Hair Geometry

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 "name_object"  
        [ 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 [x_min y_min z_min x_max y_max z_max] ]  
        [ motion box [x_min y_min z_min x_max y_max z_max] ]  
        [ max displace value ]  
        [ samples min  max ]  
        [ data null|"name_data" ]  
        [ tag number_label_int ]  
        hair  
            [ material "name_material" ]  
            [ radius radius ]  
            [ approximate segments ]  
            [ 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 group...end group 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:

The material specifies the material to use when shading a hair. It is shared by all hairs in this object. If missing, the material is inherited from the instance.
The radius specified the global radius of all hairs. This number may be overridden by hair or vertex radii if defined. It is defined in object space and typically a very small number. The default is 1.
The approximate specifies the number of segments used to approximate each Bézier hair segment, if the degree is 2 or 3. The default is 1.
The degree is the degree of the Bézier curves that approximate each hair. Valid values are 1 (linear), 2 (quadric), or 3 (cubic). Each hair must have (1 + degree · segments) vertices, where the number of segments is a positive number.
The max size defines the leaf size of the hair BSP tree that is created by mental ray to facilitate finding hairs during rendering. The default is 32. For a discussion of BSP trees, see the options block, which defines parameters for the BSP tree used for triangle intersection.
The max depth defines the depth of the hair BSP tree. The default is 20.
The hair n statement, if present, specifies that each hair has an associated normal vector consisting of three scalars.
The hair m statement, if present, specifies that each hair has hm associated motion vectors consisting of three scalars each.
The hair t statement, if present, specifies that each hair has ht associated texture scalars.
The hair u statement, if present, specifies that each hair has hu associated user scalars.
The hair radius statement, if present, specifies that each hair has an associated radius. No global radius may be present.
The vertex n statement, if present, specifies that each vertex has an associated normal vector consisting of three scalars. If present, hair n is ignored.
The vertex m statement, if present, specifies that each vertex has vm associated motion vectors consisting of three scalars each. If present, hair m is ignored.
The vertex t statement, if present, specifies that each vertex has vt associated texture scalars.
The vertex u statement, if present, specifies that each vertex has vu associated user scalars.
The vertex radius statement, if present, specifies that each vertex has an associated radius. No global radius may be present. If present, hair radius is ignored.
The scalar list begins with the number of scalars that follow in angle brackets, followed by that many scalar values.
The hair list begins with the number of hairs that follow plus 1 in angle brackets, followed by that many hair offsets.

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:

Header:

If there was a hair n statement, three scalars for the hair normal (X, Y, Z).
hm motion scalars (X, Y, Z, X, Y, Z, ...).
ht texture scalars.
hu user scalars.
If there was a hair radius statement, one radius scalar.

Vertices:

Three mandatory scalars that define the location of the vertex in object space (X, Y, Z).
If there was a vertex n statement, three scalars for the vertex normal (X, Y, Z).
vm motion scalars (X, Y, Z, X, Y, Z, ...).
vt texture scalars.
vu user scalars.
If there was a vertex radius statement, one radius scalar.

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 n, then m, then t, then u, then radius.

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 needed. 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 (11 22, 33 44, and 55 66, 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[0].x and tex_list[0].y, and the vertex texture scalar ends up in tex_list[0].z. It is best to re-cast tex_list to a miScalar pointer in the hair shader.

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, mib_illum_hair that implements a much more effective hair shading model.

When rendering hair with the raytracer, 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 raytracer 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.