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 one technique called dithering Some early graphics applications actually used this approach 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 Blanking is active I O port address 0x3DA color display or 0x3BA monochrome display void vertical retrace sync void spin if retrace is already underway while inb 0x3DA 8 8 then spin until a new retrace starts while inb 0x3DA 8 0 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 5 of time So safe drawing time for screen update is about 1 60 5 100 1 1200 second What if your screen updates take longer Animation may exhibit tearing of images Retrace visualization Our demo program instatus cpp provides a visualization for the volatile state of the VGA s Input Status Register One It repeatedly inputs this register s contents and writes that value to the video memory The differences in pixel coloring show how much time is spent in the retrace states You can instrument its loop to get percent Programming techniques Your application may not require that the whole 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 Offscreen VRAM CRTC Start Address default 0 Our classroom machines have 16 megabytes of video display memory The amount needed by the CRT for a complete screen image depends upon your choice of the video display mode Example For a 1280 by 960 TrueColor graphics mode e g 32 bits per pixel the visible VRAM is 1280x960x4 bytes which is less than 5 megabytes VRAM for the visible screen image Extra VRAM available 16MB on our Radeon X300 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 In class exercises 1 Investigate the effect of the function calls to our vertical retrace sync routine by turning it into a comment and recompiling Add a counter to the loop in instatus cpp which is incrementd whenever bit 3 is set to find the percentage of loop iteractions during which vertical blanking was active Adding sound effects We can take advantage of Linux s support for synchronizing digital audio with graphic animation adds realism to pong game But for this we will need to understand the basic principles for using PC soundcards Linux supports two APIs OSS and ALSA Open Sound System by 4Front Technology Advanced Linux Sound Architecture GNU A PC s Timer Counter Most PCs have a built in Timer Counter that is capable of directly driving the PC s internal speaker if suitably programmed By using simple arithmetic a programmer can produce a musical tone of any pitch and so can play tunes by controlling the sequencing and duration of those tones But we can t hear the speaker in our class Square Wave output But we can listen to the external speakers that attach to the Instructor s workstation or listen to a stereo headset that you plug in to your individual machine s soundcard The same basic principle used by the PC internal speaker and Timer Counter if understood can be used in our software to generate square wave musical tones Our flipflop cpp demo We created a graphics animation that will show you the basic idea for generating a square wave output stream to vibrate an external speaker or headset earphones at any given frequency humans can hear This animation also illustrates principles of VGA graphics animation programming for a 4bpp 16 color planar memory model Our makewave cpp tool We created a tool that will generate actual audio files which play notes of designated frequencies under Linux or another OS e g WinXP that understands a standard Waveform Audio File wav You can see what application code you d need to write to play a wav file using the simple OSS Linux programming interface by looking at our pcmplay cpp program In class exercises 2 Use our makewave program to produce some wav files that contain square wave data for musical tones of different pitches Use our pcmplay program to play these audio files and listen with your headset We will learn more about Waveform files in our next lecture bring your earphones if you have them
View Full Document
Unlocking...