When volume rendering with ray casting (known also as ray tracing), the system traces rays from the viewpoint through each pixel of the image and determines what cells each ray intersects. If we desire global effects such as shadows or scattering, then we can trace also other rays from cell intersections to light sources or other cells (although computational cost quickly becomes prohibitive). Because the rendering system, in its outer loop, iterates over all the pixels in the output image, ray casting is known also as image-order rendering.