1. Achromatic and Colored Light
Chapter 11

2. Chapter 11 -- Achromatic and Colored Light
It is crucial that the student of modern computer graphics understand the theory and application of light and color.
Even the judicious use of just a few shades of gray can greatly enhance the appearance of a rendered object.
But it is color that is responsible for much of the impact of images.
Color is an immensely complex subject -- it draws from many fields
physics, physiology, psychology, art, graphic design,…
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The color of an object depends not only on the object itself, but also on
the light source,
the color of the surrounding area,
and the human visual system
Furthermore, certain objects reflect light
wall, desk, paper
And others transmit light
cellophane
glass
But we are going to begin our discussion with achromatic sensations -- Black, Gray, and White
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1. Achromatic Light
Achromatic light (literally the absence of color) is what we see on a black and white television set.
An observer of achromatic light experiences none of the sensations we associate with red, blue, yellow, and so on.
Quantity of light is the only attribute of achromatic light.
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Quantity of light can be discussed in different ways.
in the physics sense of energy, in which case certain terms are used:
intensity
luminance
in the psychological sense of perceived intensity, in which case the following term is used:
brightness
These concepts are related, but are not the same
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It is useful to associate a scalar with with different intensity levels
0 as black
1 as white
intensities between 0 and 1 representing different grays
Devices
Some can produce many different intensities at a single pixel
black-and-white television
Others can not.
printers
Chapter Achromatic and Colored Light

7. 1.1 Selection of Intensities
Suppose we want 256 different intensities
Why was 256 selected?
because it can be represented in 8 bits.
Which intensities do we want to use?
we don’t want half between 0.0 and 0.1, and the other half between 0.9 and 1.0.
the transition between 0.1 and 0.9 is big.
We might distribute them evenly
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We might distribute them evenly
this choice ignores the important characteristic of the eye -- it is sensitive to ratios of intensity levels, not absolute differences.
0.10 to 0.11 is viewed as being as different as 0.5 to 0.55
Therefore, the intensity levels should be spaced logarithmically rather than linearly.
Finding the intensities to display:
First find I0 -- the minimum intensity of the device.
For a CRT it is not 0, it is typically between and of Max intensity because of the phosphor.
The ratio between maximum and minimum intensity is called the dynamic range
Now you can select the values.
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Gamma Correction:
correctly displaying intensities is difficult.
particularly when the device has nonlinearities
CRT, film,…
These difficulties can be overcome with a technique called gamma correction.
How many intensities are enough?
By “enough” we mean the number need to reproduce a continuous tone black-and-white image such that the reproduction appears to be continuous.
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How many intensities are enough?
There is a formula for the number of intensities you need based upon the ratio of (1/I0)
n=log1.01(1/I0)
Here is the results for typical display media
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How many intensities are enough?
The book provides an example of a continuous tone photograph
original
with 4 intensity levels
countouring is very obvious
with 32 intensity levels
contouring is barely detectable
They observe that for this text, 64 levels would be acceptable.
Chapter Achromatic and Colored Light

