The Assistant Performer, which is started from Moritz’ opening dialog, can perform a score alone or under the control of a live performer. Various options can be set before beginning to perform.

An Assistant Performer dialog associated with the Study 2c3.1 score might, for example, look like this:

The popup menu at the top contains a sorted list of Moritz Performance Options (.mpox) files associated with the current score. Each .mpox file contains a particular set of performance options applicable to the current score. Each score can be associated with any number of .mpox files (which are saved in the same folder as the original score file). This popup is populated automatically from the Windows file system when the dialog is loaded, and whenever an .mpox file is created or deleted (using the appropriate button at the bottom of this part of the dialog). The popup’s width is dynamically allocated, depending on the width of the file names it has to accommodate. If no .mpox file currently exists for this score, a new one is created containing default options and having a file name ending ‘.1.mpox’. Any edits made in the dialog are immediately saved to the .mpox file on disk. The file does not have to be saved explicitly.
The selection panel below the pop-up is populated with the list of staff/voice names from the score (in this case from Study 2c3.1). Each voice corresponds to a MIDI channel (Moritz currently supports up to 16 voices corresponding to the standard 16 MIDI channels, and up to two voices per staff.) After selecting one or more channel names in the list, they can be allocated to the Assistant (black), Performer (blue), or neither (red) using the buttons below the list.
New .mpox file: Creates a new set of options for this score, copying the current set by default. Adds the new .mpox file name to the list of files in the .mpox file selector at the top of the dialog.
Delete .mpox file: Deletes the current .mpox file, after getting confirmation from the user. Any of the available .mpox files can be deleted. When the last one is deleted, it is immediately replaced by a new one whose name ends with ‘.1.mpox’. (The user is told what is going on, of course.)

The first input field is used to set the moment number at which the performance starts. The number is restricted to the number of performer’s moments, calculated using the options above and the score.
If this is an assistant-only performance, the number of assistant’s moments is used.
If the Repeat? checkbox is checked, the performance will repeat (from the beginning of the score).
If the Save MIDI file? checkbox is checked, the performance will be recorded, and can be saved as a Standard MIDI file.
The Minimum ornament chord duration field can be used to raise this threshold from its composed value.
(See the help text on the dialog.)

These radio buttons are self explanatory. The selected option is highlighted in blue.
All these options are disabled if none of the score’s staves is set to be performed live.

Clicking the Performer’s Keyboard Options button opens the Performer’s keyboard settings dialog (see below).
Currently (July 2012), The Performer’s Keyboard Options have no effect. They are supposed to be controlled by special instructions written into the score, but these do not currently exist. The dialog has, however, been kept for future use. This option is disabled if none of the score’s staves is set to be performed live.

The radio buttons are self explanatory. The selected option is highlighted in blue.
Selecting Relative Symbols allows a live performer to influence the assistant's speed of execution.
These options are disabled if none of the score’s staves is set to be played by the assistant.

When either of the two buttons on the right is clicked, the other buttons are disabled, and they are replaced by a large Stop button as in the following screen-shot. If there is no live performer, the performance begins immediately, otherwise it waits for the performer. During a performance, the two counters at the top of this section show the current time. The moment position is the default position of the current performer’s moment in milliseconds from the beginning of the performance.
(The message on the stop button depends on which start button was clicked.)

This dialog appears when the Performer’s Keyboard Options button is clicked. The keyboard settings are stored in the current Moritz Performance Options (.mpox) file, but they currently have no effect. They have been kept here for future use.

Originally, the particular keyboard in use in the score was set by attaching a Moritz keyboard number control text to any chord in the score. When the Assistant Performer saw the control text, it switched keyboards.
Control texts were invented as a way of augmenting the data in .capXML (standard notation) files, but I am now using SVG and non-standard notation, and most such control texts have become redundant. Wanting to start with a clean sheet, I have temporarily deleted them completely from Moritz’ code-base.
When visible control texts are re-introduced, the number and meanings of the key types may change. Also, continuous MIDI control messages (expression, volume etc.) coming from homophonic MIDI controllers such as AKAI’s EWI need to be dealt with more fully...

By default, the dialog appears in its full width (128 keys) with one default keyboard :

Middle-C is marked with a small blue notch. The top and bottom of a standard piano keyboard are marked with small red notches. The top and bottom of my 49-key EMU XBoard49 are marked with small magenta notches.
Individual key types can be edited using the mouse and dialog buttons (see the help text).

Key types:

Black-and-White: The Assistant hears the key. The key’s MIDI events are sent to the Assistant Performer, using the Performer’s Options. This is the default setting.
Green : Solo. The Assistant does not hear the key. The key’s MIDI events are all sent to the MIDI output device, using the Performer’s Options. This is useful, for example to allow the performer to trill or tremolo, or to perform some improvisation.
Blue : Solo, but the assistant hears the MIDI NoteOn. The key’s MIDI events are all sent to the MIDI output device, using the Performer’s Options. This is a variation of the Green option. Useful, for example to allow the performer to trill or tremolo, or to perform some improvisation.
Red : A dead key. The Assistant does not hear the key, and Moritz swallows the event, sending nothing to the MIDI output device. Useful for constructing special glissandi etc.

Keyboards can be added and deleted using the buttons provided. If the list contains more than 7 keyboards, the dialog's height is fixed and the list becomes scrollable.

For convenience, the dialog exists in two widths: 128 standard MIDI keys (as above), and the width of a standard piano (as below). Here is how the dialog looks in “Piano width”, with a second keyboard containing edited key types: