Vertical Retrace Interval
An introduction to VGA techniques
for smooth graphics animation
The CRT Display
Screen’s image consists of horizontal scanlines, drawn in top-down order,
and redrawn about 60-70 times per second (depending on display mode).
Image “persistence”
• The impression of a steady screen image
is purely a mental illusion of the viewer’s
• The pixels are drawn on the CRT screen
too rapidly for the human eye to follow
• And the screen phosphor degrades slowly
• So the brain blends a rapid succession of
discrete images into a continuous motion
• So-called ‘motion pictures’ are based on
these phenomena, too (30 frames/second)
Color “dithering”
• The mind’s tendency to “blend” together
distinct images that appear near to one
another in time can be demonstrated by
using two different colors -- alternately
displayed in very rapid succession
• This is called “dithering”
• Some early graphics applications actually
used this technique, to show extra colors
Timing mechanism
• Today’s computers can “redraw” screens
much faster than a CRT can display them
• We need to “slow down” the redrawing so
that the CRT circuitry will be able keep up
• Design of VGA hardware allows programs
to “synchronize” drawing with CRT refresh
• Use the “INPUT STATUS REGISTER 1”
accessible (read-only) at I/O port 0x3DA
Input Status Register One
7
6
5
4
3
2
1
0
Vertical Retrace status
1 = retrace is active
0 = retrace inactive
Display Enabled status
1 = VGA is reading (and displaying) VRAM
0 = Horizontal or Vertical Retrace is active
I/O port-address: 0x3DA (color display) or 0x3BA (monochrome display)
void vsync( void )
{
// wait for current retrace to finish
while ( ( inb( 0x3DA ) & 8 ) != 8 );
// wait until the next retrace begins
while ( ( inb( 0x3DA ) & 8 ) == 8 );
}
// This function only returns at the very beginning
// of a new vertical blanking interval, to maximize
// the time for drawing while the screen is blanked
Animation algorithm
1)
2)
3)
4)
5)
Erase the previous screen
Draw a new screen-image
Get ready to draw another screen
But wait for a vertical retrace to begin
Then go back to step 1.
How much drawing time?
•
•
•
•
Screen-refresh occurs 60 times/second
So time between refreshes is 1/60 second
Vertical blanking takes about 15% of time
So “safe” drawing-time for screen-update
is about: (1/60)*(15/100) = 1/400 second
• What if a screen-update takes longer?
• Animation will exhibit “tearing” of images
Programming techniques
• Your application may not require that the
full screen be redrawn for every frame
• Maybe only a small region changes, so
time to “erase-and-redraw” it is reduced
• You may be able to speed up the drawing
operations, by “optimizing” your code
• Using assembly language can often help
Using off-screen VRAM
• You can also draw to off-screen memory,
which won’t affect what’s seen on-screen
• When your ‘off-screen’ image is finished,
you can quickly copy it to the on-screen
memory area (called a ‘BitBlit’ operation)
• Both CPU and SVGA provide support for
very rapid copying of large memory areas
Our ‘animate1.cpp’ demo
• We can demonstrate smooth animation
with a “proof-of-concept” prototype
• It’s based on the classic “pong” game
• A moving ball bounces against a wall
• The user is able to move a “paddle” by
using an input-device (such as a mouse,
keyboard, or joystick)
• We didn’t implement user-interaction yet