Texturing/shading models (multi-spectral)

Mesh facial models (either polygonal or parametrically based) may be given realism or texture by means of surface mapping and shading. Shading can smooth a polygonal model. Various methods are available and they may be applied alone or in combination depending on the desired appearance of the model:

• Flat shading: the pixels in a polygon are all the same color with no variation. If the model is faceted, each facet will be distinguishable. Flat shading is useful only as a low-cost rendering method.

• Gouraud (smooth): This is a shade-interpolating method of shading that will make the object appear smooth, instead of faceted. This method doesn't work well with highlights or local light sources and one can often still see polygonal edges on the object.

• Phong: This is a normal-interpolating method. The object appears very smooth. This method goes a step further than Gouraud. A new shade is computed for each point, point by point before it shades.

• Bump mapping: is another method for producing maps of rough or textured surfaces, but it does not have the edge or shadow accuracy of displacement mapping.

• Displacement mapping: is a method for distorting a surface to produce an embossed or debossed surface that produces geometry with accurate edges and shadows. The displacement map specifies how the surface is to be moved before being mapped.

• Reflection mapping: gives the illusion of reflection or a mirrored effect.

• Environment mapping: is a method by which the model's surface reflects the environment on its surface.

• Opacity mapping: involves using the grayscale of a 2D object to define an object's transparency or opacity.

• Transparency mapping: gives the illusion of transparency, like looking through glass. This is particularly useful for skin pallor, as it is semi-transparent. For example, [65] includes an emotion model which expresses emotion through the vascular system, such as paleness due to fear or blushing due to embarrassment.

• Texture mapping: is the process by which the bitmap is applied, or decaled, on to the geometric model. Textures may be applied as either 2D bitmaps or scans [80][148]. Photographs may be applied to mesh models as maps also as seen in [75]. The mapped textures may also be shaded in accordance with the lighting and surface geometry.