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MATERIALS AND GEOMETRY FORMAT |
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SCCSid "$SunId$ LBL" |
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|
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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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|
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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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|
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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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|
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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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|
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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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|
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Without further ado, here are the proposed entities and their interpretations: |
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|
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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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|
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These are the context dependencies of each entity: |
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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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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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files are not affected. |
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|
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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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|
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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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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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applying this transformation twice, and so on up to N-1 applications. |
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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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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|
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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: |
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m bright_emitter |
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cyl cv1 .15 cv2 |
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m dark |
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ring cv1 0 .15 |
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ring cv2 0 .15 |
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|
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The following is a more typical example, which relies on a material library: |
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|
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# Include our materials: |
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i material.mgf |
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# Modify red_plastic to have no specular component: |
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m red_plastic |
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rs 0 0 |
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# Make an alias for blue_plastic: |
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m outer_material = blue_plastic |
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# Make a new material based on brass, with greater roughness: |
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m rough_brass = brass |
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c brass_color |
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rs 0.9 0.15 |
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# Load our vertices: |
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i lum1vert.mgf |
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# Modify appropriate vertices to make luminaire longer: |
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v v10 |
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p 5 -2 -.1 |
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v v11 |
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p 5 2 -.1 |
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v v8 |
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p 5 2 0 |
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v v9 |
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p 5 -2 0 |
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# Load our surfaces, rotating them -90 degrees about Z: |
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i lum1face.mgf -rz -90 |
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# Make a 2-D array of sequins covering the face of the fixture: |
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m silver |
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i sequin.mgf -a 5 -t .5 0 0 -a 4 -t 0 .75 0 |
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|
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Note that by using libraries and modifying values, it is possible to create |
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a variety of fixtures without requiring large files to describe each one. |
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|
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Interpretation |
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============== |
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Interpretation of this language will be simplified by the creation |
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of a general parser that will be able to express the defined entities |
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in simpler forms and remove entities that would not be understood by |
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the caller. |
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|
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For example, a caller may ask the standard parser to produce only |
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the entities for diffuse uncolored materials, vertices without normals, |
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and polygons. The parser would then expand all include statements, |
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remove all color statements, convert spheres and cones to polygonal |
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approximations, and so forth. |
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|
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This way, a single general parser can permit software to operate |
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at whatever level it is capable, with a minimal loss of generality. |
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Furthermore, distribution of a standard parser will improve |
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both forward and backward compatibility as new entities are added |
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to the specification. |
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|
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Rationale |
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========= |
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Why create yet another file format for geometric data, when so many |
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others already exist? The main answer to this question is that we |
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are not merely defining geometry, but materials as well. Though the |
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number of committee and de facto standards for geometric data is large, |
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the number of standards for geometry + materials is small. Of these, |
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almost all are non-physical in origin, i.e. they are based on common, |
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ad hoc computer graphics rendering practices and cannot be used to create |
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physical simulations. Of the one or two formats that were intended |
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for or could be adapted to physical simulation, the syntax and semantics |
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are at the same time too complex and too limiting to serve as a suitable |
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standard. |
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|
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Specifically, establishing the above, new standard has the following |
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advantages: |
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|
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o It is easy to parse. |
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o It is easy to support, at least as a least common denominator. |
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o It is ASCII and fairly easy for a person to read and understand. |
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o It supports simple color, material and vertex libraries. |
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o It includes a simple yet fairly complete material specification. |
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o It is easy to skip unsupported entities (e.g. color, vertex normals) |
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o It supports transformations and instances. |
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o It is easy to add new entities, and as long as these entities can |
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be approximated by the original set, backwards compatibility |
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can be maintained through a standard parsing library. |
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|
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Most of the disadvantages of this format relate to its simplicity, but |
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since simplicity was our most essential goal, this could not be helped. |
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Specifically: |
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|
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o There is no general representation of curved surfaces (though |
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vertex normals make approximations straightforward). |
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o There are no general surface scattering functions. |
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o There are no textures or bump-maps. |
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|
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If any of these seems particularly important, I will look into adding them, |
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though they will tend to complicate the specification and make it more |
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difficult to support. |
