LOSTCIRCUITS

Full Scene Anti-Aliasing (FSAA)

There is a wealth of comprehensive information on FSAA available so I’ll limit this here to a very brief description of the general principle. As resolution decreases, more pixels on the screen are pulled together to square or rectangular blocks. This means that, e.g., on a monitor capable of displaying 1600 x 1200 pixels, a resolution of 400 x 300 pixels, will cause each pixel generated to show up as a block of 4 x 4 = 16 pixels. Running a diagonal line across the monitor will, therefore, produce a ladder or stepping line instead of a smooth line. One way of avoiding the jagged edges is to supersample the images at twice the linear dimensions, or 4 times the amount of pixels and then perform anti aliasing, which means calculating the RGB values for each pixel according to the four surrounding pixels in x/y direction. The result is a slightly blurred image, nonetheless, more pleasing than the original jagged edges.

The same line drawn at low resolution in Photoshop with and without AntiAliasing enabled (2 x magnification)

at 4 x magnification, the aliasing / antialiasing differences become even more obvious, the same effect occurs when playing games at low resolution

Since each pixel has to be divided into 4 subpixels, the amount of work required by FSAA increases with the square of the resolution. In addition, approaching the resolution limit of the monitor will yield diminishing returns for FSAA since the number of virtual pixels approaches the number of physical pixels, each of which can only display one RGB value. Therefore, FSAA may be useful for resolutions up to 800 x 600 pixel resolution, above that, it pretty much becomes a matter of personal taste and above 1280 x 1024 pixel resolution, FSAA becomes completely useless.

Anisotropic Texture Filtering

One relatively unusual way of doing things in the Radeon is the anisotropic texture filtering. Conventional antialiasing employs, as mentioned above, a 2 x 2 matrix for medianing of adjacent pixel RGB values. Since the x and y coordinates are the same size, such a square is called isotropic. A more sophisticated way of filtering can be achieved by introducing a variable algorithm for the ratio between x and y length. As a result, the filtering mask becomes rectangular or anisotropic. In the Charisma engine, the anisotropy can reach a value of 12:1 to achieve optimal anti aliasing at any given angle.

Bump Mapping

The Radeon does support all three current forms of bump mapping, the easiest of which is embossing which is realized essentially by subtracting identical frames with a shift by one or more pixels to one side (away from the light source). This effect can be simulated easily in Adobe Photoshop and needs no further explanation (just try it yourself)

DOT product 3

More complex than embossing is DOT product 3 where a bump texture map is used to create a 3D vector representing the slope of the surface. This 3D vector is combined in a DOT product operation with a vector pointing to the light source to generate reflexions and shadows. The drawback of this method is that it doesn’t work well with reflective surfaces.

Environment Mapped Bump Mapping (EMBM)

The feature pioneered by Matrox with the release of the G400 is, by far, the most advanced technique for creating bumps and 3-dimensional surfaces using a perturbation map. Starting with Microsoft DX6, EMBM has been an integral feature of all DirectX versions. Like DOT product 3, EMBM does require a special bump texture map, however, it has the advantage of combining bumps with shiny surfaces which is why it is the favorite tool to model rippled water surfaces.

The three different modes of bump mapping and their typical applications

These features are the most important ones for gaming, there is, however, more information and illustration on the ATi website on Texture Transformations, Environment Mapping, Spherical Environment Mapping, Dual-Paraboloid Environment Mapping, Projective Textures, Shadow Mapping, Range-Based Fog, Soft Reflections and Soft Shadows, Motion Blur and other effects. Suffice it to say that the intelligent features in the Charisma engine are a good example for the mind over matter principle. Hardware implementations, however, are only as good as the drivers that unleash them.

Next Page:    => finally, some benchmarks =>