Clay SOP
Summary
The Clay SOP deforms faces and surfaces by pulling points that lie directly on them. As opposed to the Point SOP or other SOPs that manipulate control points (CVs), the Clay SOP operates on the primitive contours themselves, providing a direct, intuitive, and unconstrained way of reshaping geometry. Thus, rather than translating CVs to change the aspect of the primitive, the Clay SOP takes the inverse approach of manipulating the primitive's skin to reposition the CVs.
The point that defines the area to be modified is called a "target point" or "target" for short. It is expressed as a (u,v) pair in the parametric space of the primitive and ranges between 0 and 1 in both U and V. The image of the target point on the primitive is a 3D point which Clay can displace in several ways. Furthermore, if the primitive is a surface, there is are options to pull only the point or a whole isoparametric curve in either U or V.
Clay does not refine the faces and surfaces unless asked to, so the complexity of the geometry does not increase. The area affected by the change varies with each primitive type and topology. In all cases it is possible to reduce to amount of change by inserting a Refine SOP before the Clay SOP and inserting detail around the target point. For other ways to increase the locality of the deformation as well as its sharpness, see U and V Sharpness below.
If a second input is present, it is possible to snap the target (u,v) point to an (s,t) point on the first primitive of the second input. Without a second input, the primitives can be made to snap to themselves. Moreover, the Clay SOP is able to snap the target to arbitrary points in space.
Both this SOP and the Align SOP can be used effectively as snapping tools and building blocks for curve networks. The main difference between the two SOPs is that Clay deforms the inputs partially, while Align translates and/or rotates the whole primitive.
The Clay SOP accepts a mix of any combination of face and surface types.
Contents
Parameters
Group  If there are input groups, specifying a group name in this field will cause this SOP to act only upon the group specified. Accepts patterns, as described in Pattern Matching in the Scripting Guide.
U  Determines whether the clay operation affects the U parametric direction. Both faces and surfaces respond to this option. If the primitives are face types and the toggle is off, clay doesn't change the inputs at all. If the inputs are surfaces and U is off, clay will pull the surfaces along the entire V direction (if V is on) or not at all.
V  Determines whether the clay operation affects the V parametric direction of the surface. If the inputs are surfaces and V if off, clay will pull the surfaces along the entire U direction (if U is on) or not at all. If both U and V are on, clay will pull the surface at the target (U, V) point specified in the U and V folders below. This option does not affect face types.
Parameters  U Page
U /u
 This value defines a point in the parametric space of a face, representing the location to be affected by the clay operation. This location is referred to as the "target". For surfaces, U defines a line of constant value in the parametric space of the primitive and requires a second coordinate  V  to specify a unique location (see the V Page below). Spline faces and surfaces have explicit parametric spaces known as domains; since domains are not restricted to the unit range [0, 1], the Clay SOP maps U to the real domain value of the primitive. For polygons and meshes, U is expressed in terms of the number or vertices and columns respectively and in terms of their relative positions.
U Bias /ubias
 Indicates whether the clay algorithm should use the bias it thinks works best for the given U parameter, or take the value explicitly stated beside the toggle. Since clay affects the CVs in the neighbourhood of the given parametric location, the bias can influence the amount of pull applied to the CVs on either side of this location. The effect is a slant of the "wave" the parametric point rides on  towards one side or the other.
U Sharpness /usharp
 This parameter affects only NURBS curves and surfaces. The pull generated by clay on these primitives can be smooth or sharp depending on the position of the target relative to the underlying domain (the farther away from a knot, the rounded the bulge) and the knot multiplicity near the target (the higher the multiplicity the sharper the pull). If the pull is too round or affects too big an area in U, the U Sharpness parameter can reduce it by inserting one or more knots at the target U value. When the U Sharpness is zero no knots are inserted. When the U Sharpness is 1, all "degree" knots are inserted and the shape becomes very sharp. The U Sharpness varies in discrete steps; the number of steps equals the U degree of the spline.
Note: The range of the U Sharpness slider varies with the degree of the spline (i.e. the closer it is to 1, the more knots it adds). Since the number of knots added forms a discrete sequence, the slider will automatically jump to the valid positions.
Parameters  V Page
V /v
 This value affects only surface types. V defines a line of constant value in the parametric space of the surface and together with U specifies a unique location in that space. Spline surfaces have explicit parametric spaces known as domains; since domains are not restricted to the unit range [0, 1], the Clay SOP maps V to the real domain value of the surface. For meshes, V is expressed in terms of the number or rows and their relative positions.
V Bias /vbias
 Indicates whether the clay algorithm should use the bias it thinks works best for the given V parameter, or take the value explicitly stated beside the toggle. Since clay affects the CVs in the neighbourhood of the given parametric location, the bias can influence the amount of pull applied to the CVs on either side of this location. The effect is a slant of the "wave" the parametric point rides on  towards one side or the other.
V Sharpness /vsharp
 This parameter affects only NURBS surfaces. The pull generated by clay on these primitives can be smooth or sharp depending on the position of the target relative to the underlying domain (the farther away from a knot, the rounded the bulge) and the knot multiplicity near the target (the higher the multiplicity the sharper the pull). If the pull is too round or affects too big an area in V, the V Sharpness parameter can reduce it by inserting one or more knots at the target V value. When the V Sharpness is zero no knots are inserted. When the V Sharpness is 1, all "degree" knots are inserted and the surface becomes very sharp. The V Sharpness varies in discrete steps; the number of steps equals the V degree of the spline.
Note: The range of the V Sharpness slider varies with the degree of the spline (i.e. the closer it is to 1, the more knots it adds). Since the number of knots added forms a discrete sequence, the slider will automatically jump to the valid positions.
Parameters  Transformation Types
The following describe the four types of transformations the Clay SOP can apply to the target (U, V) points or isoparametric curves (U or V). All the transformations are exclusive, and all reduce to a signed translation along a direction. The difference between the various transformations is the way the translation and the direction are specified.
Matrix  The matrix method relocates the target point based on a transformation matrix. For a description of transforms, see the Transform SOP.
Vector  The folder method provides a distance and a vector to translate along.
Distance /dist
 Specifies the translation distance along the given vector.
Normal  The translation will be performed along the primitive normal at the target (U, V) point if the Normal button is On. Otherwise an explicit direction is required (see Direction below). If the input contains face primitives, the primitive normal is independent of the target (U) point. Primitive normals can be displayed from the Display Options dialog.
Direction /dirx /diry /dirz
Normal
Point  The point method provides a 3D point in object space that the target point must snap to.
Coordinates /coordx /coordy /coordz
 Specifies the absolute 3D location the (U, V) target must translate to.
Primitive  The primitive method allows the target point to be snapped to a point on another primitive. Any primitive type is allowed, including metaballs, all quadrics, and even particle systems. The point on the destination primitive is expressed parametrically as a (U, V) pair.
If the Clay SOP has a second input, then the destination primitive is going to be the first primitive in that input. If there is no second input, the primitives in the first input will snap to themselves (each face or surface to itself).
UV /uvsnap1 /uvsnap2
