This export converter writes out ASCII XGL formatted files as well as
their ZGL compressed equivalents. Triangular mesh data (with vertex normals
and vertex uv texture coordinates), material properties, embedded texture
images and object hierarchy are exported from the XML-style XGL file.
The XGL file format is designed to represent 3D information for the
purpose of visualization. It attempts to capture all of the 3D information
that can be rendered by SGI's OpenGL rendering library. XGL uses XML 1.
0 syntax, so it is easy to write and be parsed. XGL is quickly becoming
the ideal format for transmitting 3D data over the Internet and enabling
applications with complex functionality, such as collaboration, configuration,
and data association. The XGL file format is defined and maintained by the
XGL Consortium.
Viewing XGL Files
The primary method to view XGL files is with the free XGL file viewer
distributed by RealityWave (www.realitywave.com).
RealityWave also provides XGL streaming capabily across the Internet using their VizStream WebKit.
Okino's NuGraf is ideal for creating, importing, editing, and photo-realistic
rendering of XGL objects. Both, NuGraf and PolyTrans, are fully capable
of importing and exporting XGL files.
Limitations of the XGL File Format
XGL is an adequate, but not ideal 3D file format to translate scene data from
one program to another. It seems to have been written under the general
assumption that the exported data will be sent to a viewing program, and to a
lesser degree to another 3D program. As such, it is not a perfect file format
to move complete scene data from one program to another. The following lists
some of the limitations of the XGL file format when used as a mechanism to
move scene data between 3D software packages:
XGL file format is not the most ideal for converting 3D scenes with object
hierarchy. The main problem is that the "transform" tag does not allow for
non-uniform scaling or off-axis scaling to be defined for an objects
transformation node. This problem can be overcome by transferring the mesh
data in absolute world space coordinates (the transformation matrix is
embedded directly with the mesh vertices), but that does not allow for the
transfer of any pivoting information.
All mesh data must be turned into triangles.
Vertex color and UV tangent attributes for mesh vertices is not provided.
No naming of objects or null hierarchy nodes is possible.
All texture maps must be embedded within the XGL file. Texture filename
references are not provided.
Transformations with off-axis scaling or non-uniform scaling are not
accomodated.
Only directional light sources are provided. Point and spot lights are not.
Otherwise, XGL is an adequate file format for moving general purposes
triangular-based mesh data between 3D programs, or a 3D program and a
destination viewing program, with associated color and texture attributes,
some lighting and hierarchy.
WEB Site References
The XGL file format specification is contained on the WEB site
www.xglspec.org.
If this option is enabled (checkmarked) then the tag will be output to the XGL file along with each mesh object. This will tell the destination XGL viewer program to allow both sides of the each polygon to be viewable. If this option is disabled then the destination viewer will cull polygons which are "back facing", those whose geometric normal faces away from the camera.
Weight Colors by Shading Coefficients
Internally within Okino's NuGraf and PolyTrans software a material is associated with a number of attributes, some of which are the basic colors (ambient, diffuse, specular and luminous) and their corresponding "shading coefficients". The shading coefficients are basically intensity controls for the materials colors. For example, lowering the diffuse shading coefficient will make the diffuse color darker during rendering.
If this option is enabled (checkmarked) then the shading coefficients will be multiplied into their corresponding material colors during export. This is the desired manner in which Okino materials are used. If this option is disabled then the shading coefficients will be ignored, and mostly probably this will result in exported materials which will appear brighter and more saturated.
Flip Model so that 'Z' Axis is Up
If this option is enabled (checkmarked) then the scene will be reoriented from Okino's internal Y-Up axis orientation to the XGL-preferred Z-Up axis orientation.
Compress output file (to ZGL file)
If this option is enabled (checkmarked) then the final ASCII file will be compressed with ZLIB to create a .zgl compressed file. This will make the file much smaller in size.
Output Linefeeds for Easier Reading ASCII Files
If this option is enabled (checkmarked) then entities output to the XGL file
will be on their own separate lines in the file, while if this option is
disabled then all entities will be output to the XGL file in one long line, with
no CR/LF's separating each line. This will reduce file size but make the
file near-impossible to read in a text editor.
Output Hierarchy for Geometry Objects
If this option is enabled (checkmarked) then object hierarchy will be
recreated inside the XGL file by using <object> tags inside each other. By
default this option is disabled in which case all mesh objects are output
in world-space coordinates and without any hierarchical relationships.
Output Enables
These checkbox options control what type of data will be output to the XGL file.
Vertex Normals
If this checkbox is enabled (checkmarked, which is the default) then vertex normals will be output to the XGL file. Vertex normals define how smooth or how faceted/angular the mesh data will appear.
Lights
If this checkbox is enabled (checkmarked, which is the default) then lights will be output to the XGL file. Only ambient and directional lights are accommodated by XGL. Point and spot lights will be turned into semi-equivalent directional light sources.
Materials
If this option is enabled (checkmarked) then material definitions will be output and associated with each mesh object. Ambient, diffuse, specular and luminous colors will be output as well as transparency and shininess.
Vertex uv Texture Coordinates
If this checkbox is enabled (checkmarked, which is the default) then (u,v) texture coordinates will be output to the XGL file. (u,v) texture coordinates are needed for texture mapping. They define which part of a flat texture map will get applied to which part of the 3D mesh object.
Embed texture images in file
If this option is enabled (checkmarked), and the 'Output Materials' option is enabled, then any texture map images will be directly embedded within the XGL file. This is the normal way of transferring texture images to an XGL viewer program.
Background Color
If this option is enabled (checkmarked) then the background color will be output to the XGL file via the tag.
Texture Map Application Mode:
These tags specify how texture pixels are combined with underlying color from the material. The texture color can either completely replace the underlying color, multiply with it, or mix with it.
The following notation is used for this section:
C Color value of the combined pixel
A Alpha value of the combined pixel
Ct Color value (R,G,B) of the pixel in the texture
Cf Color value (R,G,B) of the pixel in the face
At Alpha value of the pixel in the texture
Af Alpha value of the pixel in the face
Replace
This tag specifies the replace texture function. The color obtained from the texture map will completely overwrite the base material color of an object.
Texture pixels will be combined with face pixels as follows:
C = Ct
A = At
Modulate
This tag specifies the modulate texture function. The color obtained from the texture map will be multiplied into the base material color of an object.
Texture pixels will be combined with face pixels as follows:
C = Cf * Ct
A = Af * At
Decal
This tag specifies the decal texture function. The color obtained from the texture map will be mixed with the base material color of an object based on the alpha channel of the texture map.
Texture pixels will be combined with face pixels as follows:
Overview