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
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