Page  377 ï~~Capella: A Graphical Interface for Algorithmic Composition Heinrich Taube Tobias Kunze Zentrum fir Kunst und Medientechnologie, Karlsruhe CCRMA, Stanford University ABSTRACT: Capella is an object-oriented graphical interface for algorithmic composition in Common Music. It defines classes of browsers and worksheets that implement a consistent set of visualization tools and serve as a graphical front end for the system. The interface currently runs on the Macintosh under Macintosh Common Lisp. Introduction Algorithmic composition is a complex activity in which both musical and technological issues must be addressed in parallel. This, in turn, places special requirements on a graphical interface that supports the process. Object-oriented composition environments such as Common Music,' DMix,2 and Mode3 place additional demands on graphical tools due to the breadth of representation and functionality that these kind of systems implement. Smalltalk environments are able to take advantage of a powerful windowing system provided by Smalltalk itself. Since Common Music was designed to be as portable as possible, without the aid of a native windowing system, almost no attempt to address visualization issues was made until recently. Until now, visual output in Common Music was completely text-based, similar to the type of display one sees when working, for example, in a Unix shell window. Common Music's command-line driven interpreter, Stella, connects to the system's toolbox similar to the manner in which a shell connects to Unix. Although it allows powerful input expressions to be formulated, Stella does not allow the inner processes to be easily understood. Capella is a response to some of the communication limitations in Stella, while keeping in mind that graphic representation and mouse based gestures are not always the best or most expedient models to choose for interacting with a complex system. Capella has been designed to be a complement, not a replacement, for the two other modes of interaction supported by the system: command processing from Stella and procedure invocation from Lisp. Common Music simply runs all three modes "in parallel" (Figure 1) and the composer is free to choose whatever is most appropriate to a particular situation. Capella is still in the early stages of development. Its primary goal is to allow a set of flexible visualization tools to be developed, but it also makes interacting with the system as a whole easier and more transparent. The need for transparency is particularly acute in algorithmic composition workshops, where participants must quickly absorb not just new theoretical concepts, but a specific implementation of them as well. Browsers and Worksheets Capella provides two basic classes of windows: browsers and worksheets. Browsers provide context sensitive views on musical objects and permit the associated data to be inspected and manipulated in some class specific manner. A basic premise behind the design of Capella is that there is not a single best way to view objects in the system, but rather, that their visualization depends upon context. Browsers in Capella are therefore not identical to the musical structures they display. Worksheets are windows that support compositional activities such as analysis gathering, musical event editing, musical structure definition, and score output processing. Each class of worksheets provides methods for a generic protocol controlling the creation, enabling, activation, and updating of worksheets and their subviews. Any number of browsers and worksheets may be open on the screen at the same time but only one browser and worksheet are said to be "active" at any given time. The active browser is called the "focus browser" and typically provides selection constraints for the active worksheet. The system currently defines classes of worksheets for data cf. Taube (1994) 2 cf. Oppenheim (1993) Scf. Pope (1992) I C MC P RO C EE D IN G S 199537 377

Page  378 ï~~\ Capella Stella Common Music \ "\\\ \\ \ GUI Interpreter Lisp Package \: ---------------- I. Score Description Toolboox Services " Item Streams " Scheduling \\\\ " Classes " API \\ ",,, " Utilities \\,......................................................... \\. Common Music Composition Environment R, EIR slot readout-fa reader-.-rage- steps ----lip-!418 Ix time 1.0 --- IL I i - - - FS5. note note e' ee 0 -127 - 111length duration 0 -zNI I Ightn. +-non -. ' 0 -166553, 128 - ><}-lik color channel 1234_56_ 70_ I8 <) tO 11213 1415 16! Otoacoground background-fn: oano-staves colors/mute: 12 x 24 1 - - ----- --. Y.. Y-., Â~ Â~ bpm - -.:.... -- -._"-".-.. '. -- set il esn eriT L o+ 1.+ ++: f t.;+.i+i S.