Achromatic and Colored Light

CS 2889/17/1998

Vic

Introduction

• It is crucial for modern computer graphics students to understand the theory and application of light and color.

• Color adds impact to images. Without the proper use of color, even the most interesting scene can be rather dull and boring.

Color and Light

• The color of an object depends not only on the object but also the light source illuminating the object, the color of the surrounding area, and on the human visual system.

• Certain objects reflect light (walls, desk, paper) while other objects also transmit light (cellophane, glass).

Achromatic Light

• Achromatic light is literally “the absence of color”.

• Can be seen as a black-and-white TV or monitor.

• An observer of achromatic light experiences none of the sensations we associate with red, green, blue, yellow, and so on.

Achromatic Light

• The only attribute of achromatic light is Quantity of light.

• Quantity of light can be discussed in the physics sense of energy, in which case we use the terms “intensity” and “luminance”, or in the psychological case, the term “brightness” is used.

Achromatic Light

• Intensity can be thought of as a scalar such as 0 defined as black and 1 defined as white

• Luminance is the amount, or intensity of light• Brightness is the perceived intensity of an

object• Black-and-white TVs can produce many

different intensities at a single pixel position

Varying Intensity Levels

Intensity (4 bit)

Intensity (1 bit)

Chromatic Color

• Presence of color!!!• Color perception involves Hue, Saturation,

and Brightness.• Other terms are tint, shade, and tone.

Hue

• Hue distinguishes between colors. • It is the pure color of the object.• Hue allows us to perceive the differences

between red, green, blue, purple, and yellow.

Saturation

• Saturation refers to how far a color is from a gray of equal intensity.

• Unsaturated colors include more white light than do the vivid, saturated colors.

• Red is highly saturated.• Pink is relatively unsaturated.• Royal blue is highly saturated• Sky blue is relatively unsaturated.

Brightness

• Brightness refers to the perceived intensity of a self-luminous object (such as a light-bulb or the sun).

• Think of brightness as being associated to a light emitting object rather than a reflecting object.

HSB Color Selector

Tints, Shades, and Tones

• 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 brightness.

• A tone is the consequence of adding both black and white pigments to a pure pigment

Psychophysics

• Color mixing methods are very subjective -- they depend on the human observer’s ability to perceive the varying colors, as well as the lighting, surrounding color, size of the sample, and the overall lightness of the environment.

• Colorimetry is the objective, quantitative branch of physics to measure color.

Colorimetry

• Dominant Wavelength is the wavelength of the color we see when viewing the light. Corresponds to the perceptual notion of hue.

• Excitation Purity is the proportion of pure light of the dominant wavelength and of the white light needed to define the color. Corresponds to saturation.

Colorimetry

• Luminance is the amount or intensity of light. Corresponds to brightness.

• A completely pure color is 100% saturated and thus contains no white light. White light and hence grays are 0% saturated, containing no color of any dominant wavelength.

Tristimulus Theory of Color Perception

• The tristimulus theory of color perception is based on the hypothesis that the retina has three kinds of color sensors (cones) with peak sensitivity to Red, Green, and Blue.

Luminous Efficiency function of the Eye

• The luminous efficiency function is the eye’s response to light of constant luminance. Our peak sensitivity is to yellow-green of wavelength around 550 nm.

Typical Spectral Energy distribution of a light

What Are Complementary Colors

• Complementary colors are those colors that combine to make white light.

Color Gamut

• The color gamut is all the colors that a device can display.