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()