1 Perception and VR MONT 104S, Fall 2008 Lecture 7 Seeing Color

2 Wavelengths and Color 1. Light is composed of photons with different wavelengths. 2. Humans are sensitive to a small range of wavelengths: nm.

3 Color Perception is not equivalent to wavelength When light hits a surface, some is reflected and some is absorbed and some may be transmitted, depending on the surface properties. Some of the reflected light may reach the eye and is focused on the retina. Neural processing of the light that hits the retina determines our perception of color. Perception of color is not an inherent object property. It depends on neural processing. Different people may perceive the same wavelengths (or spectra) differently (e.g colorblind people).

4 Color Space Human color perception is defined in terms of 3D color space. The 3 dimensions of this space are Hue, Saturation and Brightness. 2 sides of color space (or spindle). brightness goes from bottom (dark) to top (light).

5 Hue and Saturation A slice through the color spindle shows a color circle of a given brightness. Hue is the angle of the color within the circle. It varies as you move around the circumference of the circle. Saturation is the distance from the center of the circle. The most vibrant colors (high saturation) are at the outside of the circle. The grayest colors (low saturation) are at the inside of the circle.

6 Trichromatic Color Theory Observation: You can match any color in color space with a linear combination of 3 appropriately chosen (non co-linear) colors. Color Matching

7 Three types of cone photoreceptors The retina has three types of cone photoreceptors. Each has a different pigment molecule that absorbs best at a given wavelength. S (Short wavelength), "Blue"=>440 nm M (Middle), "Green" => 530 nm L (Long), "Red" => 560 nm A single wavelength stimulus will stimulate each type of cone by some amount. A mixed color match is a set of stimuli that stimulate the cones by the same amounts as the single wavelength.

8 Color Opponency Observation: Color appears to be organized in terms of 3 sets of complementary colors. (This was first established by Hering) Red/GreenBlue/YellowWhite/Black Evidence: 1) We don't see a Reddish-Green when mix Red and Green light. Instead, we see less red and green and more yellow. 2) Color adaptation: If you stare at a Green surface for a while, then look at a white background, it will appear red.

9 Color Adaptation Stare at the white dot

10 Color Adaptation Stare at the black dot

11 Color Opponent Cells Retinal Ganglion Cells and LGN cells have center-surround receptive fields that show color opponency. R+ G- R+ G- R- G+ R- Concentric single-opponent (G+R)- (G+R)+ (G+R)- Concentric Broadband B+ (R+G)- B- (R+G)+ Co-extensive single-opponent

12 Types of Single-opponent cells Concentric single-opponent: Signal Red vs. Green. These are also useful for edge detection. Concentric broad-band: Useful for overall luminance changes (responsive to white light changes). Useful for edge detection. Co-extensive single-opponent: Signal Blue vs. Yellow. Blue cones are probably not involved in edge-detection because of chromatic aberration. The image of the blue wavelength light is more distorted on the retina than of red or green wavelength light.

13 Responses of Single-opponent cells

14 Color Contrast Observation: The appearance of a color depends on the surrounding colors. There is only one shade of pink in this picture.

15 Color Constancy Some surfaces appear the same color, even when the ambient lighting changes. The wavelength of the blue squares on the left matches the wavelength of the yellow squares on the right!

16 Color Constancy This is the same picture as before, but the surrounding squares have been masked out to show the "true" color of the blue and yellow squares.

17 Retinex Theory The perception of a color depends on the colors of all objects in the scene. Edwin Land came up with Retinex Theory to predict what appearance colors would have for a given scene. Retinex theory integrates colors across space by measuring the ratios of color changes across color borders. Because we can only measure the relative luminance of surfaces, the retinex theory uses the brightest region in the scene as an anchor. It sets this region to white, and all other regions are computed relative to the anchor.

18 Color Blindness A small proportion of the population is missing one of the types of cones. Another small proportion has an anomalous pigment. Trichromats: 3 pigments: Normal color vision Dichromats: 2 pigments: Red/Green color blind (1% of males and 0.01% of females) Missing red cones: protanopia Missing green cones: deuteranopia Blue/Yellow color blind (0.02% of males, 0.01% of females) Missing blue cones: tritanopia Color anomalies: One of the pigments has a different absorption spectrum.

19 Colorblindness test Can you see the numbers?