GLOD - OpenGL-level creation, management, and rendering of
multiresolution geometry.
The level of detail pipeline consists of three basic stages: geometric
simplification, adaptation, and rendering. The simplification
process takes in ``flat'' geometry and produces from it a
multiresolution hierarchy. There are several types of
mulresolution hierarchies. Discrete hierarchies are a list of
ever-simplified versions of your input model, and can be thought of as
a close analog to mip-maps for texturing. Discrete hierarchies are
extremely computationally efficient as far as rendering goes, but
have limited efficiency for close-up viewing of large-scale
geometry. Continuous hierarchies, while more computationally
complex during rendering, address many of the limitations of discrete
level of detail.
Before render this hierarchy, GLOD must pick a level within that hierarchy
which cooresponds to how much detail you want in your rendered
output. This process is called adaptation. You can adapt a
hierarchy to a variety of goals --- a triangle budget for
instance, or an error threshhold. Error thresholds are used to produce
a lower-triangle version of your model that differs by at most a
certain amount of visual detail from the original object. Error is
computed with respect either (A) to screen space, meaning the number
of pixels of error on your screen, or (B) to object space, meaning the
number of units of error in your object's coordinate space. Usually,
you don't actually adapt a single object blindly. Instead, you usually
adapt a group of objects together so that you meet some global goal.
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GLOD represents an extremely lightweight approach to Geometric Level
of Detail toolkits. You will find that GLOD is not another scene graph
library. Instead, GLOD is designed to closely mesh with the standard
OpenGL programming model in such a way that using GLOD should be just
like using standard OpenGL vertex arrays.
The most primitive element of GLOD is a patch, which represents
the smallest drawable element within GLOD. GLOD performs
simplification on objects -- collections of
patches. Creating an object with multiple patches gives you the
opportunity to change your OpenGL drawing state multiple times while
drawing an object.
GLOD does not actually provide a mechanism to adapt a single
object. Instead, you adapt a group of objects. This is motivated by the
idea that you can always place an object in its own group and that the
adaptation of groups of objects to some metric is usually quite a bit
more complicated than it is to adapt a single object. Accordingly,
objects are placed in groups when you first create an object. You can
set a group of objects to various standard refinement modes,
including triangle budgets and error budgets.
Drawing a GLOD object is a relatively straightforward process once its
group has been adapted. Importantly, GLOD follows the same assumptions
that OpenGL vertex arrays follow. GLOD does not modify your OpenGL
state when you draw a patch. This allows you full control over object
rendering. Some small exceptions to this rule are noted in the
documentation.
This release of GLOD provides a number of facilities that make working
with GLOD better match traditional LOD workflows, including
readback and memory management. GLOD readback facilities are
designed to allow a few additional things. Hierarchy readback
allows you to perform simplification as an pre-process, making it
possible to load up multiresolution objects in an efficient manner for
scenarios like level-loading. In addition, GLOD supports draw
readback, which allows you to read back ``what glod would draw''. This
facility is provided to allow you to draw the output of GLOD objects
yourself.
Memory management is not fully-implemented in the pre-release of GLOD, but
will be supported by the final stable release. When this is
complete, you will be able to restrict and customize GLOD's usage of
video memory, for example. Some facilities may be provided to manage
GLOD's usage of main memory as well.
- glodInit
-
Initializes GLOD. This call is mandatory.
- glodShutdown
-
Shuts down GLOD, freeing up memory, etc.
- glodGetError
-
Reports the current error and resets the error flag.
- glodNewObject
-
Creates and names an object in GLOD
- glodInstanceObject
-
Creates an additional named instance of a GLOD object, allowing multiple
adaptations of the same hierarchy
- glodInsertElements
-
Inserts geometry into a GLOD object's patch using the same calling
conventions as
glDrawElements()
- glodInsertArrays
-
Inserts geometry into a GLOD object's patch using the same calling
conventions as
glDrawArrays()
- glodBuildObject
-
Performs the simplification process that transforms the inserted
geometry into a specific type of multiresolution hierarchy
- glodDeleteObject
-
Deletes a particular object instance
- glodReadbackObject
-
Reads an already-simplified object into a specified buffer
- glodLoadObject
-
Loads an object from a specified buffer that was previously created
using
glodReadbackObject()
- glodFillElements
-
Reads the current geometry of an object into the current OpenGL vertex
arrays and additionally a specified index array
- glodFillArrays
-
Reads the current geometry of an object into the current OpenGL vertex
arrays
- glodNewGroup
-
Creates a new group of a specified name
- glodBindObjectXform
-
- glodObjectXform
-
When you have set a group to screen-space-aware adaptation, GLOD will
need to know where the objects in that group are located. This call
binds the current OpenGL viewport and matrix states to a particular
object so that, when you do call
glodAdaptGroup() , this adapation
becomes possible.
- glodAdaptGroup
-
Causes a particular group of objects to be adapted using your current
adaptation settings
- glodObjectParameter[if]
-
- glodGetObjectParameter[if]v
-
- glodGroupParameter[if]
-
- glodGetGroupParameter[if]v
-
Set and get object and group parameter settings
Last updated by Nat Duca, n@jhu.edu, June 2003
For more information, visit the GLOD web site:
http://www.cs.jhu.edu/~graphics/GLOD
