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+=======================
+Cue lists and playbacks
+=======================
+
+Anatomy of a cue
+================
+
+Here is cue 4.3 representing a 4 second crossfade to a dim lighting wash::
+
+  (cue 4.3                     ;; Cue number 4.3
+    (crossfade 4               ;; Cross-fade in 4 seconds
+      (lighting-state          ;; To a lighting state...
+        (at washL washR 30)    ;; ... consisting of 'washL' and 'washR' at 30% intensity
+        (at washM 40))))       ;; ... and 'washM' at 40% intensity.
+
+Other types of transition are possible.  For example, the simplest transition
+of all is a "snap" (a hard, immediate change)::
+
+  (cue 4.3
+    (snap
+      (lighting-state
+        (at washL washR 30)
+        (at washM 40))
+
+The simplest transition effect is ``snap``, which produces a hard zero-time
+transition to the cue.  The usual one is ``crossfade``, which produces a smooth
+fade in the time you specify (in seconds).  It gives you a lot of control, for
+example, to fade intensities up in 4 seconds but down in 2 seconds::
+
+  (crossfade 4 2 state)
+
+To delay the up fade part by 2 seconds relative to everything else::
+
+  (crossfade 4 #:up-delay 2 state)
+
+To delay the down fade part, use ``#:down-delay``.  To control the fade times
+for non-intensity parameters, use ``#:attr-time`` and ``#:attr-delay``.
+Putting it all together for a complicated crossfade::
+
+  (crossfade 3 4
+             #:up-delay 2
+             #:attr-time 1
+             #:attr-delay 1.5
+             (lighting-state ...))
+
+You can write your own transition effects, if you want some other kind of
+transition.  Documentation pending, but look at snap-transition.scm and
+crossfade.scm for some examples.
+
+The lighting state does not have to feature a literal ``lighting-state`` form.
+Let's say you defined a state somewhere else::
+
+  (define my-state
+    (lighting-state
+      (at spotSR spotSL 50)
+      (at spotCS 100)))
+
+You can refer to that state in a cue like this::
+
+  (cue 3
+    (crossfade 3 5 my-state))
+
+You can also layer changes on top of the state, by using ``apply-state`` (which
+does exactly what its name suggests)::
+
+  (cue 3
+    (crossfade 3 5
+      (lighting-state
+        (apply-state my-state)
+        (at upstage-spot))))
+
+You can even make those changes conditional::
+
+  (define spot-needed #f)
+
+  (cue 3
+    (crossfade 3 5
+      (lighting-state
+        (apply-state my-state)
+        (when spot-needed
+          (at upstage-spot 100)))))
+
+
+Multi-part cues
+===============
+
+To make certain fixtures fade differently during the same cue, simply add a
+separate transition for each part.  Here's an example (from a real show)::
+
+  (cue 3
+
+    (crossfade 6 #:up-delay 14
+      (lighting-state
+        (at highsideL intensity 100.0)
+        (at highsideR intensity 100.0)
+        (at front-leds colour (cmy 0 93 80))
+        (at splitL splitR 70)
+        (at washL washR 100)
+        (at washM 50)))
+
+    (crossfade 3
+      (lighting-state
+        (at portrait-spot 100)))
+
+    (crossfade 3 #:up-delay 16
+      (lighting-state
+        (at front-leds 100))))
+
+In this example, the ``portrait-spot`` fades up first, in 3 seconds.  The main
+part of the scene fades up more slowly, in 6 seconds after a delay of 14
+seconds.  The ``front-leds`` (a group containing all of the front-light LED
+fixtures) fades up a further 2 seconds after that.  Note that the cue parts
+don't need to appear in chronological order.  However, the first cue part is
+"special", because it's the one *into* which the other parameters will track
+(see below).
+
+
+Cue lists
+==========
+
+A cue list is simply a list of cues.  For example::
+
+  (cue-list
+
+    (cue 0.5
+      ;; Tab warmers
+      (snap
+        (lighting-state
+          (at washL washR 30)
+          (at washM 40))))
+
+    (cue 0.8
+      (crossfade 6 blackout))
+
+    (cue 1
+      ;; Act 1, Scene 1
+      (crossfade 3
+        (lighting-state
+          (at front-wash 80)
+          (at moverL colour (cmy 21 0 0))
+          (at moverL 25))))
+
+    (cue 2
+      (crossfade 3 4   ;; Separate up/down fade times
+        (lighting-state
+          (at washM 100))))
+
+    (cue 2.5
+      (crossfade 2
+        (lighting-state
+          (apply-state home-state)
+          (at moverR 100)))))
+
+Just so you know, the cue list is represented internally as a Scheme *vector*,
+not a real list.
