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Graphics Hardware Revisted Graphics Cards AKA Video cards , Video boards , Video display boards , Graphics boards , Graphics adapter cards , Video adapter cards Also 3D Cards, 3D accelerators Graphics Cards

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Graphics cards l.jpg

AKA Video cards, Video boards, Video display boards, Graphics boards, Graphics adapter cards, Video adapter cards

Also 3D Cards, 3D accelerators

Graphics Cards

  • Basic Role is to convert digital information produced by the computer and generate analog information for Monitor.

  • For LCD monitors output remains digital

  • Modern graphics consist primarily ofdedicated memory and a graphics processor designed specifically to handle the intense computational requirements of displaying graphics. Most have special command sets for graphics manipulation built right into the chip.


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Basic Components

  • Memory: For frame buffer

  • Computer Interface: through which the CPU can modify the contents of the frame buffer. This is normally done by connecting the graphics card to the card bus on the motherboard. The computer can send signals through the bus to alter the memory. On modern cards, the GPU can directly control the framebuffer. Instructions are communicated only indirectly from CPU to framebuffer.

  • Video Interface: to generate the signals for the monitor/output device. The card must generate analog colour signals that drive the cathode ray tube (CRT) electron beam, as well as synchronization signals. This is done by the RAMDAC (Random Access Memory Digital to Analog Converter.



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The Graphics Processor

  • Originally the CPU did all the processing including accessing frame buffer and converting to analog.

  • The quantity of data transferred across the bus to the graphics card placed excessive burdens on the system.

  • Also, ordinary DRAM graphics memory couldn't be written to and read from simultaneously, meaning that the RAMDAC would have to wait to read the data while the CPU wrote, and vice versa.

  • Modern graphics cards have evolved to take some or all of this load off the microprocessor. A modern card contains its own high-power processor that is optimized for graphics operations.

  • Depending on the graphics card, this CPU will be either a graphics coprocessor or a graphics accelerator.


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Graphics Processors

  • Instead of sending a raw screen image across to the frame buffer, the CPU sends a smaller set of drawing instructions, which are interpreted by the graphics card's driver and executed by the card's on-board processor.

  • Operations including bitmap transfers and painting, window resizing and repositioning, line drawing, font scaling and polygon drawing can be handled by the card's graphics processor,

  • It is designed to handle these tasks in hardware at far greater speeds than the software running on the system's CPU.

  • It then writes the frame data to the frame buffer. As there's less data to transfer, there's less congestion on the system bus, and the PC's CPU workload is greatly reduced

  • The use of dual-ported memory allows the processor to simulataneously read off the graphics memory while it is being written to


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RAMDAC

  • For displaying the framebuffer on CRT’s the Random Access Memory Digital to Analog Converter (RAMDAC) reads the contents of the video memory, converts it into an analogue RGB signal and sends it over the video cable to the monitor.

  • It does this by using a look-up table to convert the digital signal to a voltage level for each colour.

  • There is one Digital-to-Analogue Converter (DAC) for each of the three primary colours the CRT uses to create a complete spectrum of colours.

  • The intended result is the right mix needed to create the colour of a single pixel.

  • The rate at which the RAMDAC can convert the information, and the design of the graphics processor itself, dictates the range of refresh rates that the graphics card can support.

  • The RAMDAC also dictates the number of colours available in a given resolution, depending on its internal architecture.


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Driver software

  • Drivers translate what the application wants to display on the screen into instructions that the graphics processor can use.

  • The quality of any given driver can greatly affect performance.

  • Modern graphics processors support a wide range of high level functionality (more than just single pixels operations) including drawing operations for complex primitves the movement of large blocks of information.

  • It is the driver's job to decide on the most efficient way to use these graphics processor features, depending on what the application requires to be displayed.

  • In most cases, a separate driver is used for each resolution or colour depth. This means thata graphics card can have markedly different performance at different resolutions, depending on how well a particular driver has been written and optimised.


