Generate color bitmap

mat spec_t; // after doing stft, super spectrogram... size: spec_t(1025,2002);

mat mag_spec_f = db(abs(spec_t)); // where db method: m = 10 * log10(m);
cube c(mag_spec_t.size(), mag_spec_t.size(), 3);
c.slice(0) = mag_spec_t; // error here
c.slice(1) = 256 - abs (256 - 2 * mag_spec_t);
c.slice(2) = 256 - mag_spec_t;
c.save ("mag_spec_t.ppm", ppm_binary);

//Error:
// error: Mat::init(): size is fixed and hence cannot be changed

//libc++abi.dylib: terminating with uncaught exception of type std::logic_error: Mat::init(): size is fixed and hence cannot be changed