+ + IA.! t!a?i < -IIl ~ Figure 1: Capella serves as a graphic "'front Figure2: A SEE browserwith its control pane expanded. Four of the five end" to Common Music. This illustration de- possible dimensions are depicted for a set of Midi-Note events. The xpicts the three parallel interaction modes possi- axis has been set to time, y-axis to pitch, color to channel, and length to ble in Common Music: gesture (Capella), com- duration. The brightness dimension was not used. The browser's value mand (Stella) and procedure (Lisp). access functions take care of mapping back and forth between symbolic, floating point or integer pitch representations. editing, creation and deletion of musical objects, query processing, global preference setting, loading and saving archives, and score processing. Several of the browsers and worksheets will be discussed in more detail in the remaining sections of this paper. Information Browser The information browser provides class specific summary information about an object (its type, status, parents, number of subobjects, and so forth) and a table for displaying and editing the current slot values in the object. This table distinguishes between internal, external and "read only" slots. Normally, only external slots are included, and only "writable" slots have their input buffer active for editing. An "inspect" mode overrides this default behavior and gives full display and editing access to all the slots in the object. Listing Browser The listing browser (Figure 3) displays a containing object together with its subobjects. Subobjects are displayed by a printer with class specific methods. Subobjects that are "events" include parameter information in their output displays, so when a listing browser displays event data it provides formatted views for each parameterized sound event. Listings can be used in conjunction with Edit worksheets for generic sequence editing. Operations on the Edit worksheet apply to the current selection in the focus listing browser. Subobjects in a listing may be selected by mouse gesture, by iterative index referencing, and by the application of musically salient predicate selection expressions. SEE Browser The SEE (Structured Event Editor) browser (Figure 2) is a visualization and analysis tool that operates on sequences of parametric, ordered data objects. These sequences may be structures declared in the system or be "virtual" sequences of merged output generated from multiple objects by the scheduler. The SEE browser provides a programmable graphics output window that supports a number of user customization and display hooks. To use a SEE browser, the composer assigns a maximum of five "data parameters" to five dimensions of visualization supported by the browser. The first two dimensions determine the element's location on a Cartesian pane (representing its x- and y-axis, respectively). The third dimension is drawn as an element's "length" in the same direction as either the x- or y-axis. Brightness is used to represent a fourth dimension, and color (ie. hue and saturation) provides a fifth dimension. None of these dimensions are required to participate in the drawing process or be used to display a specific type of data. 378 I7 CMC PROCEEDINGS 1995

Page  379 ï~~Note Amplitude Start ~ ie (motes (steps -2 initially-from 'c6 returning note) g I: \, b3 14 95 e16 Channel in rotation change (changes width (1 2))) Lengh 01:kill 13) End \ (mot (member note '(b3 f4 g5 e16))) Add Form it accented 2 1) Figure 4: Algorithm browers hold two code editors, one for initialization statements, the other for runtime statements. Since the object's initialization values for rhythm and duration in this example are not updated in the run time code, they act as constant values for the algorithmic process. The runtime expression for the note parameter executes 13 periods of a pattern and then terminates the process. The amplitude for each Midi-Note event depends on the particular note value chosen. The system automatically maps symbolic and floating point data to integer equivalents suitable for MIDI. Figure 3: Listing browsers and Edit worksheets used together support generalized sequence editing. Edit actions apply to the currently selected (highlighted) objects in the active browser. The objects in this example were selected by applying a musically salient selection expression to the set of subobjects. Value statements in the Edit worksheet may involve Lisp expressions in addition to constant values. If the OK button were pressed, the selected events (events whose frequencies fell between 200Hz and 880Hz) would have their duration scaled by a value between 1.2 and 1.5 and their amplitude scaled as a function of their frequency. Algorithm Browser The algorithm browser (Figure 4) is a structured editor for displaying and modifying program code associated with musical algorithm and generator objects. The