ShadeVis
From vcg Library
ShadeVis: Computing Ambient Occlusion the ShadeVis tool computes a simple static, but much more correct, per-vertex ambient term. This effect, commonly known as ambient occlusion, is aimed to provide more faithful simple realtime rendering of rather complex meshes.
Standard Opengl Shading: purely diffuse, lambertian, surface with a constant ambient term
Enhanced shading with an ambient occlusion term that is per vertex statically precomputed
In practice rather than considering the ambient lighting to exist uniformally throughout a scene, this approach determines the ambient brightness of each part of a surface to be proportional to the extent to which the surface has "its outward view of its environment" free i.e. occluded, by other surfaces of the object. For example, the upper part of the David forehead is less exposed to its local environment than the nose and as a consequence it will be proportionally darker.
The most common techniques to compute ambient occlusion is based on shooting rays from each surface element to evaluate the quantity of light that reach it. Shadevis tools use OpenGL to accellerate this process by simply rendering the scene ortographically from a set of uniformely distributed directions and checking against the zbuffer if each vertex is occluded or not.
The Shadevis tool is oriented to handle rather large meshes, we have successfully used on a 8M triangle mesh of the David on a 512Mb machine.
It should be noted that in the context of the visualization of models of real statues the standard OpenGL lighting model is not very satisfactory. Cast shadows and darkenings of internal areas are very important in this context. Infact the artist usually take into account these lighting effects during the creation of a statue. The most common example is the shape of the pupil of the David’s eye. To create the effect of something similar to a shiny reflection Michelangelo sculpted the pupil as an hole with a upper small triangular inset that remains lighted against the dark background of the hole. Without the correct lighting these subtle effects are lost and the most evident effect is that the head of the david seems to change its expression.
Papers We used and described this technique for computing the ambient occlusion term in the following paper.
R. Borgo, P. Cignoni, R. Scopigno
An easy to use visualization system for huge cultural heritage meshes
VAST’01 Conference, Greece, Nov. 28-30, 2001.
pdf
We have used this approach (and a slightly modified version of this code) to computing occlusion or visibility information also for estimating the best direction where shooting rays when recovering the detail (normal maps) of a High-res mesh over a Lowres mesh. This approach is the one now used by ATI in their normalmapper tool (when specifying the slow bent rays option). We introduced this approach in:
M. Tarini, P. Cignoni, R. Scopigno
Visibility based methods and assessment for detail-recovery
Proc. of Visualization 2003, October 19-24, Seattle, USA.
pdf
