7   Opticals

Fade

The term "optical" is used to cover the commonly used transistion effects: fade in, fade out, dissolve (also called "mix").

The simplest optical is a fade in or out. In the first example, we dope a simple fade out. In this example, a half second fade out is indicated. (Note that the correct use of the word "fade" means that the entire image goes to or comes from black. If any part of the image remains, you will need to do a dissolve, of which see more below).

Note: the last "arm" cel is ringed in order to draw the compositor's eye to the fact that this is a repeat use of Arm 1  It is a good idea to ring any cel where something happens that may catch a compositor out. For example, you might have a sequence of cels A1,A2,A3.......etc. Suppose that for some reason A23 is not used. Then it would be a good idea to ring A24 to draw attention to the skipped drawing. A note can also be made in the right margin of the dopesheet e.g. "No A23".

Dissolve or Mix

A dissolve is really two fades on top of one another - one in and one out, hence the crossing red lines on the dopesheet below. Although in digital technology the dissolve is actually achieved by the software simply making an image more or less transparent, I recommend sticking to the old convention: regard a dissolve as being two cross fades; it will make more complex opticals easier to deal with.

Here, through the magic that is animation, a man's hat fades off in half a second

The sloping line under the H1 cel draws attention to its change in opacity.

Although the hat is fading off, the correct way to show this optical is as a mix (dissolve), hence the red cross shape.
This shape represents a mix (or fade) off:  
...and this of course represents a fade in or mix in:   

We are mixing off the man's hat and mixing on a blank cel.

(Although blank cels no longer exist as real things, again I recommend sticking to the traditional method and pretending that there is a blank cel - I promise you that the end result will be a greater clarity)

Since a blank cel is invisible, the nett effect is that the hat becomes progressively more transparent over the 12 frames until it has disappeared.

Finally, an example of a mix where one object replaces another. The hat artwork is held as it mixes off and the pineapple is animating as it mixes on, then holds on PA2. Hopefully, the rest is self-explanatory:

"In camera" opticals are virtually unique to animation. In the early days of live action cinema, opticals were all done in the camera. When film laboratories got more sophisticated, opticals would be done there as they were far easier to control. However, straightforward opticals were always done in the animation camera. You can of course have opticals done after the animation process in your post-production house, but it simply adds more work to the post-production process. The upshot is that most straightforward effects and opticals are still done "in-camera" (in software) in animation. 

Conclusion

The few simple methods shown in the pages here cover the basics of writing a dopesheet. For any special effects or anything else out of the ordinary, use the right hand column of the dopesheet to make notes about what is required; wherever anything can be easily explained with a drawing, do it that way and use the dopesheet to indicate the presence of the drawn guide.
Identify all artwork with the production details scene number etc. so that if the drawing goes astray, its function can be known.
Again, I would like to emphasise that nothing you see here is a cast-iron rule; you will come across countless variations - the most important single rule is:

BE CLEAR !