Vertical Retrace IntervalThe CRT DisplayImage “persistence”Color “dithering”Timing mechanismInput Status Register Onevoid vertical_retrace_sync( void )Animation algorithmHow much drawing time?Retrace visualizationProgramming techniquesUsing off-screen VRAMOffscreen VRAMOur ‘animate1.cpp’ demoIn-class exercises #1Adding sound effectsA PC’s ‘Timer-Counter’‘Square Wave’ outputOur ‘flipflop.cpp’ demoOur ‘makewave.cpp’ toolIn-class exercises #2Vertical Retrace IntervalAn introduction to VGA techniques for smooth graphics animationThe CRT DisplayScreen’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 colorsTiming 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 0x3DAInput Status Register One 7 6 5 4 3 2 1 0Vertical Retrace status 1 = retrace is active 0 = retrace inactiveDisplay Enabled status1 = VGA is reading (and displaying) VRAM0 = Horizontal or Vertical Blanking is activeI/O port-address: 0x3DA (color display) or 0x3BA (monochrome display)void vertical_retrace_sync( void ){// spin if retrace is already underwaywhile ( ( inb( 0x3DA ) & 8 ) == 8 );// then spin until a new retrace startswhile ( ( 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 blankedAnimation algorithm 1) Erase the previous screen2) Draw a new screen-image3) Get ready to draw another screen4) But wait for a vertical retrace to begin5) 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 imagesRetrace 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 percentProgramming 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 helpUsing 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 areasOffscreen VRAMExtra VRAM availableCRTC Start_Address (default = 0)VRAM for the visible screen-image 16MB on ourRadeon X300Our 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 modeExample: 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)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 yetIn-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 activeAdding 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 tonesOur ‘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
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