Overlays

For overlays, you must use both the Øsemlayer and øsemcolor style options (see page [*]). Overlays have two purposes. First, you can use overlays to gradually add layers of information on a slide during a presentation. Second, you can use overlays to make color layers; each layer is printed in black-on-white on paper, and then you use some kind of color copying service to put each layer on a transparency in a different color. This is not nearly as nice as having a color printer or using a color printing service, but it's the next best thing. "seminar.sty" lets you make both types of overlays (and the main slide) from a single slide environment. The commandYou can also write
\begin{Ex} ''\begin{overlay}{<n>}'' $\ldots$\ ''\end{overlay}'' \end{Ex}

\begin{MD} \overlay{n} \end{MD}
puts whatever is in its scope (TEX group) on overlay n, where n = 0,…, 9. Actually, overlay 0 refers to the main slide, but you might use "0" because overlay commands can be nested. As implemented by the "semcolor" style option, these overlay commands can be used just about anywhere, including in math mode, tables, and around an included graphics file (if it is a conforming EPS file, at least). Also, it is all right to use non-consecutive overlay numbers. For color layers, you have to start by defining some colors using the command
\begin{MD} \colorlayers{colors} \end{MD}
<colors> should be a comma separated list of color names, without spaces, as in 
  \colorlayers{red,green,blue}
Then you can use the command "" just like the command "<n>"; everything in its scope goes on a red color layer. The "" command obeys the usual rules on scope. You can use this command any time, including in a slide environment. The command is cumulative, meaning that previously defined color layers continue to exist. The command was purposely defined so that it does not complain when a color name is already defined; this makes it easier to switch from some other color system to a layer system. However, you should be careful not to inadvertently redefine some command that you need. Fortunately, there are no TEX primitives whose names are the names of colors. When you print out the slides, the main slide is printed, followed by each of the color layers (if any) for the main slide. Then each of the overlays is printed, together with each of its color layers. Only overlays or color layers that are actually used (i.e., that are not empty) are printed. You can turn overlays and color layers on and off with the commands
\begin{MD} \overlaystrue\\ \overlaysfalse\\ \layerstrue\\ \layersfalse \end{MD}
These commands can be used at any time, and they obey the usual scoping rules. The default is for overlays to be active in the "slides" format and suppressed in the "article" format. The counter overlay keeps track of the overlays. The default definition of is: 
  \theslide-\alph{overlay}
Overlays can be cross-referenced. The command is set to the name of the current color layer. makes a label for layers; its default definition is: 
  \theoverlay-\currlayer
Color layers cannot be cross-referenced. For example, if slide 7 has overlays 1 and 2 and colors "red" and "green", then the main slide is numbered 7, followed by layers 7-red and 7-green, followed by overlay 7-a, followed by layers 7-a-red and 7-a-green, followed by overlay 7-b, followed by layers 7-b-red and 7-b-green. The caption used in the slide styles is for overlays and for layers. The defaults are, respectively, 
  \bf Overlay \theoverlay
  \bf Layer \thelayer
By default the overlays and layers use the same page styles in the "slides" format as their ``owner.'' You can specify special page styles with the commands:
\begin{MD} \overlaypagestyle{style}\\ \layerpagestyle{style} \end{MD}
Finally, by default the overlays and layers use the same frame style as their ``owner.'' You can specify special frame styles with the commands:
\begin{MD} \overlayframe[commands]{style}\\ \layerframe[commands]{style} \end{MD}