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greg |
1.1 |
MATERIALS AND GEOMETRY FORMAT
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SCCSid "$SunId$ LBL"
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Introduction
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============
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The following file format is a simple ASCII representation of surface
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geometry and materials for the purpose of visible-light simulation
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and rendering. The overall objective of this format is to provide
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a very simple yet fairly complete modeling language that does not
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place unreasonable demands on the applications programmer or the
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object library creator.
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Similar to Wavefront's .OBJ file format, our format utilizes a
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number of object entities, one per line, some of which establish
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a context for the entities that follow. Specifically, there is
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a context for the current vertex, the current color, and the
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current material. The current vertex is used only for setting
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values related to that vertex. The current color is used for
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setting values related to that color, as well as by certain
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material attributes which take an optional color setting.
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The current material is used for setting material-related
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parameters, and for establishing the material for the following
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geometric entities. In addition to these three named contexts,
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there are two hierarchical (i.e. cumulative) contexts, the
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current transform and the current object name.
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Each entity is given by a short keyword, followed by space- or tab-
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delimited arguments on a single line. A single entity may be extended
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over multiple lines using a backslash ('\') character right before the
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end of line, though no extended line may exceed 512 characters in total
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length. (Given the current set of entities, even approaching 80
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characters would be highly unusual.)
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Entities and Contexts
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=====================
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There are three contexts in effect at all times, current vertex,
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current color and current material. Initially, these contexts are
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unnamed, and have specific default values. The unnamed vertex is the
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origin. The unnamed color is neutral gray. The unnamed material is a
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perfect absorber. The unnamed contexts may be modified, but those
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modifications will not be saved. Thus, reestablishing an unnamed
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context always gets its initial default value. To save a new context
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or modify an old one, it must first be named. Entities associated with
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named contexts (i.e. "v", "c" and "m") may be followed by an identifier
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and an equals sign ('='), indicating a new context. If there is no
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equals, then the context must already be defined, and the appearance of
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the entity merely reestablishes this context. If the context id is
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followed by an equals, then a new context is defined, destroying any
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previous instance of that context name. Redefining or changing values
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of a context does not affect earlier uses of the same name, however.
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Contexts are always associated with a name id, which is any non-blank
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sequence of printing ASCII characters. An optional template may be
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given following the equals, which is a previously defined context to
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use as a source of default values for this definition. If no template
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is given, then the unnamed context of that type is used to set initial
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values. Named contexts continue until the next context definition of
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the same type.
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Hierarchical Contexts
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=====================
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Two entities define a second type of context, which is hierarchical.
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These are the transform ("xf") entity and the object ("o") entity.
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The object entity is used simply for naming collections of surfaces.
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An object entity with a name applies to the following surfaces up
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until an object entity with no name, which signifies the end of this
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object's scope. Object entities may be nested to any level, and
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can be thought of as parts and subparts of an enclosing global object.
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Note that this is strictly for ease of identification, and has no
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real meaning as far as the geometric description goes. In contrast,
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the transform entity is very significant as it determines how enclosing
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objects are to be scaled and placed in the final description.
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Without further ado, here are the proposed entities and their interpretations:
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Keyword Arguments Meaning
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------- --------- -------
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# anything a comment
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i filename [xform] include file (with transformation)
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ies filename [-m f][xform] include IES luminaire (with transformation)
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v [id [= [template]]] get/set vertex context
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p x y z set point position for current vertex
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n dx dy dz set surface normal for current vertex
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c [id [= [template]]] get/set color context
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cxy x y set CIE (x,y) chromaticity for current color
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cspec l_min l_max v1 v2 .. set relative spectrum for current color
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cmix w1 c1 w2 c2 .. mix named colors to make current color
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m [id [= [template]]] get/set material context
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rd rho_d set diffuse reflectance for current material
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td tau_d set diffuse transmittance for current material
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ed epsilon_d set diffuse emittance for current material
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rs rho_s alpha_r set specular reflectance for current material
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ts tau_s alpha_t set specular transmittance for current material
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o [name] begin/end object context
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f v1 v2 v3 .. polygon using current material, spec. vertices
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sph vc radius sphere
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cyl v1 radius v2 truncated right cylinder (open-ended)
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cone v1 rad1 v2 rad2 truncated right cone (open-ended)
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prism v1 v2 v3 .. length right prism (closed solid)
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ring vc rmin rmax circular ring with inner and outer radii
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torus vc rmin rmax circular torus with inner and outer radii
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xf [xform] begin/end transformation context
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These are the context dependencies of each entity:
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Entities Contexts
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-------- --------
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p, n vertex
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cxy, cspec, cmix color
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rd, td, ed, rs, ts color, material
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f, sph, cyl, cone, ring, torus, prism material, object, transformation
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Transformations
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===============
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A rigid body transformation is given with the transform entity, or as
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part of an included file. The following transformation flags and
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arguments are defined:
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-t dx dy dz translate objects along the given vector
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-rx degrees rotate objects about the X-axis
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-ry degrees rotate objects about the Y-axis
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-rz degrees rotate objects about the Z-axis
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-s scalefactor scale objects by the given factor
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-mx mirror objects about the Y-Z plane
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-my mirror objects about the X-Z plane
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-mz mirror objects about the X-Y plane
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-i N repeat the following arguments N times
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-a N make an array of N geometric instances
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Transform arguments have a cumulative effect. That is, a rotation
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about X of 20 degrees followed by a rotation about X of -50 degrees
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results in a total rotation of -30 degrees. However, if the two
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rotations are separated by some translation vector, the cumulative
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effect is quite different. It is best to think of each argument as
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acting on the included geometric objects, and each subsequent transformation
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argument affects the objects relative to their new position/orientation.
