In computer graphics, image order algorithms iterate over the pixels in the image to be produced, rather than the elements in the scene to be rendered. For typical rendering applications, the scene contains many fewer elements (for example, geometric primitive) than the image (pixels). In those cases, object order algorithms (below) are usually most efficient. But when the scene complexity exceeds that of the image, such as is the case often in volume rendering, then image order algorithms (for example, shear warp) will usually be more efficient.
Object order algorithms are those that iterate over the elements in the scene to be rendered, rather than the pixels in the image to be produced. For typical rendering applications, the scene contains many fewer elements (for example, geometric primitives) than the image pixels). In those cases, object order algorithms are usually most efficient.