Z buffer: A buffer that stores for each pixel during the rendering phase, how far its current content is away from the camera. If the newly calculated content would be further away, it will be invisible, therefore the color does not need to be changed. A finer Z buffer requires more memory, but will reduce possible glitches in plane intersections.
Anti-Aliasing:
The image is rendered at a higher resolution, and before displaying, the resolution is calculated down with specific averaging calcilations. This can reduce tecture flickering in the distance, and \"aliasing\" (staircasing) of slight diagonal lines, at the cost of much graphic memory and longer rendering time (lower FPS).
Multitexturing:
If supported by hardware, overlaying several textures can be done quickly and enhance the visible quality remarkably (e.g. adding the color texture - sometimes more than one, the lighting, probably bump maps, ...). If the graphic chip doesn\'t support it well, multitexturing may reduce the speed.
This is a good context to describe \"-nolightmapping\": Usually, texturing and lighting is a required multitexturing part of game graphics. Mapping textures onto planes is one step, mapping lighting is another (all texels get multiplied by a factor between 0.0 and 1.0, darkening it depending on the lightness map; with \"-nolightmapping\", all texels will remain with full brightness.).
Multitexturing may speed up both steps into one rendering cycle, if the graphic card supports it (most current chips should). If multitexturing is not supported by the chip, both steps need to be calculated in distinct cycles.
But I guess that the \"Multitexturing for quality\" checkbox is there for a different reason, it may rather control multiple textures on the same plane, like basic and detail textures overlaying each others...
Stencil buffer:
used for transparency effects with sharp lines, for several different occasions. I don\'t know which kinds of effects are indeed used by CrystalSpace, though... -- Sorry, no idea about the threshold.
Anisotropy:
If you look over a slanted plane with texture at a flat angle, the graphic card may calculate a too rough mipmap texture which makes the texture too blurred. Anisotropic texture resampling makes it possible to downsample textures in different directions by different amounts, blurring the texture differently depending on the direction you look at it - along one coordinate, or across another. This way, the texture will not be either too blurred or too flickering.
Downsampling:
Uses textures with lower resolutions, reducing the time needed to reload texture images while changing a zone, reducing the required texture buffer in the graphic card, speeding up the movement through a world with many different textures.