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For example, rotating an object about its center requires translating
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the object back to the origin, applying the desired rotation, and translating
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it again back to its original position.
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Rotations are given in degrees counter-clockwise about a principal axis.
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That is, with the thumb of the right hand pointing in the direction
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of the axis, rotation follows the curl of the fingers.
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The transform command itself is also cumulative, and a transform
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command with no arguments is used to return to the previous
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greg |
1.2 |
condition. It is necessary that transforms and their end statements
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("xf" by itself) be balanced in a file, so that later or enclosing
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greg |
1.1 |
files are not affected.
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Transformations apply only to geometric types, e.g. polygons, spheres, etc.
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Vertices and the components that go into geometry are not directly affected.
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This is to avoid confusion and the inadvertent multiple application of a
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given transformation.
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Arrays
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======
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The -a N transform specification causes the following transform
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arguments to be repeated along with the contents of the included
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greg |
1.2 |
objects N times. The first instance of the geometry will be in its
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initial location; the second instance will be repositioned according
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to the named transformation; the third instance will be repositioned by
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greg |
1.1 |
applying this transformation twice, and so on up to N-1 applications.
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Multi-dimensional arrays may be specified with a single include
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entity by giving multiple array commands separated by their
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corresponding transforms. A final transformation may be given
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by preceeding it with a -i 1 specification. In other words, the
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scope of an array command continues until the next -i or -a option.
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Other Details
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=============
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End of line may be any one of the sequences: linefeed ('\n'), carriage-
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return ('\r'), or a carriage return followed by a linefeed.
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Blank lines are ignored on the input, as are any blanks preceeding
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a keyword on a line. Indentation may improve readability, especially
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in context definitions.
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The comment character ('#') must be followed by at least one blank
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character (space or tab) for easy parsing. Like any other line,
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a comment may be extended to multiple lines using a backslash ('\').
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Include filename paths are relative to the current file. Absolute
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paths are expressly forbidden. UNIX conventions should be used for the
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path separator ('/') and disk names should not be used (i.e. no
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"C:\file"). To further enhance portability across systems, directory
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names should be 8 characters or fewer with no suffix, filenames should
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fit within an 8.3 format, and all characters should be lower case.
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(They will be automatically promoted to upper case by DOS systems.)
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We suggest the standard suffix ".mgf" for "materials and geometry format".
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The XYZ coordinate system is right-handed, and lengths are always in
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SI meters. This is not really a limitation as the first statement
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in the file can always be a transform with the -s option to convert
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to a more convenient set of units. Included IES files will also start
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out in meters, and it is important to specify a transform into the
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local coordinate system. The -m option (preceeding any transform)
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may be used to specify an output multiplication factor.
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Vertex normals need not be normalized, and a normal equal to (0,0,0) indicates
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that the exact surface normal should be used. (This is the default.)
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Color in this system does not include intensity, only hue and
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saturation. Intensity, such as reflectance or emittance, is explicitly
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included in the other material parameters. All colors are absolute,
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e.g. spectral reflectance or transmittance under uniform white light.
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A CIE xy chromaticity pair is the most basic color specification.
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A full spectrum is the most general specification, and the starting
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(i.e. minimum) and ending (i.e. maximum) wavelengths are given along
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with a set of evenly spaced values. Wavelengths are given in nanometers,
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and must be within the range of 380-780. The spectral values themselves
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are located starting at the first wavelength and proceeding at even
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increments to the ending wavelength. The values in between will be
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interpolated as necessary, so there must be at least two specified points.
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The color mixing entity is intended not only for the mixing of named
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colors, but also for color specifications using an arbitrary set
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of basis functions. The actual totals for spectral and mixing
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coefficients is irrelevant, since the results will be normalized.
