Shading models

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TOPIC 8

• Shading models

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Calculating the angles ? and ?
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Gouraud shading
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Problems with Gouraud shading
1        Mach banding in which the polygon edges
appear unrealistically bright. This is a
physiological response of the human eye arising
from the eyes ability to detect changes of
intensity.   2        Problems are encountered in
animation sequences because the intensity
calculations are carried out in screen
co-ordinates from vertex normals calculated in
world co-ordinates.     The process of averaging
normals to produce vertex normals can lead to
loss of information since several different
combinations of normals can lead to the same
average
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Phong shading
The first two objections can be overcome by using
Phong shading developed by Bui-Tuong Phong in
1975. The first stage of the process is the same
as Gouruard shading until the vertex normals have
been calculated.   Instead of now calculating
vertex shading values the vertex normals
themselves are interpolated in two dimensions
across the surface. This interpolated normal is
then used to obtain a shading value at each
polygon pixel position.   On average Phong
shading is typically more computationally
intensive by a factor of 5 1
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Shading in MATLAB
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The properties of the light The properties of the
object The shading algorithm
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Using the light command
light('Position',1 0 0,'Style','infinite','color
','white')
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Parameters for use with patch or surf
colorspec Any of the allowed MATLAB colour
specifications for example green or an RGB
triplet 0.55, 0.7, 1 etc.   flat The colour
data at the first vertex determines the colour of
the whole face.   none Do not draw the face or
edge as appropriate.   interp This is a
bilinear interpolation of the colour across the
face from the vertices of the patch or surface
element.
surf(X,Y,Z,'FaceColor','red','EdgeColor','none')
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Using the material command-1
Shiny sets the reflectance properties so that
the object has a high specular reflectance
relative the diffuse and ambient light and the
colour of the specular light depends only on the
colour of the light source.   Dull sets the
reflectance properties so that the object
reflects more diffuse light, has no specular
highlights, but the colour of the reflected light
depends only on the light source.   metal sets
the reflectance properties so that the object has
a very high specular reflectance, very low
ambient and diffuse reflectance, and the colour
of the reflected light depends on both the colour
of the light source and the colour of the object.
material shiny
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Using the material command-2
material(ka kd ks n sc)
ka, kd ks 0ltklt1
n a positive integer
sc a normal colour spec.
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Using the lighting command
lighting flat lighting gouraud lighting
phong lighting none
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Putting it all together
function LightDemo To demonstrate some of the
properties of lighting Create a sphere with a
red colour and draw it with the surf
command X,Y,Z sphere(30) surf(X,Y,Z,'FaceColo
r','red','EdgeColor','blue') Try changing the
material to dull, shiny, metal and see the
effect material (0.4 0.7 0.95 100) Light is
incident along the X axis and is located an
infinite distance away Try changing the colour
of the light (e.g. 'white', 'green', 'blue'
etc.) light('Position',1 0 0,'Style','infinite',
'color','white') Try the effects of flat,
gourard or phong shading lighting phong axis
square rotate3D on
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flat
gouraud
Phong
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