Photographic Tone Reproduction for Digital Images

1. Photographic Tone Reproduction for Digital Images Brandon Lloyd COMP238 October 2002

2. The range of light in the real world spans 10 orders of magnitude! A single scene’s luminance values may have as much as 4 orders of magnitude difference A typical CRT can only display 2 orders of magnitude Tone-mapping is the process of producing a good image of HDR data Problems with High Dynamic Range (HDR)

3. Zone System • Used by Ansel Adams. Utilizes measured luminance to produce a good final print • Zone: an approximate luminance level. There are 11 print zones • Middle-grey: Subjective middle brightness region of the scene, typically map to zone V • Key: Subjective lightness or darkness of a scene

4. Zone System • Measure the luminance on a surface perceived as middle-gray - map to zone V • Measure dynamic range from both light and dark areas. • If dynamic range < 9 zones then full range can be captured in print • Otherwise dodging-and-burning must be used to bring out details • Dodging-and-burning: Witholding or adding light in development to lighten or darken the final print

5. Algorithm • First apply a scaling to the whole image. This similar to setting the exposure for mapping to middle-gray • Apply automatic dodging-and-burning to compress dynamic range if necessary

6. Luminance Scaling • Use log-average luminance to approximate the key of the scene • In a normal-key image middle-gray maps to a key value a = .18 suggesting the function:

7. Luminance Scaling

8. Luminance Scaling • Control burn out of high luminances

9. Automatic Dodging-and-Burning • Think of this as local adaptation, choosing a key value for every pixel • Need a properly chosen neighborhood • Dodging-and-burning is applied to regions bounded by large contrasts • Use center-surround functions to measure local contrast at different scales

10. Automatic Dodging-and-Burning • The effects of using different scales s1 s2 s3 s1 Center s2 Surround s3

11. Automatic Dodging-and-Burning • Use difference of Gaussians for center-surround function

12. Automatic Dodging-and-Burning • Choose largest neighborhood around a pixel with fairly even luminances • Take the largest scale that doesn’t exceed a contrast threshold: • Final local operator

13. Automatic Dodging-and-Burning Details recovered by using dodging-and-burning

14. Results • Used FFT to compute the Gaussians • 8 discrete scales ranging from 1 pixel to 43 increasing by a factor of 1.6 •  = 0.05

15. Results

16. Results

17. Results

18. Comparison Durand et al. Reinhard et al.

19. Comparison Durand et al. Reinhard et al.