->How to describe colors
Instead of perceptual cognition, let’s discuss it seriously and explore the essence of the problem - - -> Color Science.
1. Color transmitter (light source emission angle)
->Understand with object-oriented thinking:
● Birth point: light source
● Object: ray. The light source emits rays, which finally enter the human eye through direct exposure, reflection, refraction, etc.
● After the human eye receives light, the human eye cells produce a series of reactions, which are finally transmitted to the brain for recognition.
->//Supplement: Color is actually human perception. How to perceive it specifically -> See the follow-up: Distribution of photoreceptor cells in the human eye.
1.1 Elements of light
1.2 Light source
● An object that produces light. (Source of light)
○ Light: a physical phenomenon, cognitive measurement: wavelength, energy distribution
1.3 wavelength
● Light is theoretically infinite, but the wavelength of light we can see is limited
● Supplement: Any kind of light - - - must correspond to a spectrum (visible light: 400-700)
○ Left of 400: Ultraviolet
○ Right of 700: Infrared
○ The sun is at its brightest, showing a white color.
1.4 Energy distribution
● Light is a kind of wave, which is real, so it also has energy
. eg: A kind of light is a combination of multiple wavelengths.
● Energy unit of light:
○ Joule
● Then when we describe color, we can just use wavelength
● ->Problem: It’s too anti-human and cannot guarantee a simple description
● So there is: spectrophotometer
● Supplement: different wavelengths - - - Corresponding to different refractive indexes
1.5 spectrophotometer
● Function:
○ ① Split light to form a single wavelength light
○ ② Measure the actual energy of a single wavelength light
● Used to describe the specific energy intensity of light
○ Through the "spectroscopic" operation, we sense and measure the interval wavelength, and the final spectrum of light The energy distribution is concentrated near 550nm (green part)
1.6 Propagation of light
● When light reaches the surface of an object, more or less part of the energy will be absorbed.
● What color of light is absorbed -> We cannot see which color of light. We only see the reflected light
○ eg: black: absorption All light is reflected
■ White: All light is reflected
2. Color receiver (light source receiving angle)
1. Relative brightness perception
The human eye's perception of light is relative
: if you look at a light in a dark and bright place, you will have different perceptions. The contrast between the light and the ambient light is a kind of optical illusion.
2. Human eye HDR
//Concepts such as aperture and exposure are mentioned here. Anyone who has played with a camera should understand it
○ Some students in the barrage described these concepts like this:
■ Exposure: understand by putting water in a water tank
■ Aperture size: the diameter of the faucet
■ Shutter: how long the faucet is open
■ IOS sensitivity: water pressure of the water
■ Exposure: how much water can be held
● It can be understood this way: the human eye can see different details and levels of bright objects.
● Same as Dark
● But there is no guarantee that both functions will take effect at the same time. (The span is too large and cannot be supported by the human eye)
3. Distribution of photoreceptor cells in the human eye (how the human eye perceives color)
○ Pupil → Aperture
○ Lens → Lens
○ Retina → Place of perception
■ Retina: Has photoreceptor cells
■ Photoreceptor cells: Rod/rod cells: Sense light intensity (grayscale image)
■ Cone cells: Sense color
● Cone cells It can be divided into three types:
● S-cone (small wavelength/high frequency), perceives the blue range
● M-cone (medium), perceives the green range
● L-cone (low frequency), perceives the red range
● Three types for different people Cells are very different (further proof that color is human perception)
● How to sense: SML These three numbers are what people see → Color
4. Cone cells
These are the cells that the human eye uses to perceive color.
5.The nature of the human eye
● Receiver of light source
● Input: external light
● Output: neural electrical signals -> brain
6. Complete calculus formula
3. Color space
1. History①Three
conjectures about 1800
○
②1905 Munsell Color System
○ Proposed by artists, not based on scientific theory
○ Use color cards to describe colors
○ Somewhat close to HSV color space
③1931 CIE RGB system
○ Find someone to test. When the RBG and F light (test light) perform the same, decide (the idea is close to color matching)
○ CIE tested light of various wavelengths and finally obtained three lines of light.
○ Image visualization:
■ As can be seen from the above, there are three rgb, and you will know it if you get two of them, so using r' as the x-axis and g' as the y-axis, you can draw the point image and describe it.
④1931 CIE XYZ (more commonly used in the scientific community)
● Solved the problem of negative values above,
○ Multiply RGB (CIE 1931RGB) by a matrix to convert, convert to XYZ, and normalize
● If you want to visualize, three-dimensional is not easy to do → Solution: Y represents brightness (x represents chroma)
○ Final effect: This is the picture Block is called color gamut
● Existing problems and shortcomings:
○ Uneven color distribution
● So we try to define the color space ourselves (analogous to 0℃ -> the temperature at which water freezes)
2.Definition of color space
Define a color space that meets at least three important indicators:
① color gamut
, coordinates of the three primary colors (can form a triangle)
② Gamma
(The interior of the triangle is divided several times toward the white point) to facilitate calculation.
● Detailed explanation:
○ GAMMA is not a color space, but a way to define how to sample colors
○ GAMMA = 1, which means uniform segmentation (linear)
○ GAMMA ≠ 1, which is non-uniform segmentation
○ The human eye’s perception of dark parts Observe more and observe less bright parts (human eyes are more sensitive to dark parts)
○ Hardware problems of early computers
○ //For the knowledge points involved, you can refer to the GAMMA correction lesson for details
③ The middle of the white point
gamut triangle Point, white -> brightest point
④ Supplement:
○ Color gamut: all possible colors represented by a color space
○ RBG is just the triangle in the middle
○
⑤Customized color space
● Changing the color gamut, white point position, and gamma value is actually a new color space.
● Linear itself is not a color space -> it just refers to whether gamma is linear
4. Commonly used color spaces and color models
1. Color model
A method of describing (arranging) colors using certain rules.
For example:
■ RGB, CMYK, LAB
2. Commonly used color spaces
Color space: meet at least three indicators: color gamut, white point, gamma
● A classic picture:
Supplement: CMYK system: (Typical subtractive color system)
5. Color space conversion
#Assignment:
1. What is the definition of color space?
● To define a color space, at least three indicators must be met: color gamut, gamma, and white point
○ Color gamut: It is the coordinates of the three primary colors. (Can be understood as all possible colors represented in a color space)
○ Gamma refers to how the triangle is divided.
○ White point, the middle point of the color gamut triangle. (Understanding: White is the result of all colors superimposed together, and the white point is the brightest point in this color space.)
2. What is the visible light range of the human eye?
Light with a wavelength of 400-700 is visible light.