Human Visual System)

Color/Brightness Perception:

Human perception of light relies on retina cells. Cones (cone cells) are responsible for the perception of light (strong light) and color. rods (rod cells) can only perceive light and cannot perceive color, but its sensitivity to light is 10 thousand times that of cones. In the light environment of Microsoft, rods plays a major role, so we cannot distinguish colors in the dark environment. The luminous shooting modes of some digital cameras also simulate this feature. Three cone cells (CONES) in the retina have overlapping frequency response curves, but the response intensity is different. They are respectively red (570nm), Green (535nm), blue (445nm) light is the most sensitive and jointly determines the color perception. Luminance is proportional to the light intensity energy received by retina cells, but human sensitivity to light with the same intensity and different wavelengths varies. Perceptible wavelength ranges from nm ~ 780nm, known as visible light. Among them, the maximum light intensity sensitivity is generated for green (550nm) light. Human beings are sensitive to brightness signals, and are not sensitive to color signals. The usage frequency of the color signal can be reduced to half of the brightness sampling frequency (or even 1/4). For example, the YUV signal can be sampled at and. (Please turn your eyes quickly to see Hermann grid transfer sion)

Perception of space:

The spatial resolution of the eye, that is, vision, is usually measured in the reciprocal of the identifiable Angle of View (degree. The minimum recognizable visual threshold of normal people is about 0.5 ", and the maximum visual range is 200 degrees (width) × 135 degrees (height ). Spatial Frequency is the speed at which images change in space. Test the stripe with spatial sine variation. The brightness y (x, y) = B (1 + mcos2 π FX), given the stripe frequency F as a fixed value (as the width ), change the amplitude M (as contrast) to test the resolution capability. Obviously, the larger the value of M, the clearer the resolution. The minimum m value that can be distinguished under different conditions (different CPDS) is tested, and 1/mmin is defined as contrast sensitivity ). Defines the angle frequency of the human eye's perception of space: CPD: Cycle/degree, indicating the number of cycles of the black and white stripes that are scanned once every rotation of the eye. For a given stripe, this value is related to the distance from the human eye to the display screen. For a screen of the same size, the longer the exit, the larger the CPD. Usually, the human eye feels like a band-pass filter. The most sensitive information is 2 ~ 5 CPDs with a spatial cutoff frequency of 30cpd. For example, when we look at oil paintings and TV screens, when the distance is too long, CPD increases, and people's eyes cannot tell the details of pixel points, they will not feel the grain sensation. When a person observes a static image, his or her eyes will not rest in one place (except for mental patients). Generally, after the image is retrieved in a few hundred milliseconds, the object will be moved to another place to retrieve the image. This continues. This kind of movement is called saccadic eye movement ). Research shows that jumping motion can increase the contrast sensitivity, but the sensitivity peak is reduced.

Awareness of time frequency:

The time frequency is the speed at which the screen changes over time. Kelly. D. H is used to test the stripe with time-varying sine. brightness yt) = B (1 + mcos2 π ft ). Change m to test the contrast sensitivity of F at different time frequencies. Experiments show that the time frequency response is also related to the average brightness. Under normal indoor light intensity, the human eye's response to time frequency is similar to a band-pass filter. For ~ The 20Hz signal is the most sensitive and has a strong flickering sensation (flick). If the response is greater than 75Hz, the system returns 0 and the flickering sensation disappears. The frequency at which the flicker disappears is called the critical fusion frequency (CFF ). In a dark environment, the CFF is low-pass and will be reduced. At this time, the 5Hz signal is the most sensitive, and the blinking over 25Hz disappears. The cinema environment is very dark, and the update rate of the projector is 24Hz, which can reduce the film usage and machine speed. However, the brightness of the computer monitor is large, and the flash will disappear after 75Hz. After the flash disappears, the Brightness Perception is equal to the average brightness time (Taru's Law ). This low-pass feature can also be resolved to a temporary visual feature, that is, when the image disappears/changes, the brain image will not disappear immediately, but will be retained for a short time. The dynamic blur that we often feel in our lives is also related to sports images. Many electronic products are designed to take advantage of this phenomenon, such as dynamic scanning of LED digital tubes and rotating led subtitles. When you observe a moving object, the eye automatically follows its motion. This is called the following motion (Eye pursuit movement ). At this time, the relative velocity of the eye and the object will decrease, and we can recognize the object more clearly. For example, watching ball games (such as baseball games), although baseball is very fast, we can still see the approximate appearance of the ball due to followers (but there will be fuzzy sports ). If we turn our eyes in the direction of the fan, we will see the details of the fan blade clearly. The maximum speed of eye followers is 4 ~ 5 degrees/second, so we cannot see a bullet fly.