Rendering hair is a difficult problem for triangle rendering because the number of hairs is typically very large (hundreds of thousands or millions), and each hair has a very large bounding box that it fills poorly. For this reason a special hair primitive was introduced which is based on a curve description of hairs with a compact data layout to avoid the storage overhead in comparison to triangle strips.
object "object_name" [ visible [on|off] ] [ shadow [on|off] ] [ shadowmap [on|off] ] [ trace [on|off] ] [ reflection [mode] ] [ refraction [mode] ] [ transparency [mode] ] [ select [on|off] ] [ tagged [on|off] ] [ caustic [on|off] ] [ globillum [on|off] ] [ caustic [mode] ] [ globillum [mode] ] [ finalgather [mode] ] [ box [xmin ymin zmin xmax ymax zmax] ] [ motion box [xmin ymin zmin xmax ymax zmax] ] [ max displace value ] [ samples min max ] [ data (null|"name") ] [ tag number ] hair [ material "name" ] [ radius radius ] [ approximate segments ] [ approximate hair curve_approx ] [ degree degree ] [ max size size ] [ max depth depth ] [ hair n ] [ hair m hm ] [ hair t ht ] [ hair u hu ] [ hair radius ] [ vertex n ] [ vertex m vm ] [ vertex t vt ] [ vertex u vu ] [ vertex radius ] scalar [ nscalars ] scalar_list hair [ nhairs ] hair_offset_list end hair end object
The object header is similar to the headers for all other
geometry types, but the standard
block is replaced with
hair...end hair. This block
begins with a common header, followed by a scalar list, followed by
a hair list. The header has the following optional statements:
brackets, followed by scalar_list containing the actual scalar values.
brackets, followed by hair_offset_list containing the actual offset integer values.
It does not make sense to specify hair n if there is also a vertex hair because all hair normals will be overridden. Similarly, if vertex radii are used, no hair radii (or the global radius) should be present. If any normals are specified, the hairs are not intersected like cylinders but like oriented flat ribbons.
The scalar list defines a sequence of hairs. Each hair consists of a certain number of scalars that describe the entire hair, followed by another number of scalars that describe each vertex of the hair. The layout of these sequences of scalars is identical for all hairs, except that each hair may have a different number of vertices. It is not possible to have one hair with three texture scalars and another with two, for example. Here is the exact sequence of scalars for a single hair:
All vertices begin with three scalars for the location. The
order of the other vertex scalars, and the order of the header
scalars, is determined by the order of hair and vertex statements.
The lists above correspond to the syntax listing at the beginning
of this section: first
Each hair has only one header but multiple vertices. As
described above, the number of vertices must be (1 + degree·segments), where segments may be
different for each hair. This number is not encoded in the hair but
in the separate hair list. Note, that hairs do not use texture
vectors like polygons and free-form surfaces but texture
scalars. It's up to the shader to interpret
state→tex_list as a list of scalars, and properly use
them in groups of one, two, three, or whatever is appropriate. This
makes it possible to avoid the third null component if only
two-component UV textures are needed, for example, which can save a
lot of memory because hair objects tend to have a very large number
of vertices, probably millions.
The hair list specifies where in the scalar list each hair begins, by offset in the scalar list such that 0 is the first scalar, 1 is the second scalar, and so on. At the end of this offset list, one extra offset specifies the last scalar plus one, where the next hair would begin if there were another one. If all scalars are used (which is normally the case), the first offset is 0 and the extra one at the end equals the number of scalars. Here is a simple example for a hair object:
object "hair1" visible trace shadow tag 1 hair material "mtl" radius 0.3 degree 1 hair t 2 vertex t 1 scalar [ 42 ] 11 22 0.0 0.0 0.0 1111 0.0 1.0 0.0 1112 1.0 1.0 0.0 1113 33 44 0.0 0.0 0.0 1114 0.0 -1.0 0.0 1115 -1.0 -1.0 0.0 1116 55 66 0.0 0.0 0.0 1117 0.0 -1.0 0.0 1118 -1.0 -1.0 0.0 1119 hair [ 4 ] 0 14 28 42 end hair end object
This example consists of three hairs, each with three vertices.
The header of each hair consists of two texture scalars
33 44, and
respectively). Each vertex consists of four scalars, three for the
location in object space and
one more for a vertex texture. The shader will receive three
texture scalars, two from the hair and one more from the vertices.
The former are copied from the hair that was hit, and the latter is
interpolated from the nearest two vertices. They are stored in
state→tex_list as if it were a scalar array, header
texture scalars first, so the two hair texture scalars end up in
tex_list.y, and the
vertex texture scalar ends up in
tex_list.z. It is
best to re-cast
tex_list to a miScalar pointer in the
Hair objects may use the same material shaders as any other object, but often special hair material shaders are used because although hair may be a cylinder, it is too thin for properly resolving the upper and lower edge and the diffuse terminator and the highlight. There is a simplified hair illumination shader in the base shader library called mib_illum_hair that implements a much more effective hair shading model.
When rendering hair with ray tracing, mental ray uses the ray origin to generate a temporary, flat surface which is as ray-facing as possible while lying on the hair axis. A corresponding normal is generated. When rendering with the rasterizer, the hair geometry is converted to camera-facing geometry, and intersected in the same manner as other geometric types. Note, that a switch from the latter to the former is performed if both the rasterizer and the ray tracing are active. Hair intersections cannot happen on the same hair strand twice in a row, thus avoiding surface acne from this switch. This does have the consequence that individual hair strands cannot cast shadows on themselves.
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