Once the clean-ups and in-between drawings for a sequence are completed, they are prepared for photography, a process known as ink-and-paint. Each drawing is then transferred from paper to a thin, clear sheet of plastic called a cel, a contraction of the material name celluloid (the original flammable cellulose nitrate was later replaced with the more stable cellulose acetate). The outline of the drawing is inked or photocopied onto the cel, and gouache, acrylic or a similar type of paint is used on the reverse sides of the cels to add colors in the appropriate shades. In many cases, characters will have more than one color palette assigned to them; the usage of each one depends upon the mood and lighting of each scene. The transparent quality of the cel allows for each character or object in a frame to be animated on different cels, as the cel of one character can be seen underneath the cel of another; and the opaque background will be seen beneath all of the cels.
When an entire sequence has been transferred to cels, the photography process begins. Each cel involved in a frame of a sequence is laid on top of each other, with the background at the bottom of the stack. A piece of glass is lowered onto the artwork in order to flatten any irregularities, and the composite image is then photographed by a special animation camera, also called rostrum camera. The cels are removed, and the process repeats for the next frame until each frame in the sequence has been photographed. Each cel has registration holes, small holes along the top or bottom edge of the cel, which allow the cel to be placed on corresponding peg bars before the camera to ensure that each cel aligns with the one before it; if the cels are not aligned in such a manner, the animation, when played at full speed, will appear "jittery." Sometimes, frames may need to be photographed more than once, in order to implement superimpositions and other camera effects. Pans are created by either moving the cels or backgrounds one step at a time over a succession of frames (the camera does not pan; it only zooms in and out).