Introduction

For over ten years, the Computer Graphics Lab has distributed a popular UNIX-based molecular modeling application, MidasPlus. The MidasPlus system is used daily in university-level research programs in order to display and manipulate macromolecules such as proteins and nucleic acids. Ancillary programs provide features such as computation of molecular surfaces and electrostatic potentials and generation of publication quality space filling images with multiple light sources and shadows. MidasPlus is distributed with documented source code to serve as a starting point and training tool for others interested in doing their own software development.

Chimera is a new molecular graphics package developed by the UCSF Computer Graphics Laboratory as the next generation of MidasPlus. The primary features of Chimera are high quality real-time molecular graphics and programmable extensibility. The emphasis on extensibility is a direct result of the success of the MidasPlus delegate extension mechanism, which has shown that the ability to incorporate user-written programs greatly enhances the modeling environment. MidasPlus was not originally designed with easy extensibility as a goal, although an extension mechanism allows applications (``delegates'') to add functionality to MidasPlus without requiring the developer to recompile the MidasPlus source code.

Chimera has been designed to provide advanced molecular graphics capabilities while retaining the functionality currently available in MidasPlus. The package has been completely redesigned rather than built upon MidasPlus. Instead of combining the functionality into a single program, we selected a subset of MidasPlus features to form the core of Chimera and then implemented the rest of the features as extensions to the core. The three major groups of core features in Chimera are the graphical display, the user interface, and the extension mechanism.