Precreated cel components are very convenient, and allow artists full use of artistic tools to create cel contents, but almost all cels must have at least one component changed or created by the program on the fly-typically the CCB, which must be changed to simply move a cel across the screen or to change the size of the cel. Other cel components may be changed or created within Portfolio as well. The typical example is a cel whose pixels come from an existing bitmap. The cel's preamble is created by the program; the preamble specifies a rectangle within the bitmap as the source data. The cel engine projects the pixels within the rectangle as the cel pixels. The program can change the preamble between cel projection calls to specify new sections of the bitmap for projection.