SET SURFACE APPROXIMATION CRITERIA indicates the minimum quality desired for the approximation of the surface's geometry. It provides an indication of the degree of approximation tolerable and the relative importance of image quality versus rendering speed. Specification of lower relative approximation quality is intended to produce relatively lower quality approximations at relatively higher speeds. Specification of higher relative approximation quality is intended to produce relatively lower quality approximations at relatively higher speeds. Implementations may produce a higher quality approximation than that requested. Approximation of the surface shall not introduce any gaps, holes or other visible spatial discontinuities.
A surface approximation criteria consists of a type and a data record. The data record contains information specific to the type and is generally unique for each type. The defined surface approximation criteria types are :
WORKSTATION DEPENDENT : A workstation dependent approximation criteria is used. There are no data record contents for this type.
CONSTANT PARAMETRIC SUBDIVISION BETWEEN KNOTS : Sample points are at equal parameter increments between each pair of u knots and v knots. The data record contains two integer values that indicate the minimum number of sample points in the u and v parameter dimensions. Both values have the following interpretation :
<=0 Sample points are at the u and v knots values.
>0 Sample points are at the u and v values and at this number of positions between each pair of knot values in each parameter dimension.
CHORDAL SIZE in WC : Generate at least sufficient sample points such that length in World Coordinates of every line segment (chord) between the sample points for both the u and v parameter directions is less than the corresponding specified real approximation value. The data record contains two approximation values, one for each parameter direction.
CHORDAL SIZE in NPC : Generate at least sufficient sample points such that the length in Normalized Projection Coordinates of every line segment (chord) between the sample points for both the u and v parameter directions is less than the corresponding specified real approximation value. The data record contains two approximation values, one for each parameter direction.
CHORDAL SIZE in DC : Generate at least sufficient sample points such that the length in Device Coordinates of every line segment (chord) between the sample points for both the u and v parameter directions is less than the corresponding specified real approximation value. The data record contains two approximation values, one for each parameter direction.
PLANAR DEVIATION in WC : Generate at least sufficient sample points such that the length in Normalized Projection Coordinates of every line segment (chord) between the sample points for both the u and v parameter directions is less than the corresponding specified real approximation value. The data record contains two approximation values, one for each parameter direction.
PLANAR DEVIATION in NPC : Generate at least sufficient sample points such that the absolute value of the maximum deviation measured in Normalized Projection Coordinates between the surface and the approximating facet is less than a specified real approximation value. The data record contains the approximation value.
PLANAR DEVIATION in DC : Generate at least sufficient sample points such that the absolute value of the maximum deviation measured in Device Coordinates between the surface and the approximating facet is less than a specified real approximation value. The data record contains the approximation value.
RELATIVE in WC : A single real approximation value between 0 and 1 indicates a relative quality of rendering to be maintained independent of scaling in World Coordinates. Higher values specify a better relative quality. The data record contains the approximation value.
RELATIVE in NPC : A single real approximation value between 0 and 1 indicates a relative quality of rendering to be maintained independent of scaling in Normalized Projection Coordinates. Higher value specify a better relative quality. The data record contains the approximation value.
RELATIVE in DC : A single real approximation value between 0 and 1 indicates a relative quality of rendering to be maintained independent of scaling in Device Coordinates. Higher values specify a better relative quality. The data record contains the approximation value.
It is not intended that there be any correspondence between the approximations produced by different approximation criteria types. For example, the set of all possible approximations achievable by CHORDAL SIZE in WC may be disjoint from the set of all possible approximations achievable by RELATIVE in WC.