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3D Acceleration

  • A graphics chip, whether it's dedicated to 3D or a dual-purpose 2D/3D chip, removes the bulk of the load off the CPU and performs the rendering of the image itself.

  • All of this rendering, or drawing, is accomplished through the graphics pipeline in two major stages: geometry and rendering.

  • The geometry stage handles all polygon activity and converts the 3D spatial data into pixels. e.g. 3D transformations

  • The rendering stagemanages all the memory and pixel activity and prepares it for painting to the monitor. e.g. rasterization, shading algorithms


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Geometry

  • 3D images are broken down into polygons. Vertex lists need to be treated for…

    • Local to Global coordinate Mapping. Coordinate Transformations.

    • Clipping.

    • Input variables per-frame may cause the global layout to change as an object is transformed its geometry must be revised and recalculated. Projection/Viewing Lighting and Texturing will need to be recalculated.

  • Known collectively as geometry set-up - this has traditionally been the last stage of the graphics pipeline performed by the main CPU before rendering is performed.

  • Computer screens refresh at more than seventy times a second. Therefore, any changes in the position of these objects must also be calculated and displayed for every refresh of the screen.

  • Triangle setup converts the data created by the geometry setup into a form that can be input into a 3D accelerator traditionally based on triangles.


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Rendering

  • In the rendering stage, performed by the hardware accelerator, the 3D engine draws the pixels.

  • The bottleneck here is memory access - how fast the pixels read and write to the frame buffer.

  • There are thousands of polygons for each frame of a scene, and these must be updated and transmitted through the memory at least 30 times a second, to give the illusion of movement.

  • This transfer to the frame buffer is known as frame rate and is measured in frames per second (fps).

  • From there, the frames are transmitted to the RAMDAC and converted into an analogue signal for the monitor where, after much mathematical manipulation, the action takes place.


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Rendering Techniques

  • Some common techniques supported in modern graphics cards:

    • Texture mapping: best described as wrapping a 2D coloured paper over a 3D object.

    • Mip mapping: cutdown version of texture mapping for distant texture

    • Anti-aliasing: Reducing “jaggies” resulting from badly sampled data

    • Gouraud shading: smoothening out areas of different illumination

    • Bump mapping: generates embossed “textures”

    • Bi-linear filtering:

    • Z-buffering: used for Hidden Surface Removal … among other things


Texture mapping l.jpg

Texture mapping: best described as wrapping a 2D coloured paper over a 3D object. This process is painstaking, as it has to be repeated for every pixel on the object and each pixel of the texture - known as a texel - which lies on top. Many textures can be wrapped over the same object, and this is multitexturing.

Texture Mapping


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Bump Mapping

  • Bump mapping is an improvement on the more common "embossing" technique used to give a "bumpy" look to surfaces.

  • Achieves this by offsetting the normals used in illumination calculations


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MIP Mapping

  • From the latin multum in parvo -- many things ina small place.

  • "MIP mapping" refers to the technique of precomputing anti-aliased texture bitmaps as seen from far away, and using them in a texture mapped renderer.

  • Each subsequent image in the MIP map is one quarter of the size of the previous (e.g. if the original image is 64x64, then the second image in the MIP map is 32x32, and the third 16x16, and so on).

  • A MIP mapped image requires 4/3 of the storage of the original (1 + 1/4 + 1/16 + ...).


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MIP MAPPING

  • Effectively the same as texture mapping process but more texels are created without performing the equivalent number of calculations.

  • If a mip-map is one fourth the size of the original texture, reading a single texel from this mip-map is the same as reading four texels from the original texture.

  • If applied using proper filters, the image quality is actually higher, as it smoothes out jagged edges.


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Bilinear Filtering

  • Tries to overcome artifacts of discrete texel mapping

  • Instead of reading one texel for a texture coordinate…

    • read four texels,

    • calculate their average - that is, the average of their relative positions - colour and so on,

    • display the result as a single-screen texel.

  • Results in blurring at close quarters, which in turn reduces an otherwise blocky, pixelated appearance. Bi-linear filtering is now standard on most PC graphics cards.