When volume rendering with cell projection, the system traverses the list of cells and projects each one onto the viewing plane. Once the cell's projection is determined, the renderer fills the pixels covering that part of the viewing plane, a process we call rasterization. Because the rendering system, in its outer loop, iterates over all the objects, cell projection is known also as object-order rendering.
The major advantage cell projection has over ray casting is that every viewing-ray intersection with a cell is computed at once when the cell is projected, making it easy to take advantage of coherency among viewing rays without the overhead of a sweep algorithm. Furthermore, because commodity graphics hardware is also an object-order rendering system, it is straightforward to implement the cell projection algorithms on graphics hardware, making them faster than CPU-bound algorithms. The major disadvantage of cell projection is its reliance on proper visibility ordering of the cells. Therefore, in addition to reviewing the most popular cell projection algorithms, I review common cell sorting algorithms.