Digital Image Processing

Transcription

1 Digital Image Processing IMAGE PERCEPTION & ILLUSION Hamid R. Rabiee Fall 2015

2 Outline 2 What is color? Image perception Color matching Color gamut Color balancing Illusions

3 What is Color? 3 Visual perceptual property with the spectral sensitivities of the eye light receptors versus visible range of the electromagnetic spectrum.

4 What is Color? 4

5 Are Black and White Colors? 5

6 What is Color? (cont d) 6 Has color any information or it s just pleasing? Grayscale Color

7 What is Color? (cont d) 7 Various Types of Radiation Radiation source Opaque Reflective object Reflected Radiation sensor Electrical Signal Self-luminous object Emitted Radiation

8 What is Color? (cont d) 8 Various Types of Color Mixing Additive Emitted Radiation Subtractive Reflected Radiation

9 Image Perception 9 Image data is composed of different quantities: Spatial frequency Luminance Image perception is composed of different qualities or attributes: Brightness Color Edges Our goal is to extract image perception qualities from image data quantities.

10 Image Perception (cont d) 10 The luminance of an object shows the amount of light radiation of the object which is independent of other objects luminance. Brightness of an object is perceived luminance of object and is affected by surround luminance.

11 Image Perception (cont d) 11 The retina contains two types of photoreceptors, rods & cones. The rods are More numerous,120 million More sensitive to luminance than the cones Not sensitive to color The cones are Much less in number, 6 to 7 million Concentrated in a very thin, rod-free, densely packed cones region In task of providing the eye's color sensitivity

12 Image Perception (cont d) 12 Optic disc or blind spot has no photoreceptors. Rods are 20 times more than cones. What is its consequences?

13 Image Perception (cont d) 13 Eye performs differently when adapted to darkness or light. Dark adapted visibility is called scotopic. Light adapted visibility is called photopic Rods can t perceive red!

14 Image Perception (cont d) 14 Eye has different luminous efficiency for different wavelengths. Green

15 Image Perception (cont d) 15 individual color-receptor mechanisms contribute differently to the total foveal sensitivity:

16 Color Matching 16 Laws of Color Matching: Any color can be matched by mixing at most three colored lights The luminance of a color mixture is equal to the sum of the luminances of its components The human eye cannot resolve the components of a color mixture A color match at one luminance level holds over a wide range of luminances

17 Color Matching (cont d) 17

18 Color Matching (cont d) 18 RGB images have red, blue and green channels Which color will be preserved more in each channel? Which color will be damaged more in each channel? Main Picture Red Channel Green Channel Blue Channel

19 Color Matching (cont d) 19 Loss of which channel results in less damage? How many bits you will use for each channel? Main Picture Loss of Green channel Loss of Blue channel Loss of Red channel

20 Color Matching (cont d) 20 XYZ color system Y corresponds to luminance As we saw luminance is more Important than color, so we may Use more bit for it.

21 Color Matching (cont d) 21 HIS color matching: Hue: Angle of color. Saturation: How much pure is color? (adding more white decrease its pureness) Value: Intensity of color

22 Color Gamut 22 CIE chromaticity diagram It covers all thr visible spectrum.

23 Color Gamut (cont d) 23 Perceptual non-uniformity of xy chromaticity Ovals show just noticeable Chromaticity differences.

24 Color Gamut (cont d) 24 Each color matching system covers a different subset of visible spectrum. This may lead to some problems, e. g. you may see different colors on screen And printed image.

25 Color Balancing 25 Converting actual colors to desired colors. Results of different color balancing methods: Stanford EE368 course

26 Illusions

27 Background effect on perception 27 Perceived colors depends on background. Are these areas different?

28 Background effect on perception 28 Perceived colors depends on background. Actually they are the same!

29 Strange elephant 29 Sometimes our brain can t perceive an image because it is impossible in real world. Elephant with unknown number of feet.

30 Impossible Cube 30 It is even possible that brain can t perceive an image of a real world event! Can such a cube exist in real world? What do you think now?

31 Penrose Triangle 31 Another real world shape that seems impossible: Sculpture of impossible Triangle in Perth (Australia)

32 Penrose Triangle 32 Another real world shape that seems impossible: Impossible triangle from other views

33 Penrose Triangle (cont d) 33 This illusion happens because different angles can have same images on retina.

34 Mach band effect 34 This effect is a result of human eye Tendency to perform Laplacian operations. It shows that brightness is not a monotonic function of luminance

35 Mach band effect (cont d) 35 Luminance versus Brightness

36 Fraser Spiral 36 Eye receive sequential visual perceptions even when we concentrate on a fixed image. It also continues its movements while perceiving. This may result in this illusion:

37 Simultaneous Contrast 37 Are these circles same color?

38 Simultaneous Contrast 38 Are these circles same color?

39 Simultaneous Color Contrast 39 Even colors may perceived differently according to background. You may still doubt that!

40 Checker-Shadow Illusion 40 A & B have the same color!

41 End of Lecture 2 Thank You! Images are taken from The copyright holder of images, release them into the public domain.