In this method the polygon surface is shown by linear interpolating intensity values. Intensity values for each polygon are matched with values of adjacent polygons. By this intensity discontinuities are eliminated. Following calculations are done for displaying polygon surface:
1) Calculate average unit normal vector at each polygon vertex.
2) Determine vertex intensity by applying illumination method at each vertex.
3) Linearly interpolate the vertex intensities.
Where, N1, N2 and N3 = Surface normals of polygon
V = Vertex of the polygon
Nv = Normal vector at vertex.
For any vertex V,
Then, we find the vertex intensities:
Where, 1,2,3 are the vertices of the polygon.
p and q are the intersection points of the scan line and the polygon.
Ip = Intensity at point a.
Iq = Intenisty at point q
A is an interior point in the polygon
IA = Intensity at point A
I = Intensity at (x,y)
Intensity along this edge for next scan line y -1:
Intensities at successful horizontal pixels positions along each scan line:
1) This method can be combined with a hidden surface algorithm to fill visible polygons along each scan line.
2) This method removes intensity discontinuities.
1) Sharp drop of intensity values can’t be displayed
2) Highlights on surface are sometimes shown with irregular shapes
3) Dark intensity streaks ( Mach bands) appear on the surface because of linear intensity interpolation.