The Voodoo Graphics hardware can accept vertex data in either fixed point or floating point formats. However, Glide provides only a floating point interface, since RISC and Pentium processors are optimized for floating point calculations. If you are porting a fixed point application to the Voodoo Graphics system, plan to convert all your data to floating point representation as part of the porting process.

The GrVertex structure contains single-precision, IEEE 754 32-bit floating point values.

Geometric Coordinates

The x and y coordinates are specified in pixel units in the range [–2048..2047]. The pixel coordinate (0.5, 0.5) represents the exact center of the first visible pixel on the screen.

The ooz coordinate should be assigned a value that is linear in screen space. That is, it should be a linear function of 1/w that can be scaled and translated such that it increases or decreases with distance from the viewer. The valid range for ooz value s is [0..65535]. To minimize z aliasing this range should be mapped to the smallest possible range of eye coordinates. For example, if w eye coordinates are within the range [2..15] and 1/w is in the range [1/2..1/15] then the mapping would be approximately

where w is eye w and ooz is the value iterated in the Voodoo Graphics subsystem.

The w coordinate is a scaled positive depth value used during perspective projection, perspective texture mapping, and depth buffering. Some graphics systems do not use homogeneous coordinates; in these instances the z depth value can be used in lieu of the w coordinate, assuming that the z value is positively increasing into the screen. The range of w is [1..65535].

Glide and Voodoo Graphics actually use the reciprocal of the homogeneous coordinate, 1/w. The valid range for 1/w is [–4096..61439]. Normally, the homogeneous coordinate is clipped to a positive range of [1, far] and so its reciprocal is in the range [1.. 1/far].
Negative values should be avoided.

Each TMU and the Pixelfx chip each have their own 1/w. Normally, the values in all the chips will be the same. However, projected textures have a different w value than non-projected textures. Projected textures iterate q/w where w is the homogeneous distance from the eye and q is the homogeneous distance from the projected source. In this case, q/w has a valid range of [–-4096..61439].

The 1/w value in Pixelfx is used only for fog calculations and w buffering, and is not used for texture mapping. It can be scaled differently than the 1/w values sent to the TMUs. The fog table spans a range in 1/w from [1/65535..1]. If w buffering is enabled, the w buffer spans a range in 1/w from [1/65528..1]. Therefore, scale the 1/w value in Pixelfx such that the range [1/65535..1] encompasses all that is interesting in the scene.

Colors

The color components are in the range [0..255] where 0 is black and 255 is maximum intensity. Colors should be clamped to this range.

Glide supports four different color byte orderings: RGBA, ARGB, BGRA, and ABGR. Color byte ordering determines how linear frame buffer writes and color arguments passed to the constant color functions (see Chapter 5) are interpreted. Color ordering is established when Glide and the Voodoo Graphics system are initialized (see Chapter ).

When the terms “RGB” and “RGBA” appear in this manual, they typically refer to any color system that represents red, green, blue, and optionally, alpha, as separate components, regardless of the byte order or component width. The exceptions will be clearly recognizable as discussions about specific color resolution and format.

Texture Coordinates

Glide uses texture coordinates in the range [–32768..32767] and refers to them as (s,t) pairs, similar to the naming convention of OpenGL. A texture contains texels with (s,t) coordinates in the range [0..255.0]; the texture may be replicated many times to cover a surface by mapping the texture coordinates modulo 256 to a texel in the texture. The Voodoo Graphics subsystem iterates s/w and t/w, so s and t must be divided by w (or multiplied by oow) before storing them in the GrVertex structure.

The w term iterated by the SST-1 is actually 1/w or the reciprocal of the homogeneous coordinate. The valid range for 1/w is [–4096..61439]. Normally, the homogeneous coordinate is clipped to a positive range of [1..far] and so its reciprocal is in the range [1..1/far]. Negative values should be avoided.

Each TMU has its own s, t, and w values. Normally, they will be the same as the w in the Pixelfx. However, in certain cases they will be different. For example, projected textures have a different w value than non-projected textures. Projected textures iterate q/w where w is the homogeneous distance from the eye and q is the homogeneous distance from the projected source. In this case, q/w has a valid range of [–4096..61439].

Mipmapping [WILL83] is a method of organizing several pre-filtered texture maps into a single logical entity used for anti-aliased texture mapping.

The term mipmap is sometimes used to describe a pyramidal organization of gradually smaller, filtered sub-t extures or an individual texture map within such an organization.

Glide adopts the original convention that defines the term mipmap to mean the entire group of textures that comprise a single pyramidal data structure. Individual textures within a mipmap ar e referred to as mipmap levels.