Computer Graphics:
Programming, Problem Solving,
and Visual Communication
Steve Cunningham
California State University Stanislaus and Grinnell College
PowerPoint Instructor’s Resource
Color and Blending
Specifying and working with color
in your images
Color is Fundamental in
Graphics
• Having full-capability color available
everywhere was a great breakthrough in
computing when it finally happened
• In order to use color, we need to know
how to specify it and how to work with it
• We also need to know how color affects
the way our audience understands our
images
The Human Visual System
• Our eyes have two main kinds of cells
involved in vision
– Rods -- sense luminance or brightness
– Cones -- sense chroma or color
• Three kinds of photosensitive chemicals in
cones
• Generally these are sensitive to red, green, and
blue light wavelengths
Computer Graphics Simulates
These Capabilities
• Images are created using red, green,
and blue primaries
• Primaries may be phosphors in screen
or film
• Primaries may be created by
complements in print
– See the hardcopy chapter for more details
The RGB Color Model
• Colors are specified by naming their
red, green, and blue components
• These are usually taken to be relative
components with real-valued numbers
in [0,1] each
• Devices will have a more discrete color
space, but the [0,1] space is mapped to
this as needed
The RGB Color Space
• It is convenient to visualize RGB space
in terms of the RGB Cube, with pure
primaries at the vertices
Color and Luminance
• It can be important to think of the
luminance of a color as well as the
chroma (RGB values) of the color
• Luminance can be approximated by
0.30*red + 0.59*green + 0.11*blue
• If a viewer has color deficiencies, he can
usually distinguish color by luminance
Luminance Example
• The RGB cube clipped by a plane,
shown in both color and grayscale
• The grayscale conversion is computerbased, not visually-based; is it right?
Other Color Models: HSV
• Hue - Saturation - Value : cone model
Hue = angle; saturation = radius;
value = height
• Views from side, top, and bottom
Other Color Models: HLS
• Hue - Lightness - Saturation
double cone model with hue = angle;
lightness = height; saturation = radius;
• Views from red, green, and blue sides
Emissive vs Transmissive
Colors
•
•
•
•
Emissive colors are from screen
Transmissive colors are from inks
RGB emissive model (left)
CMYK transmissive model (right)
Color Depth
• Depth is number of bits to store color
• Current standard is at least 8 bits/color
• With smaller color depth, you can get
more mach banding
(some may always
be there!)
• See them in figure
Color Gamut
• The color gamut is
the range of colors
that can be created
compared to the
number that can be
seen
• The CIE color space
helps to see the
gamut of any device
(see triangle)
Color Blending and the
Alpha Channel
• The RGB color model is extended to the
RGBA model by adding an alpha value
• This value represents the proportion of
color this contributes when it is blended
with other colors
• 1 = complete coverage; 0 = no coverage
• Blending models transparency but does
not actually provide it
Creating Transparency with
Blending Takes Some Work
(left) Three planes with alpha = .5, order B-G-R
(middle) Three planes as above, no depth testing
(right) Three planes as at top, varying alpha values
But if you divide each plane into four
parts and draw them back to front,
you get an accurate model of
transparency
Color for Visual Communication
• Color can be used in many ways
• Highlight color (left) and background
color (right) are two important ones
Color for Visual Communication (2)
• Pseudocolor is another very important
use of color for communication
• Use color to show the value of data
Pseudocolor and Color Ramps
• Create a color ramp by associating values
of each of R-G-B with values in [0,1]
• For example:
– From 0 to .30, increase red to go from (0,0,0)
to (1,0,0)
– From .30 to .89, increase green to go from
(1,0,0) to (1,1,0)
– From .89 to 1.0, increase blue to go from
(1,1,0) to (1,1,1)
• This is the uniform-luminance color ramp
Pseudocolor and Color Ramps
• Comparing color ramps
– rainbow (left) vs uniform-luminance (right)
Color and Shape
• You can use either color or shape to
encode values (or both)
• Compare these examples: which works?
Other Color Issues
• Color has some very specific meanings
to people
• You need to respect those if you want
your images to communicate well to a
number of audiences
• Meanings can be professional, cultural,
or even emotional
Perception Issues With Color
• There are problems with sharp contrasts
in color along edges
• These come from the way our vision
system works