The RenderMan Interface - Paul Bourke

The code array is a sequence of machine language-like instructions describing the
object’s primitive blob fields and the operations that combine them. Floating point
parameters of the primitive fields are stored in the floats array. File names of the
depth files of repellers are in the strings array. The integer nleaf is the number of
primitive blobs in object, also the number of items in each varying or vertex parameter.
Parameters with storage class constant or uniform have exactly one data value
for the entire RiBlobby primitive.
Each instruction has a numeric opcode followed by a number of operands. Instructions
specifying primitive fields start at 1000 and are listed in Table 5.2.
Opcode Operands Operation
1000 float constant
1001 float ellipsoid
1002 float segment blob
1003 float, string repelling plane
1004-1099 reserved
Table 5.2: RiBlobby opcodes for primitive fields.
For all four of these operators, the operands are indices into the appropriate arrays.
For opcode 1000 (constant) the operand indexes a single floating-point number
in the floats array. The index of the first item in the array is zero.
• For opcode 1001 (ellipsoid) the operand indexes the first of 16 floats describing
a 4x4 matrix that transforms the unit sphere into the ellipsoidal bump in object
space.
• The operand of opcode 1002 (segment blob) indexes 23 floats that give the endpoints
and radius of the segment and a 4x4 matrix that transforms the segment
into object space.
• Opcode 1003 (repelling ground plane) takes two indices. The first gives the index
of the name of a depth map in the strings array. The second indexes the first
of 4 float parameters of the repeller’s repulsion contour. The value of the field
generated by a repeller is a function of the vertical distance from the evaluation
point to the z-file, in the view direction in which the z-file was generated. The
four float parameters control the shape of the repelling field. Let’s call the four
parameters A, B, C and D. A controls the overall height of the repeller. The
field value is zero whenever the height above the ground plane is larger than A.
B controls the sharpness of the repeller. The field looks a lot like −B/z (except
that it fades to zero at z = A, and remains at a large negative value when z < 0),
so smaller values of B place the knee in the curve closer to z = 0. Added to
this negative-going barrier field is a bump that has its peak at z = C, and whose
maximum value is D. The bump component is exactly zero outside the range
0 ≤ z ≤ 2C.
There are several more opcodes that compute composite fields by combining the results
of previous instructions in various ways. Every instruction in the code array has
a number, starting with zero for the first instruction, that when used as an operand
refers to its result. The combining opcodes are given in Table 5.3.
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