Interface and method of interfacing between a parametric modelling unit and a polygon based rendering system

A 3-d graphics system comprising apparatus for determining a surface normal vector for a vertex of a surface patch used in modelling of an object to be shaded by the system, the apparatus comprising:

• means for subdividing the patch to produce a plurality of sub-patches each having corner vertices;
• means for deriving the locations of control points required for derivation of a surface normal for a vertex;
• means for deriving a plurality of candidate tangent vectors at a vertex from the control point locations;
• and
• means for deriving a surface normal from the candidate tangent vectors.

A method for determining a surface normal vector for a vertex of a surface patch used in modelling of an object to be shaded in a 3-d graphics system, the method comprising the steps of:

• subdividing the patch to produce a plurality of sub-patches each having corner vertices;
• deriving the locations of control points required for determining a surface normal for a vertex;
• deriving a plurality of candidate tangent vectors at a vertex from the control point data;
• and
• deriving a surface normal from the candidate tangent vectors.

A method for determining surface normal vectors for vertexes of a surface patch used in modelling of an object to be shaded in a 3-d graphic system, the method comprising the steps of:

• deriving surface normals for four corner vertices;
• partially subdividing the patch in a first parameter dimension to derive a first subset of control points of a first sub-patch;
• partially subdividing in a second parameter dimension to derive a second subset of control points of a second sub-patch;
• performing a third partial subdivision to derive a third subset of control points from the first and second subsets of control points;
• deriving surface normals for two mid-point vertices using the first subset of control points;
• deriving surface normals for the remaining two mid-point vertices from the second subset of control points;
• deriving a surface normal for a vertex at the centre of the patch from the third subset of control points.