3.5-4.5 Texture Tiling
 
For non-real-time applications such as intelligence analysis of large photo-mapped terrain
areas, there is one other mode supported for SkyWriter in which very large textures can be tiled
across a surface.  In this mode, textures of up to 32Kx32K texels may be used.
 
3.5.4.6 Texture Mapping Functions
 
The image engines in the Raster Subsystem are very complex ASICs which are highly
programmable (using microcode).  These processors support many different algorithms for
texture mapping onto 3D surfaces to offer flexibility to users in trading image quality against
performance.
The two basic categories into which these functions fall are:

o   Non-Mip Mapped

o   Mip Mapped

3.5.4.7 Non Mip Mapped Texture Functions
 

There are advantages and disadvantages to functions in both categories.  The primary
advantage of the non-Mip Mapped textures is that they are supported with resolutions of up to
256x256 while Mip Mapped textures are limited to 128x 128.  There are, however, only two
mapping functions available:

o   Point Sampled

o   Bilinear Interpolated

Point sampling offers the lowest quality of texturing at the best performance on previous
machines such as PowerVision.  There is, however, no performance benefit on SkyWriter, and
this mode exists purely for compatibility with earlier generation software.  When point
sampling, the system takes the iterated texture coordinates for the current pixel being drawn
and samples the specified texture to find the nearest texel to the current pixels' S and T
coordinates.

The quality problem occurs when you move the object slightly and the sampling provides a
completely different texel for that same pixel. This rapid change in texel values can cause
scintillation and "swimming" as textures are applied to moving scenes or objects.

The next best mode is Bilinear interpolated textures without Mip Mapping.  This mode at least
looks at the S and T coordinates and find the nearest four texels and blends them together to
generate the final color value to applied to the pixel being drawn. On SkyWriter, this is just
as fast as Point Sampling.