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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”.
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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 • 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
