Skinning Adventures / Architecture Review
Developer Diary
- Date:
- Wednesday, September 15, 2004
I was planning to use ARB_vertex_blend OpenGL extension and implement a mid-level path along with hw shader and sw deformer. Unfortunately, I realized that, this path requires some change in the mesh data structure. I needed to have submeshes connected to all deformers, something that I don't want to implement really. Actually, my current structure is much better than that already, and also both hw and sw paths do not need that kind of structuring. Alternatively, you can use ARB_matrix_palette extension along with vertex_blend to overcome this, but it is not supported by any of the card vendors anymore! People say, this is because skinning by extensions is replaced by hw shader versions. So, as a result, I also decided to forget about this path totally.
As soon as I gave up that path, I realized that I do NOT need to change my architecture that much, I do NOT need to implement skinning in the way it is explained. I don't need to store transformation matrices and bone structures for every deformation point as I planned. Here is my architecture.
XFace::FaceBase class controls the whole system as before. We have a new class for geometry to handle the deformations, XEngine::DeformableGeometry. Each XEngine::Drawable object now has a XEngine::DeformableGeometry bound to it. As before, XEngine::Drawable's represent the building blocks of the head, i.e. the face, hair, tongue, etc.. FaceBase, has a list of XEngine::Drawable objects, during the initialization phase, the meshes are loaded from disk into XEngine::DeformableGeometry objects and attached to XEngine::Drawable objects. In reality, only the names of XEngine::DeformableGeometry objects are stored in XEngine::Drawable, and they are retrieved through XEngine::MeshManager singleton object. During the initialization phase, also FDP file is loaded and XFace::FDPItems are filled. XFace::FDPItem objects represent MPEG-4 feature points defined by the standard, and store the characteristics of the feature point. Namely, we store, for each feature point;
the mesh it is affecting, control point that represents the feature point, the region (vertices) influenced by the control point, a set of influence calculators
Influence calculators store weights for each vertex in the zone of influence, where weight represents how the coresponding vertex will be affected by the deformation of that feature point. Interface for the influence calculators are defined by XFace::IInfluenceCalculators class. Currently, we have a set of raised cosine based influence calculator concrete classes. XfaceEd automatically detects influence calculators, thanks to pluggable factory pattern implemented inside. See XFace::InfluenceCalculatorMaker for more information on that. Each XFace::IInfluenceCalculator derived object also stores a coefficient to control the deformation limits, and the FAP id that the weights are used for (or in other words, the FAP that the influence calculator responds to).
So during the initialization, when the FDP file is loaded, first static meshes (OBJ/VRML/X3D or any other supported format) are loaded to XEngine::DeformableGeometry instances, and feature point info are loaded to XFace::FDPItems, and XFace::IInfluenceCalculators are created and the weights for the zone of influence are calculated. Then those weights are routed to XEngine::DeformableGeometry objects.
XEngine::DeformableGeometry objects store two sets of vertices, one for the still (static, no emotion/movement) and one for the deformed. During runtime, when playback of FAPs starts, for each new frame, 68 FAP values are sent to XEngine::DeformableGeometry::updateAnimationParams method, but the deformation is delayed until the rendering. During the rendering process, if a geometry needs an update, its update method is called for the moment, though I plan to move this piece of code out of XEngine::DeformableGeometry. This lazy evaluation (in the rendering pipeline) enables us to call a shader instead of updating on software. Though I haven't implemented the shader pipeline, all the structuring is done.
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