In this stage of the graphics pipeline, the grid points are also called fragments, for the sake of greater distinctiveness. Each fragment corresponds to one pixel in the frame buffer and this corresponds to one pixel of the screen. These can be colored (and possibly illuminated). Furthermore, it is necessary to determine the visible, closer to the observer fragment, in the case of overlapping polygons. A Z-buffer is usually used for this so-called hidden surface determination. The color of a fragment depends on the illumination, texture, and other material properties of the visible primitive and is often interpolated using the triangle vertice properties. Where available, a fragment shader (also called Pixel Shader) is run in the rastering step for each fragment of the object. If a fragment is visible, it can now be mixed with already existing color values in the image if transparency or multi-sampling is used. In this step, one or more fragments become a pixel.