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WirelessDevNet: Microwindows - Enabling graphical applications on embedded Linux systems

May 04, 2000, 22:13 (6 Talkback[s])
(Other stories by Gregory Haerr)

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"In most embedded designs, the X Window System is overkill, especially when running a dedicated graphics application. For this reason, the Microwindows Project was developed. Microwindows is an open source project aimed at producing desktop-quality graphics functionality for small devices. The architecture allows for ease in adding different display, mouse, touchscreen and keyboard devices, as I will explain below. Starting with Linux version 2.2.0, the kernel contains code to allow user applications to access graphical display memory as a framebuffer, which ends up being a memory-mapped region in a user process space that, when written to, controls the display appearance. This allows graphics applications to be written without having to have knowledge of the underlying graphics hardware, or use the X Window System. This is the way that Microwindows typically runs on embedded systems."

"Microwindows was designed to attempt to bring applications to market quickly, with a minimum of effort. In order to accomplish this, I felt that designing yet another graphics applications programming interface (API) would steepen the learning curve, thus discouraging interest and increasing time-to-market. Microwindows implements two popular graphics programming interfaces: the Microsoft Windows Win32/WinCE graphics display interface (GDI), used by all Windows CE and Win32 applications, and an Xlib-like interface, known as Nano-X, used at the lowest level by all Linux X11 widget sets. This allows the extremely large group of Windows programmer talent to be used in developing the graphical side of the application, as well as being familiar to the core group of Linux graphics programmers used to working with X11."

"Microwindows runs in 50-250k of memory, an order of magnitude smaller than the X Window System. This is primarily the result of using a single routine for each of the drawing functions in the engine layer. The engine layer checks for clipping and calls a driver-level routine for drawing a pixel or line if unclipped. The X Window System duplicates the entire drawing routine for each pixel depth and has clipped and unclipped versions for speed. Microwindows fully supports the new Linux kernel framebuffer architecture, and currently has support for 1, 2, 4, 8, 16, 24, and 32 bits per pixel displays, with support for palettized and truecolor display color implementations, as well as grayscale. With both APIs, all colors are specified in the portable RGB format, and the system has routines to convert to the nearest available color, or shade of gray for monochromatic systems. Although Microwindows fully supports Linux, it's internal, portable architecture is based on a relatively simple screen device interface, and can run on many different RTOS's as well as bare hardware. This brings a big benefit: graphics programming by the customer can be shared between projects, and even run on different targets with different RTOS's, without having to rewrite the graphics side of the application."

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