surface detection methods

Back face detection

A point (x,y,z) is inside a polygon surface with plane parameters A,B,C and D if

               Ax+By+Cz+D < 0

       When an inside point is along the line of sight to the surface , the polygon must be a back-face

Conditions for back face:

   A polygon is a back-face if  V.N > 0 

Depth buffer method

Steps

Initialize the depth buffer and refresh buffer so that for all the buffer positions (x,y)  depth(x,y) =  0 , refresh(x,y) = I backgnd     

For each position on each polygon surface listed on the  polygon table calculate the depth value and compare the depth vaslue to the previously stored values in the depth buffer to determine visibility

        Let the calculated depth be  Z for each position (x,y)

        If Z > depth(x,y) , then set )  depth(x,y) = Z , refresh(x,y) = Isurf (x,y)

Scan-line method-concept-example-diagram

Extension of scan line algorithm for filling polygon interiors

All polygon surfaces intersecting the scan lines are examined

Depth calculations are made for each overlapping surface across every scan line to determine the nearest surface to the view plane

After the visible surface is determined the intensity value for the position is entered into the refresh buffer

Depth-sorting method

Steps:

Surfaces are ordered according to the largest Z value

Surface S with greatest depth is compared with other surfaces to determine whether there are any overlaps in depth

If no depth overlap occurs , S is scan converted

This process is repeated for the next surface as long as no overlap occurs

If depth overlaps occurred additional comparisons are used to determine whether reordering of surfaces are needed or not

 Ray casting method

    - it is a variation of depth buffer method

    - process pixels one at a time and calculate   depths for all surfaces along the projection path to that pixel

Wireframe method

       visible edges are displayed and hidden edges are either eliminated or displayed differently from the visible edges .Procedures for determining visibility of object edges are referred to as wireframe visibility methods / visible line detection methods / hidden line detection methods