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160 Chapter 8 Video Scopes
IPT
The IPT color space is a perceptually weighted color space, the purpose of which is to
more accurately represent the hues in an image distributed on a scale that appears
uniformly linear to your eye.
While the RGB, HSL, and Y´C
B
C
R
color spaces present three-dimensional analyses of the
image that are mathematically accurate, and allow you to see how the colors of an
image are transformed from one gamut to another, they don’t necessarily show the
distribution of colors as your eyes perceive them. A good example of this is a
conventionally calculated hue wheel. Notice how the green portion of the hue wheel
presented below seems so much larger then the yellow or red portion.
The cones of the human eye which are sensitive to color have differing sensitivities to
each of the primaries (red, green, and blue). As a result, a mathematically linear
distribution of analyzed color is not necessarily the most accurate way to represent
what we actually see. The IPT color space rectifies this by redistributing the location of
hues in the color space according to tests where people chose and arranged an even
distribution of hues from one color to another, to define a spectrum that “looked right”
to them.
In the IPT color space, I corresponds to the vertical axis of lightness (desaturated black
to white) running through the center of the color space. The horizontal plane is defined
by the P axis, which is the distribution of red to green, and the T axis, which is the
distribution of yellow to blue.
Here’s an analysis of the test image within this color space.