Lecture No.24 Lighting II
Clamping Color Values
Now it's perfectly fine to end up with an oversaturated color and pass this result along to
the graphics engine. What happens in the pipeline is an implicit clamping of the color
values. Any value that's greater than one is clamped to one, and any less than zero are
clamped to zero. So this has the benefit of requiring no effort on the part of the shader
(technology that is being used today for lighting and shading supported by Graphics
hardware) writer. Though this may make the rendering engine happy, it probably isn't
what we want. Intuitively, we'd think that shining orange and green lights on a white
surface would yield a strong green result. But letting the hardware clamp eradicates any
predominant effect from the green light. Clamping is fast, but it tends to lose fidelity in
the scene, particularly in areas where we would want and expect subtle changes as the
light intensities interact, but end up with those interactions getting eradicated because the
differences are all getting clamped out by the graphics hardware.
Scaling Color Values by Intensity
Instead of clamping, we might want to scale the color by dividing by the largest color
value, thus scaling the rgb values into the [0,1] range. In the example from figure 1, the
final color values were [1.0,1.49,0.49] meaning our largest color value was the green, at
1.49. Using this approach, we divide each element by 1.49, yielding a scaled color of
[0.671,1.0,0.329]. Thus any values greater than one are scaled to one, while any other
values are also scaled by the same amount. This maintains the hue and saturation but
loses the intensity. This might not be acceptable because the contrast with other colors is
lost, since contrast perception is nonlinear and we're applying a linear scaling. By looking
at the three results, we can see the large difference between the resulting colors.
Figure 1: Adding colors can result in colors that are outside the displayable range.
Shifting Color Values to Maintain Saturation
One problem with clamping or scaling colors is that they get darker (lose saturation). An
alternative to scaling is to maintain saturation by shifting color values. This technique is
called clipping, and it's a bit more complicated than color scaling or clamping. The idea is
to create a gray-scale vector that runs along the black-white axis of the color cube that's
got the same brightness as the original color and then to draw a ray at right angles to this
vector that intersects (i.e., clips) the original color's vector. We need to check to make
sure that the grayscale vector is itself within the [0,1] range and then to check the sign of
the ray elements to see if the color elements need to be increased or decreased. As we are
probably wondering, this can result in adding in a color value that wasn't in the original
color, but this is a direct result of wanting to make sure that the overall brightness is the
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