Graphics Reference
In-Depth Information
other channels. Thus, the framebuffer/color buffer distinction steers the high-level
system toward an efficient low-level implementation while abstracting the details
of that implementation.
3
2-bit pixe
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In the real world, light sources emit photons. These scatter through the world and
interact with matter. Some scatter from matter, through an aperture, and then onto
a sensor. The aperture may be the iris of a human observer and the sensor that per-
son's retina. Alternatively, the aperture may be at the lens of a camera and the sen-
sor the film or CCD that captures the image. Photorealistic rendering models these
systems, from emitter to sensor. It depends on five other categories of models:
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24-bit channels
8-bit channels
Figure 14.8: The
GL_DEPTH24
STENCIL8
buffer format encodes
a 24-bit normalized fixed point
“depth” value with eight stencil
bits used for arbitrary masking
operations.
1. Light
2. Light emitters
3. Light transport
4. Matter
5. Sensors and their imaging apertures and optics (e.g., cameras and eyes)
We now explore the concepts of each category and some high-level aspects that
can be abstracted to conserve space, time, and implementation complexity. Later
in the chapter we return to specific common models within each category. We must
defer that until later because the models interact, so it is important to understand
all before refining any.
Although the first few sections of this chapter have covered a great many
details, there is a high-level message as well, one that we summarize in a prin-
ciple we apply throughout the remainder of the chapter:
T
HE HIGH
-
LEVEL DESIGN PRINCIPLE
:
Start from the broadest possible
view. Elements of a graphics system don't separate as cleanly as we might
like; you can't design the ideal representation for an emitter without considering
its impact on light transport. Investing time at the high level lets us avoid the
drawbacks of committing, even if it defers gratification.
14.4.1.1 The Visible Spectrum
The energy of real light is transported by photons. Each photon is a quantized
amount of energy, so a powerful beam of light contains more photons than a weak
beam with the same spectrum, not more powerful photons. The exact amount of
energy per photon determines the frequency of the corresponding electromagnetic
wave; we perceive it as color. Low-frequency photons appear red to us and high-
frequency ones appear blue, with the entire rainbow spectrum in between (see
Figure 14.9). “Low” and “high” here are used relative to the visible spectrum.
There are photons whose frequencies are outside the visible spectrum, but those
can't directly affect rendering, so they are almost always ignored.
The human visual system perceives light containing a mixture of photons
of different frequencies as a color somewhere between those created by the
