This opens the file once (overwriting if it already exists) and writes your results one line at a time.
fid = fopen( 'results.txt', 'wt' );
for image = 1:N
[a1,a2,a3,a4] = ProcessMyImage( image );
fprintf( fid, '%f,%f,%f,%f\n', a1, a2, a3, a4);
end
fclose(fid);
Alternatively, you could open the file in 'append' mode, write the line, and close it again each time through the loop.
doc fopen
