Graphics / Postprocessing - Overviev

The following technical summary of the graphics / postprocessing package introduces various features of the package, describes in more detail some of the entities that make the package especially useful for interactive 3D visualization, describes the particular devices supported, and gives a brief overview of the internal structure of the package.

Examples

The following examples are included to show some of the capabilities of the PHLEX postprocessor. See also other examples included in this web site with the specific solver packages.All images are shown in separate window

Solution visualization (60Kb)
Error estimation (31Kb)
HP mesh and manual adaptation (35Kb)
Solution on a slice (75Kb)
Solution probe (30Kb)
Loads and boundary conditions (83Kb)
Deformed configuration (111Kb - stereo image; requires red-blue glasses)
XY plot (21Kb)
Contact surface (specialized capability) (40Kb)
Selective plots - by bounding box (35Kb)

Postprocessor Interfacing

To an application developer, the PHLEX postprocessor appears not as a library but rather as a finished package, with a few simple interface functions required to provide data to the package.

For the purpose of user interaction, the developer has to define the names of all the components of the solution (stresses, deflections, temperature, etc.), and provide a single function, which computes the derived components based on primary solution components (for example, in CFD the primary components could be: the density, energy, and velocities, while the derived components are the Mach number or temperature. For solid mechanics problems they are usually respectively: deformation and stresses). These equations can also be defined at runtime as interpreted formulas.
After defining the components, the initialization consists of one single call with no parameters, preceded optionally by calls to customization functions (their purpose is to enable or disable specialized functionality of the postprocessor, eg. particle traces or velocity vectors).
In addition, developers can create their own forms and drawing modules, or add other objects to the postprocessor. Some examples include: well properties for an oil reservoir simulation package, contact surfaces for a nonlinear structural mechanics code, or special element types. This capability is very useful for creating various ad hoc forms used to visualize internal structures in the database (hp-constraints, enrichment patterns, etc)

This interface was designed to be as general as possible so that the postprocessing package remains an independent library and can be immediately included in other numerical programs. All functions of the preprocessor are controlled from various GUI forms, but it is also possible to control postprocessor from simple Tcl scripts, which can be used e.g. to prepare 'canned' demo runs, or for automatic generation of series of images for reports or animations.

D3: graphics library

PHLEX postprocessor is built on top of our proprietary graphics library. The entire package is called d3, and it consists of following modules:

graphics module, which contains routines to define, manipulate and display graphical entities,
device drives, interfacing graphics module with specific system dependent drawing routines (see Devices, below)
graphics widget module, provides routines to build and define form widgets, and to define and implement particular mouse actions (such as zoom, tumble, and trackball). This module is also considered part of the GUI, or, Graphical User Interface. To avoid operating system dependencies, this module is built on top of the Tcl/Tk package

D3 library is fully platform independent (compiles cleanly - out of the box on Windows NT/95 platforms and various flavors of Unix). It can be used as a completely independent package, useful in building various graphics utilities, and also (by utilizing only GUI module) to build various small utilities equipped with a graphical user interface utilizing Widgets and Forms. Altair has built a small preprocessor (FEM mesh generator) QM (quad-mesh), allowing end user to prepare interactively 2D and 2.5D meshes for PHLEX applications, as well as a specialized preprocessor for Oil reservoir projects.

Devices

There are three classes of supported devices: hardware graphics , X11, and PostScript. Each device represents a different way of "doing" graphics, but they all have in common the raster model of hardware. This implies that they can fill polygons, as opposed to just drawing lines like a pen plotter. Each device is implemented in "immediate" mode, in which the application re-draws the entire picture for each new view of data.

The hardware graphics device represents the top end in interactive graphic capabilities. The hardware features used by the graphics package include viewing transforms, Z-buffering, Gouraud shading, alpha transparency, full RGB color and double buffering. Currently D3 library supports following hardware graphics:

Silicon Graphics: GL
Hewlet Packard: Starbase
multiplatform: Open GL.

The X11 class of devices includes most Unix workstations. Due to the inadequate design of graphics in X11, the best interactive performance on such devices is achieved by letting the X server do the graphics, rather than the client. In practice, this limits applications to lines and flat-filled polygons. Due to the software implemented hidden line algorithm, this device may be slow for the display of graphics with a large number of details. The X11 device does not support Gouraud shading and alpha transparency (screen door transparency is implemented instead). As an alternative to hardware graphics, the software implementation of OpenGL (MESA), is implemented on all platforms, providing images of much better quality than X11 graphics, and usually faster (except for remote display mode). MESA is implemented in parallel to hardware OpenGL, so e.g. on SGI platforms either one can be selected (to draw on remote, non-OpenGL display, or to produce screen dump images in the batch mode).

The PostScript device is the standard device for hardcopy. A PostScript file is produced by interpreting all icon graphics currently visible and creating a file containing the equivalent moves and draws in the PostScript language to produce a hardcopy of the image. Both grayscale and color PostScript files may be produced. A hardcopy represents a copy of the actual interactive image, however transparency and Gouraud shading are not supported.

OpenGL and MESA devices also have the capability to produce RGB (bitmap) images ("screen dump"), which could be externally converted into other formats (gif, jpeg, etc) or into the animated sequences (animated gif, mpeg, and other formats). Both devices also can be swithed to grayscale stereo mode (using red/blue glasses) to aid in the visualization of complex three-dimensional geometries.