Bitmap Display Hardware
Modern display hardware uses a bitmap display,
to show raster graphics.
The image on the screen consists of a matrix of colored dots.
The main features of Graphical Display Systems are:
- The cpu can pass commands to a graphics board. That board performs
many actions independently (while the cpu can be working on other tasks).
Examples are setting display resolution and timings, changing the way
colors are handled (truecolor, hicolor or with colormaps), changing the
shape and/or position of a cursor, etc.
- The cpu can pass data to the graphics board's memory by means of a
shared part of their address space. That memory is called the
frame buffer.
Data copied to the frame buffer immediately result in an image on the screen.
A typical bitmap display has about a million dots, and each dot takes up
between 1 and 24 or even 32 bits of (color) data.
- The graphics board copies the frame buffer to the screen 30 to 100
times per second. For a stable image, the refresh rate should be 70 Hz
or higher.
- Bitmap display hardware is simple, but the software (executed by
the workstation's cpu) needs to do a lot of work to compose and update
all images that need to be shown.
- Bitmap displays have a number of intrinsic limitations:
- Dots can only be placed at positions on the "grid".
Truly smooth curves and shapes cannot be drawn, only approximated.
The error becomes more apparent as the images get smaller or the
display resolution lower.
This limitation implies that very small icons will be hard to recognize.
- Letters are a special kind of shape: they too don't look sharp,
which causes slower reading.
The following four images show the letter A
at different resolutions. The leftmost image is enhanced by means
of anti-aliasing.
This limitation means that text in small fonts will be hard to read.
- Some displays use a color map to let software select
a small set of (256) colors from a large collection (16 million).
When different applications wish to use different color maps,
switching between windows may cause a switch between color maps,
creating awkward effects.
- Some displays use interlacing to hide a slow refresh rate.
This causes horizontal lines that are 1 pixel wide to flicker.
- The "3 million" rule describes the "standard" requirement for
workstation-level graphics hardware:
- 1 million pixels (or dots).
Some typical display sizes are VGA with 640x480 or only 307.200 pixels;
SVGA with 800x600 for 480.000 pixels of 1024x768 for 786.432 pixels.
The first popular resolution which achieves the million pixels is
1152x900.
- 1 million bytes (resulting in 8 bits per pixel at 1 million pixels).
This amount refers to memory dedicated for the frame buffer, a possible
font cache, hardware cursor, etc., not to main memory of the workstation.
- 1 million instructions per second (MIPS)
This speed is all it takes to be able to redraw the entire screen in
under 1 second.
Modern workstations have cpu's capable of running at 100 MIPS,
but the overhead of window system software is often a hundred fold
(especially when using Java).