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Dynamic Range Compression & Color Constancy

Dynamic Range Compression & Color Constancy. Democritus University of Thrace 2006. Dynamic Range. Dynamic Range : The ratio between the maximum and the minimum tonal values in an image (cd/m 2 )

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Dynamic Range Compression & Color Constancy

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  1. Dynamic Range Compression& Color Constancy Democritus University of Thrace 2006

  2. Dynamic Range • Dynamic Range: The ratio between the maximum and the minimum tonal values in an image (cd/m2) • Commercial cameras have only a dynamic range of 256:1 (the maximum value is 256 times greater than the minimum value that they can capture) • Scenes with grater dynamic range than 256:1 are not captured correctly (intensities are clipped)

  3. The Dynamic Range Problem • The dynamic range of natural scenes is a lot more than 256:1(object in frond of a backlight) • The camera can capture correctly only the bright or the dark area, but never both of them (underexposure, overexposure) Normaly exposured Normaly exposured Underexposured (no visible details) Overexposured (no visible details)

  4. Images with dynamic range problem

  5. Center Surround Our approach –center surround • Every pixel (center) is compared with its neighborhood (surround) and is assigned a new value, in order to maximize the contrast in the dark regions of the image • Surround is the average intensity value of the neighborhood (0-255) • Center is the intensity of the pixel (0-255) center surround

  6. Before: Surround 18 Center 23 After: Surround 18 Center 85 Center Surround Center-surround transfer function • In a dark image region (surround is small) When the center is dark 85 18 23

  7. Before: Surround 18 Center 241 After: Surround 18 Center 245 Center Surround Center-surround transfer function • In a dark image region (surround is small) When the center is bright 245 241 18

  8. Before: Surround 240 Center 36 After: Surround 240 Center 36 Center Surround Center-surround transfer function • In a bright image region (surround is high) When the center is dark 36 240 36

  9. Before: Surround 240 Center 244 After: Surround 240 Center 244 Center Surround Center-surround transfer function • In a bright image region (surround is high) When the center is bright 244 244 240

  10. Center Surround Conclusion • In a dark image region (shadows or underexposured areas) the value of the pixel is increased relatively its neighborhood, increasing the local contrast • In a bright image region (normally exposured areas) the value of the pixel is unchangeable

  11. Results

  12. Results

  13. Results

  14. Results

  15. Results

  16. The unknown illuminant problem white white The HVS has a degree of color constancy white

  17. Images under color illuminant Incandescent lights Fluorescent light Green water

  18. The specularity problem • Specularities and direct light sources have a greater intensity than the response to pure white Which intensity is the response to pure white?

  19. Our approach: estimate the white response

  20. Results

  21. Results

  22. Results

  23. Results

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