# Off-screen Rendering

Mesa's off-screen rendering interface is used for rendering into user-allocated blocks of memory. That is, the GL_FRONT colorbuffer is actually a buffer in main memory, rather than a window on your display. There are no window system or operating system dependencies. One potential application is to use Mesa as an off-line, batch-style renderer.

The OSMesa API provides three basic functions for making off-screen renderings: OSMesaCreateContext(), OSMesaMakeCurrent(), and OSMesaDestroyContext(). See the Mesa/include/GL/osmesa.h header for more information about the API functions.

There are several examples of OSMesa in the `progs/osdemo/` directory.

## Deep color channels

For some applications 8-bit color channels don't have sufficient precision. OSMesa 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.

Before version 6.5.1, Mesa had to be recompiled to support exactly one of 8, 16 or 32-bit channels. With Mesa 6.5.1, Mesa can be compiled for either 8, 16 or 32-bit channels and render into any of the smaller size channels. For example, if Mesa's compiled for 32-bit channels, you can also render 16 and 8-bit channel images.

To build Mesa/OSMesa for 16 and 8-bit color channel support:

          make realclean
          make linux-osmesa16

To build Mesa/OSMesa for 32, 16 and 8-bit color channel support:

          make realclean
          make linux-osmesa32

You'll wind up with a library named libOSMesa16.so or libOSMesa32.so. Otherwise, most Mesa configurations build an 8-bit/channel libOSMesa.so library by default.

If performance is important, compile Mesa for the channel size you're most interested in.

If you need to compile on a non-Linux platform, copy Mesa/configs/linux-osmesa16 to a new config file and edit it as needed. Then, add the new config name to the top-level Makefile. Send a patch to the Mesa developers too, if you're inclined.