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Adding Bumps to 3D Objects
In its simplest form, bump mapping adds realism to
textures and objects by creating the illusion of bumps, or
variations in surface depth, on an otherwise flat surface.
This means that a flat plane can be transformed into the
dimpled surface of a golf ball, the gnarled bark of a tree
or the rough surface of a rock.
For bump mapping to be convincing, the eye must perceive
variations in surface depth even though the surface really
is flat. The viewer's perception of depth is influenced by
the amount of light reflected into the eyes by the surface
being looked at, and the amount of light reflected in
any given direction depends on the surface reflecting it.
A smooth, level surface will reflect more light than a
bumpy one. Likewise, a surface that is perpendicular to
the viewer's line of vision will reflect more light than
one that is not directly facing the viewer.
Consider the situation in figure 2. A flat surface (2a)
directly facing the viewer reflects more light back into
the viewer's eyes than the bumpy surface does. The
viewer perceives all areas of the surface as being of
equal brightness and consequently, completely flat. However, the bumpy surface (2b) reflects less light back into
the viewer's eyes because all areas of that surface are
not directly facing the viewer. The part of the bump that
is directly in the viewer's line of vision appears brighter
than the parts that are not - meaning that the sides of
the bump appear darker, and may also cast shadows.
Reflection of light from flat and bumpy surfaces.
In 3D graphics, the illusion of bumps relies solely on
color, lighting and shading. The lighting/shading effect is
easily achieved by modifying the color of each of the
texels, or textured pixels, being drawn on the screen.
Using a predetermined algorithm, the color of each texel
is modified to represent the amount of light that would
be reflected from any given point on a bumpy surface.
This depends on the angle of the light source relative to
the angle of the surface at the point of reflection.
In figure 3, the different altitudes of the Earth's surface are
made obvious by the variation in lighting. Areas that are
brighter are directly facing the light. Other areas are made
darker either because they do not face the light source, or
because they lie in the shadow of more elevated areas.
Either way, the areas that are darker receive less light.
Bump mapping uses lighting and shading effects on the
texture to create the appearance of surface depth, but the
variation in surface depth is really just an optical illusion.
Figure 3 is a good example of this. Although the surface
of the 3D model on the left appears jagged and in relief,
its outer edge is not marked by bumps. To create the illusion of bump mapping, the only thing that has been
modified is the color of the texture on the surface of the
sphere on the right.
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The top image is bump mapped, the lower one is not.
To create this effect, all areas of the surface must be
specified as either elevated or sunken. To do this, a
second texture or "bump map" is required to define
2depth or height for every pixel in the conventional texture
map.
In the simplest case, this bump map would be a
monochrome intensity map with brighter shades representing
the more elevated areas and the darker shades representing
the lower areas. A gradual change in intensities
represents a moderate slope, and a sudden change in
intensities represents an abrupt change in depth. Alternatively, the bump map could be called a height map.
To complete the effect, an "environment map" is
required. The environment map represents what should
be reflected off the surface. It can be as simple as a
bright circle representing a single light source or a
complex environment map consisting of multiple light
sources and effects due to other environmental factors
such as clouds and shadows.
Multi-pass Alpha-blending
and Its Limitations
Some existing 3D hardware can simulate one bump mapping effect known as ‘embossing’ with multiple alpha-blending passes. However, this is a per-polygon technique
which produces artifacts, is limited to monochrome lighting
and is unable to simulate any of the more interesting
environmental effects.
Embossing with multiple alpha-
blending passes is accomplished when a monochrome
version of the conventional texture map is shifted
in one direction, subtracted from the original texture,
shifted back, and then blended into the original texture
map. This gives the illusion of shadow on one side and
bright light on the other. The direction and amount of
shift is determined by the direction and intensity of the
light source.
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