrix into PPM format. I tried to simplify the saving process by using the matlab function ‘dlmwrite’ such that I only need to write the header for this PPM format. However, before streaming the image data, different channels of the input image must be interleaved. I experimented two ways to do this. Both work just fine, and the use of matlab function ‘cat’ (or ‘permute’) make the implementation very simple.

function write_ppm(im, fname, xval)

[height, width, c] = size(im);
assert(c == 3);

fid = fopen( fname, 'w' );

%% write headers
fprintf( fid, 'P3\n' );
fprintf( fid, '%d %d\n', width, height);
fprintf( fid, '%d \n', xval); %maximum values

fclose( fid ); 

%% interleave image channels before streaming
c1 = im(:, :, 1)';
c2 = im(:, :, 2)';
c3 = im(:, :, 3)';
im1 = cat(2, c1(:), c2(:), c3(:));

%% data streaming, could be slow if the image is large
dlmwrite(fname, int32(im1), '-append', 'delimiter', '\n')

The implementation in Python is even simpler. ‘f.write()’ is capable of writing the entire image in one function call with the help of ‘\n’.join().

def write_ppm(fname, im, xval):
    
    height, width, nc = im.shape
    assert nc == 3
    
    f = open(fname, 'w')
    
    f.write('P3\n')
    f.write(str(width)+' '+str(height)+'\n')
    f.write(str(xval)+'\n')
    
    # interleave image channels before streaming    
    c1 = np.reshape(im[:, :, 0], (width*height, 1))
    c2 = np.reshape(im[:, :, 1], (width*height, 1))
    c3 = np.reshape(im[:, :, 2], (width*height, 1))
    
    im1 = np.hstack([c1, c2, c3])
    im2 = im1.reshape(width*height*3)

    f.write('\n'.join(im2.astype('str')))
    f.write('\n')

    f.close()