The SLIDE and Tcl/Tk Relationship

The SLIDE language uses Tcl as its scripting language. The SLIDE renderer, slide, is actually a Tcl wish program which has been augmented with real time 3D rendering capabilities using OpenGL. This relationship has many benefits.

• Tcl/Tk provides a system independent windowing environment windowing API which the slide viewer is built on. This makes the slide viewer highly portable to different architectures and operating systems.
• Tcl provides a robust scripting language interpreter which is the engine for SLIDE scene dynamics.
• Tk provides a rich set of GUI widgets which can be created directly from a SLIDE description file. This makes it possible for users to create application specific UI's for their scenes.
• The Tcl/C procedure interface enables a Tcl scripting interface to the SLIDE language parser which builds the scene graph. This makes it possible to create procedurally constructed models programmed in Tcl and called from with in the tclinit blocks of SLIDE.
• In the future, SLIDE will take advantage of the Tcl load command which allows the run time loading of separately compiled C/C++ shared object libraries. This will make it possible to augment the functionality of the slide viewer with user-created, compiled extensions without altering the core rendering engine.

Tcl Dynamics

There are four different ways to incorporate Tcl calls into a SLIDE scene description.

• Any floating point or flag value of SLIDE can be replaced by a Tcl macro expression enclosed by %{ <expr> }%. These expressions will be called once during intialization in an unspecified order in Tcl's global variable scope. The following is an example of a point on a circle:

  point p0 ( %{expr cos($SLF_PI/4)}% %{expr sin($SLF_PI/4)}% 0 ) endpoint

• Any floating point or flag value of SLIDE can be replaced by a Tcl expression enclosed by curly braces ( { } ). These expressions will be called once per frame in an unspecified order in Tcl's global variable scope. The following is an example of a point which travels in a circular path over time:

  point p0 ( {expr cos($SLF_TIME)} {expr sin($SLF_TIME)} 0 ) endpoint

  SLF_TIME is a Tcl variable whose value is updated by the slide renderer. There are a number of other variables that are maintained by the renderer:

  SLF_TIME		The run time seconds of the simulation
  SLF_FRAME		The number of the current frame counted from the last dynamics restart. This is useful for creating batch processed animations.
  SLF_tcltag__SCALE		The scale factor exported by a render statement's crystal ball interface
  SLF_tcltag__AXIS_X SLF_tcltag__AXIS_Y SLF_tcltag__AXIS_Z		The axis of rotation exported by a render statement's crystal ball interface
  SLF_tcltag__ANGLE		The rotation angle in degrees exported by a render statement's crystal ball interface

  Also, all of the SLIDE Enumerated Type values are exported as Tcl variables. For example, the value of SLF_GOURAUD can be referenced by $SLF_GOURAUD in the global variable scope.

• One time initialization Tcl code can be run in SLIDE tclinit { } blocks. This code will be run directly after parsing, before any windows are created. The user can write any Tcl they wish between the curly braces, and the string presented there will be evaluated by the Tcl interpreter as is. The user can even source Tcl files or load C/C++ shared libraries. This is where users should create all procedurally defined geometry, create all Tk widgets, and initialize any simulation state variables.
• Once per frame Tcl update code can be run in SLIDE tclupdate { } blocks. These blocks will be run at the beginning of each time slice in the order in which they were encountered while parsing. They will be run before any Tcl expression which are replacing static SLIDE values.

Dynamic Expressions

Embedded TCL

Dynamic Control Statements

These top level statements allow you to specify arbitrary TCL code that will be run at a particular point in the rendering program. As with other TCL statements, the tcl_code blocks should be enclosed in curly braces {}; the braces will be stripped off before the code is sent to the TCL interpreter.

The tclinit statement adds a block of TCL code to be run when the SLIDE file is first read in. All of the tclinit blocks will be run in the order in which they were encountered in the file. They will be run exactly once. This is a good place to define procedures and create widgets.

The tclupdate statement adds a block of TCL code to be run at the beginning of each update to the scene graph just before rendering. All of the tclupdate blocks will be run in the order in which they were encountered in the file. They will be run once per update. This is a good place to assign values to global variables that will be used by many elements in your SLIDE scene.

SLIDE API

The SLIDE API allows the user to dynamically interact with SLIDE through TCL commands.
The API allows SLIDE objects to be created and deleted as follows:

slide  create  type   id   [parameters] slide  delete  type  id

For specific parameter syntax see the SLIDE Specification page.

Example:

tclinit { slide create sphere Ball -radius 3 slide create point p1 { 0 2 1 } slide delete sphere Ball slide delete point p1 slide create sphere Ball -radius 6 slide create point p1 { 1 0 0 } }