Editing Shading (section)

====Smooth shading====
In contrast to flat shading where the colors change discontinuously at polygon borders, with smooth shading the color changes from pixel to pixel, resulting in a smooth color transition between two adjacent polygons. Usually, values are first calculated in the vertices and [[bilinear interpolation]] is used to calculate the values of pixels between the vertices of the polygons. Types of smooth shading include [[Gouraud shading]]<ref>{{cite journal|last=Gouraud|first=Henri|title=Continuous shading of curved surfaces|journal=IEEE Transactions on Computers|year=1971|volume=C-20|issue=6|pages=623–629|doi=10.1109/T-C.1971.223313|s2cid=123827991 }}</ref> and [[Phong shading]].<ref>B. T. Phong, [https://users.cs.northwestern.edu/~ago820/cs395/Papers/Phong_1975.pdf Illumination for computer generated pictures], ''Communications of ACM'' 18 (1975), no. 6, 311–317. ([[PDF]])</ref>

=====Gouraud shading=====
# Determine the normal at each polygon vertex.
# Apply an [[illumination model]] to each vertex to calculate the light intensity from the vertex normal.
# Interpolate the vertex intensities using [[bilinear interpolation]] over the surface polygon.

Problems:
* Due to lighting being computed only at vertices, the inaccuracies (especially of specular highlights on large triangles) can become too apparent.  
* T-junctions with adjoining polygons can sometimes result in visual anomalies. In general, T-junctions should be avoided.

=====Phong shading=====
Phong shading is similar to Gouraud shading, except that instead of interpolating the light intensities the normals are interpolated between the vertices and the lighting is evaluated per-pixel. Thus, the specular highlights are computed much more precisely than in the Gouraud shading model.

# Compute a normal N for each vertex of the polygon.
# Using [[bilinear interpolation]] compute a normal, N<sub>i</sub> for each pixel. (Normal has to be renormalized each time.)
# Apply an [[illumination model]] to each pixel to calculate the light intensity from N<sub>i</sub>.