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Diffuse emittance is always given in SI units of lumens/meter^2. Note that
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this is emittance, not exitance, and does not include light reflected or
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transmitted by the surface.
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The roughness associated with specular reflectance and transmittance
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is the RMS surface facet slope. A value of 0 indicates a perfectly
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smooth surface, meaning that reflected or transmitted rays will not
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be scattered.
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The sum of the diffuse and specular reflectances and transmittances
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must be strictly less than one (with no negative values, obviously).
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The object entity establishes a hierarchical context, consisting of
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this identifier and all those preceding. It has no real meaning except
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to group the following surfaces up until an empty object statement
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under a descriptive name for improved file readability.
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Surfaces are one-sided, and appear invisible when viewed from the
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back side. This means that a transmitting object will affect the
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light coming in through the front surface and ignore the characteristics
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of the back surface. As long as the characteristics are the same,
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the results should be correct. If the rendering technique does not
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allow for one-sided surfaces, an approximately correct result can
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be obtained for transmitting surfaces by using the square root of
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the given tau_s and half the given alpha_t.
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The surface normal of a face is oriented by the right-hand rule.
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Specifically, the surface normal faces towards the viewer when the
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vertices circulate counter-clockwise. Faces may be concave or convex,
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but must be planar. Holes may be represented as concave polygons with
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coincident sides (i.e. seams).
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A prism consists of a set of coplanar vertices specifying an end-face,
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and a length value. The prism will be extruded so that the end-face
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points outward, unless the length value is negative, in which case the
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object is extruded in the opposite direction, resulting in inward-
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directed surface normals.
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A sphere, cylinder or cone with negative radii is interpreted as having
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an inward facing surface normal. Otherwise, the normal is assumed
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to face outwards. (It is illegal for a cone to have one positive and
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one negative radius.)
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The central vertex for a ring or torus must have an associated normal,
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which serves to orient the ring. The inner radius must be given first,
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and must be strictly less than the outer radius. The inner radius may
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be zero but not negative. There is an exception for a torus with
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inward-pointing normal, which is identified by a negative outer radius
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and a non-positive inner radius.
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Examples
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========
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The following is a complete example input file (don't ask me what it is):
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# Define some materials:
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m red_plastic =
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c red =
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cxy .8 .1
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rd 0.5
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# reestablish unnamed (neutral) color context:
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c
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rs 0.04 0.02
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m green_plastic =
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c green =
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cxy .2 .6
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rd 0.4
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c
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rs .05 0
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m bright_emitter =
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c
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ed 1000
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m dark =
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c
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rd .08
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# Define some vertices:
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v v1 =
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p 10 5 7
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v v2 =
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p 15 3 9
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v v3 =
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p 20 -7 6
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v v4 =
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p 20 10 6
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v v5 =
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p 10 10 6
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v v6 =
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p 10 -7 6
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v cv1 =
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p -5 3 8
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n 0 0 -1
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v cv2 =
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p -3 3 8
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n 0 0 1
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# make some faces:
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m green_plastic
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f v1 v3 v4
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m red_plastic
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f v3 v4 v5
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f v5 v6 v7
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m bright_emitter
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f v3 v4 v5 v6
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# make a cylindrical source with dark end caps:
|
| 324 |
|
|
m bright_emitter
|
| 325 |
|
|
cyl cv1 .15 cv2
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| 326 |
|
|
m dark
|
| 327 |
|
|
ring cv1 0 .15
|
| 328 |
|
|
ring cv2 0 .15
|
| 329 |
|
|
|
| 330 |
|
|
The following is a more typical example, which relies on a material library:
|
| 331 |
|
|
|
| 332 |
|
|
# Include our materials:
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| 333 |
|
|
i material.mgf
|
| 334 |
|
|
# Modify red_plastic to have no specular component:
|
| 335 |
|
|
m red_plastic
|
| 336 |
|
|
rs 0 0
|
| 337 |
|
|
# Make an alias for blue_plastic:
|
| 338 |
|
|
m outer_material = blue_plastic
|
| 339 |
|
|
# Make a new material based on brass, with greater roughness:
|
| 340 |
|
|
m rough_brass = brass
|
| 341 |
|
|
c brass_color
|
| 342 |
|
|
rs 0.9 0.15
|
| 343 |
|
|
# Load our vertices:
|
| 344 |
|
|
i lum1vert.mgf
|
| 345 |
|
|
# Modify appropriate vertices to make luminaire longer:
|
| 346 |
|
|
v v10
|
| 347 |
|
|
p 5 -2 -.1
|
| 348 |
|
|
v v11
|
| 349 |
|
|
p 5 2 -.1
|
| 350 |
|
|
v v8
|
| 351 |
|
|
p 5 2 0
|
| 352 |
|
|
v v9
|
| 353 |
|
|
p 5 -2 0
|
| 354 |
|
|
# Load our surfaces, rotating them -90 degrees about Z:
|
| 355 |
|
|
i lum1face.mgf -rz -90
|
| 356 |
|
|
# Make a 2-D array of sequins covering the face of the fixture:
|
| 357 |
|
|
m silver
|
| 358 |
|
|
i sequin.mgf -a 5 -t .5 0 0 -a 4 -t 0 .75 0
|
| 359 |
|
|
|
| 360 |
|
|
Note that by using libraries and modifying values, it is possible to create
|
| 361 |
|
|
a variety of fixtures without requiring large files to describe each one.
