Video Hardware, Part 3 - Texture Mapping Filtering Enhancements (Page 11 of 12 ) To improve the quality of texture maps, several filtering techniques have been developed, including MIP mapping, bilinear filtering, trilinear filtering, and anisotropic filtering. These techniques and several others are explained here: Bilinear filtering. Improves the image quality of small textures placed on large polygons. The stretching of the texture that takes place can create blockiness, but bilinear filtering applies a blur to conceal this visual defect. MIP mapping. Improves the image quality of polygons that appear to recede into the distance by mixing low-res and high-res versions of the same texture; a form of antialiasing. Trilinear filtering. Combines bilinear filtering and MIP mapping, calculating the most realistic colors necessary for the pixels in each polygon by comparing the values in two MIP maps. This method is superior to either MIP mapping or bilinear filtering alone.
Note -Bilinear and trilinear filtering work well for surfaces viewed straight-on but might not work so well for oblique angles (such as a wall receding into the distance).
Anisotropic filtering. Some video card makers use another method, called anisotropic filtering, for more realistically rendering oblique-angle surfaces containing text. This technique is used when a texture is mapped to a surface that changes in two of three spatial domains, such as text found on a wall down a roadway (for example, advertising banners at a raceway). The extra calculations used take time, and for that reason, it can be disabled. To balance display quality and performance, you can also adjust the sampling size: Increase the sampling size to improve display quality, or reduce it to improve performance. T-buffer. This technology eliminates aliasing (errors in onscreen images due to an undersampled original) in computer graphics, such as the "jaggies" seen in onscreen diagonal lines; motion stuttering; and inaccurate rendition of shadows, reflections, and object blur. The T-buffer replaces the normal frame buffer with a buffer that accumulates multiple renderings before displaying the image. Unlike some other 3D techniques, T-buffer technology doesn't require rewriting or optimization of 3D software to use this enhancement. The goal of T-buffer technology is to provide a movie-like realism to 3D-rendered animations. The downside of enabling antialiasing using a card with T-buffer support is that it can dramatically impact the performance of an application. This technique originally was developed by now-defunct 3dfx. However, this technology is incorporated into Microsoft DirectX 8.0 and above. Integrated transform and lighting (T&L). The 3D display process includes transforming an object from one frame to the next and handling the lighting changes that result from those transformations. T&L is a standard feature of DirectX starting with version 7. The NVIDIA GeForce 256 and original ATI RADEON were the first GPUs to integrate the T&L engines into the accelerator chip, a now-standard feature. Full-screen antialiasing. This technology reduces the jaggies visible at any resolution by adjusting color boundaries to provide gradual, rather than abrupt, color changes. Whereas early 3D products used antialiasing for certain objects only, accelerators from NVIDIA (GeForce 4 Ti, GeForce FX, and the 6800 series) and ATI (RADEON 9xxx and the X800 series) use various types of highly optimized FSAA methods that allow high visual quality at high frame rates. Vertex skinning. Also referred to as vertex blending, this technique blends the connection between two angles, such as the joints in an animated character's arms or legs. NVIDIA's GeForce2, 3, and 4 series cards use a software technique to perform blending at two matrices, whereas the ATI RADEON series chips use a more realistic hardware-based technique called 4-matrix skinning. Keyframe interpolation. Also referred to as vertex morphing, this technique animates the transitions between two facial expressions, allowing realistic expressions when skeletal animation can't be used or isn't practical. See the ATI Web site for details. Programmable vertex and pixel shading. Programmable vertex and pixel shading became a standard part of DirectX starting with version 8.0. However, NVIDIA introduced this technique with the GeForce3's nfiniteFX technology, enabling software developers to customize effects such as vertex morphing and pixel shading (an enhanced form of bump mapping for irregular surfaces that enables per-pixel lighting effects), rather than applying a narrow range of predefined effects. The NVIDIA GeForce4 Ti's nfiniteFXII pixel shader is DirectX 8 compatible and supports up to four textures, whereas its dual vertex shaders provide high-speed rendering up to 50% faster than the GeForce3. The ATI RADEON 8500 and 9000's version, SmartShader, is supported by DirectX 8.1. DirectX 8.1 supports more complex programs than nfiniteFX and provides comparable quality to nfiniteFXII. ATI 9700, 9800, and 9500 support DirectX 9's floating-point pixel shaders and more complex vertex shader. NVIDIA GeForce FX cards also support DirectX 9 pixel and vertex shaders, but they add more features. NVIDIA's 6800 series supports the new DirectX 9 Shader Model 3.0. Floating-point calculations. Microsoft DirectX 9 supports floating-point data for more vivid and accurate color and polygon rendition. ATI Radeon 9500, 9600, 9700, and 9800-series GPUs support standard DirectX 9 floating-point data, whereas the NVIDIA GeForce FX series supports DirectX 9 and has additional precision. The NVIDIA 6800 series further increases precision beyond the basic DirectX 9 requirements. The Matrox Parhelia supports this, but doesn't support other DirectX 9 features.
Single- Versus Multiple-Pass Rendering Various video card makers handle application of these advanced rendering techniques differently. The current trend is toward applying the filters and basic rendering in a single pass rather than multiple passes. Video cards with single-pass rendering and filtering typically provide higher frame-rate performance in 3D-animated applications and avoid the problems of visible artifacts caused by errors in multiple floating-point calculations during the rendering process. Next: Hardware Acceleration Versus Software Acceleration >>
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More By Addison-Wesley/Prentice Hall PTR |  This chapter is from Upgrading and Repairing PCs, 16th edition,by Scott Mueller. (Que Books, 2004, ISBN: 0789731738). Check it out at your favorite bookstore today. Buy this book now.
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