browser maintains two similar but completely independent editors for the object's initialization and "run time" statements. The editors are separated by a divider; moving the divider up or down controls the percentage of space allocated to each editor in the browser. Each editor manages its program statements using a control menu and an internal code table. Statements are usually indexed in the menu under the name of the parameter they effect. Selecting a statement from the control menu installs it in the associated code table. Once a statement is installed in the table its index in the control menu is "greyed out" and cannot be reselected until the statement is deleted or deinstalled from the table. The control menu can hold alternate expressions affecting the same parameter. This allows a composer to easily compare or test different expressions affecting the same parameter. Code tables permit their structure as well as their contents to be easily modified. Statements can be moved in the table simply by control-mouse-dragging them to a new position. C ode tables also perform automatic syntax checks on their input expressions and will "pretty print" their contents upon request. Once code has been developed, the browser can either redefine the algorithm object associated with it, or else "decompile" the code tables into a Lisp expression that, if evaluated, would redefine the algorithm object. This Lisp expression is automatically dumped to a new window containing a Lisp editing buffer. Output Browser Output browsers permit mixtures of sequential and simultaneous arrangements of object references (layouts) to be IC MC P R O C E E D I N G S 199537 379

Page  380 ï~~- ------ Midi Port A home:test.eps\, CLMI CMN Container t1 Stella File Figure 5: Layout contents are depicted and edited in the graphic pane at the bottom of the browser; the selected stream may be customized by double-clicking its name. Once a layout and stream have been selected or created from their "Select" menus, output processing is enabled. Pressing return then activates output from the current layout to the selected output stream. "layed out" and then processed to a specific output stream (Figure 5). The Stream and Layout panes allow the user to create, load, save and select layouts and streams from a menu. Once a particular layout has been selected it is graphically depicted in the layout pane itself. A Layout is defined in terms of one or more "object references". Each reference may represent a single object (for example an algorithm, thread or merge), or a subset (chunk) of subobjects from some containing object. A reference is graphically depicted as a box (block) in the layout pane. Blocks are mouse-draggable and self-adjust to the width of their textual content. If a block is moved it "snaps" to its horizontal and vertical neighbors after it has been released. All actions-the creation of new object references, duplication of exisiting ones, moving, selection, and changing the content-may be performed directly on the layout pane using standard command key combinations. Once the layout contains one or more references and an output stream has been selected, the Return key may be used to activate output processing. Layout processing moves from left to right across the pane: objects within a column are scheduled relative to one another and movement across columns represents "sectional" (non overlapping) divisions. Within a column, timeshifts relative to other objects may be specified using the @<time> qualifer, and block repetition is possible using the repeat qualifier. Conclusion Although Capella is a working, functional interface, the project is still in its early stages and much work remains to be done. Short term goals include developing new classes of browsers for musical pattern display and statistical analysis. Intermediate goals include insuring the modularity of native windowing code, and isolating as much graphic behavior as possible in methods on generic windowing operators. Long term goals include porting Capella to other windowing systems and at least one public domain Lisp implementation. The first port will most likely involve X-Windows on the Silicon Graphics line of machines. References - [Taube 1994] Taube, Heinrich K. "Stella: Persistent Score Editing in Common Music," Computer Music Journal Vol. 17:4, Cambridge, Massachussetts: MIT Press. -[Oppenheim 1993] Oppenheim, Daniel. "DMLX-A Multi Faceted Environment for Composing and Performing Computer Music; its Design, Philosophy, and Implementation," Proceedings for the SEAMUS Conference, Austin, Texas. -[Pope 1992] Pope, Steven T. "The Interim DynaPiano: An Integrated Tool and Instrument for Composers," Computer Music Journal Vol. 16:3, Cambridge, Massachussetts: MIT Press. 380 I C MC PROCEEDINGS 1995