|
| 362 |
|
|
|
| 363 |
|
|
Interpretation
|
| 364 |
|
|
==============
|
| 365 |
|
|
Interpretation of this language will be simplified by the creation
|
| 366 |
|
|
of a general parser that will be able to express the defined entities
|
| 367 |
|
|
in simpler forms and remove entities that would not be understood by
|
| 368 |
|
|
the caller.
|
| 369 |
|
|
|
| 370 |
|
|
For example, a caller may ask the standard parser to produce only
|
| 371 |
|
|
the entities for diffuse uncolored materials, vertices without normals,
|
| 372 |
|
|
and polygons. The parser would then expand all include statements,
|
| 373 |
|
|
remove all color statements, convert spheres and cones to polygonal
|
| 374 |
|
|
approximations, and so forth.
|
| 375 |
|
|
|
| 376 |
|
|
This way, a single general parser can permit software to operate
|
| 377 |
|
|
at whatever level it is capable, with a minimal loss of generality.
|
| 378 |
|
|
Furthermore, distribution of a standard parser will improve
|
| 379 |
|
|
both forward and backward compatibility as new entities are added
|
| 380 |
|
|
to the specification.
|
| 381 |
|
|
|
| 382 |
|
|
Rationale
|
| 383 |
|
|
=========
|
| 384 |
|
|
Why create yet another file format for geometric data, when so many
|
| 385 |
|
|
others already exist? The main answer to this question is that we
|
| 386 |
|
|
are not merely defining geometry, but materials as well. Though the
|
| 387 |
|
|
number of committee and de facto standards for geometric data is large,
|
| 388 |
|
|
the number of standards for geometry + materials is small. Of these,
|
| 389 |
|
|
almost all are non-physical in origin, i.e. they are based on common,
|
| 390 |
|
|
ad hoc computer graphics rendering practices and cannot be used to create
|
| 391 |
|
|
physical simulations. Of the one or two formats that were intended
|
| 392 |
|
|
for or could be adapted to physical simulation, the syntax and semantics
|
| 393 |
|
|
are at the same time too complex and too limiting to serve as a suitable
|
| 394 |
|
|
standard.
|
| 395 |
|
|
|
| 396 |
|
|
Specifically, establishing the above, new standard has the following
|
| 397 |
|
|
advantages:
|
| 398 |
|
|
|
| 399 |
|
|
o It is easy to parse.
|
| 400 |
|
|
o It is easy to support, at least as a least common denominator.
|
| 401 |
|
|
o It is ASCII and fairly easy for a person to read and understand.
|
| 402 |
|
|
o It supports simple color, material and vertex libraries.
|
| 403 |
|
|
o It includes a simple yet fairly complete material specification.
|
| 404 |
|
|
o It is easy to skip unsupported entities (e.g. color, vertex normals)
|
| 405 |
|
|
o It supports transformations and instances.
|
| 406 |
|
|
o It is easy to add new entities, and as long as these entities can
|
| 407 |
|
|
be approximated by the original set, backwards compatibility
|
| 408 |
|
|
can be maintained through a standard parsing library.
|
| 409 |
|
|
|
| 410 |
|
|
Most of the disadvantages of this format relate to its simplicity, but
|
| 411 |
|
|
since simplicity was our most essential goal, this could not be helped.
|
| 412 |
|
|
Specifically:
|
| 413 |
|
|
|
| 414 |
|
|
o There is no general representation of curved surfaces (though
|
| 415 |
|
|
vertex normals make approximations straightforward).
|
| 416 |
|
|
o There are no general surface scattering functions.
|
| 417 |
|
|
o There are no textures or bump-maps.
|
| 418 |
|
|
|
| 419 |
|
|
If any of these seems particularly important, I will look into adding them,
|
| 420 |
|
|
though they will tend to complicate the specification and make it more
|
| 421 |
|
|
difficult to support.
|