+
+
+Playback objects
+================
+
+The cue list doesn't do anything on its own.  To actually see the contents on
+the stage, it needs to be loaded into a playback.  In practice, the best way to
+work is to put the cue list in a file on its own and create the playback with
+a reference to that file::
+
+  (define pb
+    (make-playback
+      #:cue-list-file "shows/my-show.qlist.scm"
+      #:recovery-file "recovery.q"))
+
+The ``#:recovery-file`` is optional but highly recommended, discussed below.
+
+Once the playback has been created like this, if you change the cue list file
+then you can re-load it::
+
+  scheme@(guile-user)> (reload-cue-list! pb)
+  $8 = cue-list-reloaded
+
+If the modifications to the cue list file involved the currently active cue,
+the state shown on the stage will *not* be updated until you say so, with::
+
+  (reassert-current-cue! pb)
+
+The playback object shows useful information when printed::
+
+  scheme@(guile-user)> pb
+  $1 = #<<starlet-playback> state: ready current-cue: 43.0 next-cue: 44.0>
+
+For completeness, know that you can also create a playback like this::
+
+  (define my-cue-list
+    (cue-list
+       (cue ...)))
+
+  (define pb (make-playback #:cue-list my-cue-list))
+
+However, this makes it much harder to make subsequent changes to the cue list.
+
+
+Running cues
+============
+
+To rapidly jump (with a snap transition) to a cue, use ``cut-to-cue-number!``.
+To run a cue using the transition specified in the cue list, use
+``run-cue-number!``::
+
+  (cut-to-cue-number! pb 1)
+  (run-cue-number! pb 4)
+
+Calling ``go!`` will run the next cue in the cue list::
+
+  (go! pb)
+
+Playbacks also implement the other familiar operations:
+
+* ``(stop! pb)`` - immediately pause any running cue.  The next call to
+  ``go!`` will continue it.
+* ``(cut! pb)`` - run the next cue, using a snap transition regardless of what
+  the cue specifies.
+* ``(back! pb)`` - go backwards one step in the cue list, using a snap
+  transition.
+
+
+Tracking
+========
+
+By default, non-intensity parameters will "track" from one cue into the next
+cue.  That helps to avoid unexpected parameter changes, e.g. a moving light
+changing position while it dims.  If you run cues *out of order*, the result
+will be the same as if you'd run the cues *in order* from the start, to get to
+the cue you wanted.  If you're lucky enough to have never encountered a system
+that works any other way, just know that it works the way you'd expect it to
+work in a theatrical system.
+
+If you additionally want to track *intensities* into a cue, add
+``track-intensities`` as the first thing after the cue number::
+
+    (cue 1
+      (crossfade 3
+        (lighting-state
+          (at front-wash 80))))
+
+    (cue 2
+      track-intensities
+      (crossfade 3
+        (lighting-state
+          (at spotC 100))))
+
+In this example, cue 2 will include ``spotC`` at full intensity, **and**
+``front-wash`` at 80% intensity.
+
+
+Fixture presetting ("auto move while dark")
+===========================================
+
+Starlet tries as hard as it can to get non-intensity parameters into the right
+state before running a cue.  In other words, it makes a big effort to avoid the
+audience seeing moving lights actually move.  If a fixture's ``intensity``
+parameter is zero after running a cue, Starlet will set all its non-intensity
+parameters to the values in the next cue.  Of course, if a non-intensity
+parameter changes while the intensity is non-zero, the audience will see the
+move!
+
+
+The recovery file
+=================
+
+The purpose of the playback recovery file is to make a rapid recovery after a
+crash (not that there will be any, of course!).  If the file specified by the
+``#:recovery-file`` keyword argument to ``make-playback`` exists when the
+playback is created, the playback will immediately jump to the cue number in
+the file.  Whenever you run (or jump to) a cue, the cue number in the file will
+be updated.  If you don't use a recovery file, the playback will revert to cue
+zero on creation and you'll have to use ``cut-to-cue-number!``.  That will
+create a blackout of a few seconds while you figure out the right cue number to
+pick up from where things went wrong.