The edge buffer : A data structure for easy silhouette rendering by John W. Buchanan and Mario C. Sousa

Goal Render edges within relatively low end PC apps –obstacle: Z-buffer reading is expensive if it exists All polygons are processed Artists can define edges in the model which must be displayed

Edge Buffer (EB) uses: Closed polyhedral objects Open polyhedral objects There are times when the Artist MUST define edges in order to produce accurate results

EB Method Attributes Polygonal mesh stored in indexed vertex mesh representation Front/back facing polygons determined Buffer takes up little additional storage space

Closed polyhedral objects V- Vertex # F- Front B- Back Vertex VFB x x x00 5 Edges passed during pre-polygonal set-up Original Object

Closed polyhedral objects Vertex VFB x x x00 5 F bits have 1 added to them in the corresponding fields

Closed polyhedral objects Vertex VFB x x x00 5 Adding 1 to the F bit twice results in a 0 representing a front-facing, absolute edge.

Closed polyhedral objects Vertex VFB x x x00 5 Back-facing bits have 1’s added to them, the first silhouette is found.

Closed polyhedral objects Vertex VFB x x x00 5

Closed polyhedral objects Vertex VFB x x x00 5

Closed polyhedral objects Vertex VFB x x x00 5

Closed polyhedral objects Vertex VFB x x x00 5 Silhouettes are found and rendered

Open polyhedral objects Original Open Object Result of edge buffer technique on Open Objects

Open polyhedral objects Original ObjectRendering edges which do not end in

Open polyhedral objects Original objectAllow Artist to define edges to be rendered

Determining back-facing artist edges Three new flags are added to bit field: A, F~ and B~. –Artist defined edge. –Front or Back Absolute >

RESULTS A F B F~ B~ = Back-Facing Absolute Edge not to be rendered

RESULTS A F B F~ B~ = Front-Facing Absolute Edge not to be rendered

RESULTS A F B F~ B~ = Artist edge sharing vertices with both back-facing and front- facing polygons will be rendered

RESULTS A F B F~ B~ = Artist Edge sharing vertices with both back-facing and front- facing polygons will be rendered

RESULTS A F B F~ B~ = Artist edge sharing vertices with both back-facing and front- facing polygons

Conclusions Little additional storage space –At least two bits per edge. Front-and back computations being done Objects stored in indexed polygonal mesh representation Artist Edge can be defined during pre- processing. Increased computation adds less than 1% to rendering time