Within this trace function the possible parallelization spot points would be here : { else { // it's a diffuse object, no need to raytrace any further for (unsigned i = 0; i < spheres.size(); ++i) { if (spheres[i].emissionColor.x > 0) { // this is a light Vec3f transmission = 1; Vec3f lightDirection = spheres[i].center - phit; lightDirection.normalize(); for (unsigned j = 0; j < spheres.size(); ++j) { if (i != j) { float t0, t1; if (spheres[j].intersect(phit + nhit * bias, lightDirection, t0, t1)) { transmission = 0; break; } } } surfaceColor += sphere->surfaceColor * transmission * std::max(float(0), nhit.dot(lightDirection)) * spheres[i].emissionColor; } } } return surfaceColor + sphere->emissionColor;}