12. 1.2 Halftone Approximation
Many displays and hardcopy devices are bilevel
They produce just two intensity levels
The question then is --
How can we expand the range of available intensities?
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The answer lies in the spatial integration that our eyes perform.
If we view a small area from a sufficiently large viewing distance, our eyes average fine detail within the small area and record only the overall intensity of the area.
This phenomenon is exploited in printing of black-and-white photographs in newspapers, magazines, and books, by a technique called halftoning.
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Each small resolution unit is imprinted with a circle of black ink whose area is proportional to the blackness of the area of the original image.
Newspaper halftones use 60 to 80 variable-sized and variable-shaped areas per inch.
Magazines and books use 110 to 200 per inch
FIGURE 11.3 pg 399
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Graphics output devices can approximate the variable-area circles by decreasing spatial resolution.
in general there is a tradeoff between spatial resolution and intensity resolution.
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This figure shows the same figure processed with the 2x2 pattern from the previous slide.
Figure 11.5
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Error Diffusion:
developed by Floyd & Steinberg
the visual effects are often satisfactory
The strategy has the effect of spreading, or diffusing, the error of one pixel over several nearby pixels.
Figure 11.6
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Error Diffusion Code:
K = Approximate(S[x][y]);
// Approximate S to nearest displayable intensity
I[x][y] = K; // Draw the pixel at (x, y)
error = S[x][y] - K; // Error term. Must be of type float
// Step 1:
// spread 7/16 of error into the pixel to the right
S[x + 1][y] += 7*error /16;
// Step 2:
// spread 3/16 of error into pixel below and to the left
S[x - 1][y - 1] += 3*error /16;
// Step 3:
// spread 5/16 of error into pixel below
S[x][y - 1] += 5*error /16;
// Step 4:
// spread 1/16 of error below and to the right
S[x + 1][y - 1] += error /16;
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2. Chromatic Color
The visual sensations caused by colored light are much richer than those caused by achromatic light.
Discussions of color perception involve three quantities:
Hue
Saturation
Lightness (sometimes Brightness is used here)
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It is necessary to specify and measure colors if we are to use them precisely.
For reflected light, we can do these tasks by visually comparing a sample of unknown color against a set of standard samples.
The Munsell color-order system includes sets of published standard colors organized in a 3D space of hue, value(lightness), and chroma (saturation)
In the printing industry they use the PANTONE MATCHING SYSTEM.
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Artists often specify color as different tints, shades and tones of strongly saturated, or pure pigments.
A tint results when white pigment is added to a pure pigment, thereby decreasing saturation.
A shade comes from adding a black pigment to a pure pigment, thereby decreasing lightness.
A tone is the consequence of adding both black and white pigments to a pure pigment.
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2.1 Psychophysics
These last two methods are subjective. An objective, quantitative way of specifying colors is needed.
To meet this need we turn to a branch of physics known as colorimetry.
Important terms here are:
dominant wavelength
excitation purity
luminance
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Colorimetry Terms:
Dominant wavelength
the wavelength of the color we see.
corresponds to hue.
Excitation purity
corresponds to the saturation of the color
Luminance
is the amount or intensity of the light.
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Basically, light is electromagnetic energy in the 400 to 700-nm wavelength part of the spectrum
How does this discussion relate to the red, green, and blue phosphor dots on a color CRT? And the tristimulus theory?
The tristimulus theory:
it is intuitively attractive because it corresponds loosely to the notion that colors can be specified by positively weighted sums of red, green, and blue.
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The tristimulus theory
This notion is almost true
the negative values mean that we cannot match the color by adding the primaries.
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26. 2.2 The CIE Chromaticity Diagram
In 1931, the Commission Internationale de l’ Eclairage (CIE) defined a new color system.
based upon X, Y, and Z, it allows you to define every visible color as a positive combination of the three.
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Color Plate 15
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28. 3. Color Models for Raster Graphics
A color model is a specification of a 3D color coordinate system.
Three hardware models are:
RGB - used with color CRT monitors
YIQ - the broadcast TV color system
CMY - used for certain color printing devices (cyan, magenta, and yellow)
unfortunately they do not relate directly to intuitive color notions of hue, saturation, and brightness.
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3.1 The RGB Color Model
The RGB color model used in color CRT monitors and color raster graphics employs a Cartesian coordinate system
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3.2 The CMY Color Model
Cyan, magenta, and yellow are the complements of red, green, and blue.
they are used as subtractive primaries.
A knowledge of CMY is important when you are dealing with hardcopy devices that deposit pigments onto paper.
there are straightforward transformations between the RGB and CMY color models
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3.3 The YIQ Color Model
This model is used in US commercial color TV broadcasting.
therefore, it is closely related to RGB.
Basically YIQ is a recoding of RGB
for transmission efficiency
and backward compatibility with B/W TVs
The recoded signal is transmitted using the NTSC standard.
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3.4 The HSV Color Model
The RGB, CMY, and YIQ models are hardware oriented. By contrast the HSV (hue, saturation, value) model is user oriented.
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33. 3.5 Interactive Specification of Color
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34. 3.6 Interpolation in Color Space
Color interpolation is necessary in at least three situations:
Gourand shading
Antialiasing
Blending two images together
fade-in/fade-out sequence.
The results of interpolation depend on the color model in which the colors are interpolated.
Chapter Achromatic and Colored Light

35. 4. Use of Color in Computer Graphics
We use color for aesthetics, to establish tone or mood, for realism, as a highlight, …
In addition, users tend to like color, even when there is no quantitative evidence that its use improves their performance.
There are several pitfalls in color usage, so the biggest rule of color usage is:
Apply color conservatively.
Chapter Achromatic and Colored Light