10/31/2012
Chapter 9: Color
• What is color?
• Color mixtures
– Intensity-distribution curves
– Additive Mixing
– Partitive Mixing
• Specifying colors
– RGB Color
– Chromaticity
Additive Primary Colors
• Red, green and blue are called
the additive primaries
• We want to select primary
colors that allow us to create
the largest possible number of
other colors using just those
primary colors
Additive Mixing: Yellow
530-nm green
yellow
650-nm red
cyan
magenta
460-nm blue
• Most colors can be described in
terms of mixtures of red, green
and blue
1
10/31/2012
Display Pixels: Partitive Mixing
Partitive Mixing: Pixels
• What it instead, we make the different colors
coming from separate, very small, very closely
spaced points
• You eye cannot see them as separate sources, so
the colors mix and you see the same color. This is
called partitive mixing
Subtractive Mixing
• In additive mixing, we added the wavelengths that
were hitting the eye: say red light and green light
Subtractive Mixing
• Suppose we have a range of wavelengths hitting some
object:
• Three things can happen to each wavelength of light:
• What about things like filters and dye pigments?
• These mix by subtractive mixing
– Reflection: that particular wavelength bounces off the object
– Transmission: that wavelength passes through the object
– Absorption: that wavelength is soaked up by the object
• In general different things happen at different
wavelengths
2
10/31/2012
Colored Filters
Colored Filters
• Recall that magenta is the additive mixture of
blue and red
• If a light looks magenta, it means that red and
blue light is reaching our eyes, thus a magenta
filter must transmit blue and red light, and
subtract, by reflection or absorption, green light.
=
=
Incident white light
Colored Filters
Magenta filter
subtracts green
Cyan filter
subtracts red
Only blue
gets
through
Subtractive Mixing: Primaries
• Another example:
• The subtractive primaries
are
– Cyan
– Magenta
– Yellow
=
Incident white light
• What happens if we layer colored filters?
• Filters subtract light by absorption or reflection
Magenta
filter subtracts
green
Yellow
filter subtracts
blue
Only red
gets
through
• In subtractive mixing,
combining complementary
colors produces black
3
10/31/2012
Subtractive Mixing: Reflection
• Colored filters subtract some colors and transmit
others.
• Subtractive mixing can also occur when a colored
surface absorb some colors and reflect others
White in
Subtractive Mixing: Reflection
A green surface absorbs (subtracts) red and blue
(magenta), and reflects green
White in
Green out
Magenta out
A magenta colored surface
absorbs (subtracts) green
and reflects red and blue,
thus appeared to be
magenta
Subtractive Mixing: Real Filters
Magenta Filter Transmission
• Real filters are non-ideal, they transmit a range of
wavelengths, not just one. We can specify which
wavelengths using an intensity distribution curve.
4
10/31/2012
Cyan Filter Transmission
Subtractive Mixing: Colored Light
• We have so far been assuming that we are
illuminating our colored filters and surfaces with
uniform white light.
• But most light is not uniform white, or even white at
all.
• How can we figure out what objects will look like in
non-ideal or non-uniform light?
Colored Light: Example
Fluorescent light emission
Combining Intensity Curves
Reflectance of a magenta shirt
To combine two
intensity curves,
you multiply the
curves at each point
to get the combined
curve
When illuminated
by this fluorescent
light, this magenta
shirt will appear
gray (colorless)
5
10/31/2012
Combining Intensity Curves: Example
Color Mixing: Ink and Paint
• Color mixing with ink and pigment is in general, a
complex mixture of additive and subtractive
mixing.
• Light rays hitting paint or ink on a piece of paper
can interact with the pigments in several ways
Color Mixing: Ink and Paint
• Some light is reflected from the surface
• Some light passes through the ink and reflects
from the paper underneath, while the ink absorbs
some wavelengths
• The rays from these two processes combine in
the eye, mixing additively
Color Mixing: Ink and Paint
• This process only occurs if the ink or paint is at
least semi transparent, such as watercolors and
some printing inks.
6
10/31/2012
Watercolors
• Watercolor is a challenging medium, because the transparent
colors have to be layered carefully to avoid a muddy looking
appearance
• As light passes through each color layer, more wavelengths are
subtracted, just like layering a set of filters
• The light also reflects off the paper underneath, so the
reflectance curve of the paper itself also subtracts some colors.
Process Ink Printing
• The transparent inks used to print colored documents
and packaging are called “process inks”
• A printer wants to use the best primaries possible to
create the largest number of colors from the smallest
number of printing plates.
• Each separate color requires a separate printing plate
or roller
Process Ink Printing
• The printing primaries are
–
–
–
–
Cyan
Magenta
Yellow
Black
Non-ideal Inks
• The reason inks cannot be ideal is that they
cannot be too saturated.
• A narrow reflectance curve (more saturated)
reflects less overall light, so saturated colors
would be very dark
• Black is necessary because the printing inks are not
ideal, and combining them together does not produce
a true black, but a slightly color-tinged black.
• This is also called the “CMYK” color system
7
10/31/2012
CMYK Printing
CMYK Printing
CMYK Printing
CMYK Printing: Halftones
• We have seen that we can generate a range of colors
by subtractive mixing our CMYK primaries
• To avoid having to add even more printing plates or
rollers, printers use a variation of partitive mixing to
generate lighter colors
• But what it we want to print lighter, less saturated
colors?
• They simply print less ink in a given area
• Diluting the inks would require a new printing plate
for each color.
• This is called “half-tone“ printing, and
is common in newspaper and magazine
printing
8
10/31/2012
Halftones
9