Mesa 1.2.4 introduced off-screen rendering, a facility for generating 3-D imagery without having to open a window on your display. Mesa's simple off-screen rendering interface is completely operating system and window system independent so programs which use off-screen rendering should be very portable. This feature effectively enables you to use Mesa as an off-line, batch-oriented renderer.
The "OSMesa" API provides 3 functions for making off-screen renderings: OSMesaCreateContext(), OSMesaMakeCurrent(), and OSMesaDestroyContext(). See the Mesa/include/GL/osmesa.h header for more information. See the demos/osdemo.c file for an example program. There is no facility for writing images to files. That's up to you.
If you want to generate large images (larger than 1280x1024) you'll have to edit the src/config.h file to change MAX_WIDTH and MAX_HEIGHT then recompile Mesa. Image size should only be limited by available memory.
For some applications 8-bit color channels don't have sufficient accuracy (film and IBR, for example). If you're in this situation you'll be happy to know that Mesa supports 16-bit and 32-bit color channels through the OSMesa interface. When using 16-bit channels, channels are GLushorts and RGBA pixels occupy 8 bytes. When using 32-bit channels, channels are GLfloats and RGBA pixels occupy 16 bytes.
To build Mesa/OSMesa with 16-bit color channels:
cd Mesa-5.x/src
make -f Makefile.X11 clean
make -f Makefile.OSMesa16 linux-osmesa16
For 32-bit channels:
cd Mesa-5.x/src
make -f Makefile.X11 clean
make -f Makefile.OSMesa16 linux-osmesa32
You'll wind up with a library named libOSMesa16.so or libOSMesa32.so.
If you're not using Linux, you can easily edit Make-config and add an appropriate configuration.
The Mesa/tests/osdemo16.c file (available via CVS) demonstrates how to use this feature.
BE WARNED: 16 and 32-bit channel support has not been exhaustively tested and there may be some bugs. However, a number of people have been using this feature successfully so it can't be too broken.