Flash Photography: 1

Transcription

1 Flash Photography: 1

2 Lecture Topic Discuss ways to use illumination with further processing Three examples: 1. Flash/No-flash imaging for low-light photography (As well as an extension using a non-visible flash) 2. Multi-flash imaging for non-photorealistic rendering 3. Dual Photography Notes are from Marc Levoy, Stanford University 2

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23 Dark Flash Photography SIGGRAPH 2009 Dilip Krishnan and Rob Fergus New York University Dilip Rob (Dilip Krishnan did his UG at NTU and Masters at NUS in Mathematics) Idea is to replace the visible flash with a Ultra-Violet and Infrared Flash They call this a dark flash, since the people in the scene can t see the flash 23

24 Example Dark Flash (you don t see the flash) Ambient light With fast exposure. Note the noise. Their reconstructed image, combining F and A. Comparison with ambient light with long exposure. 24

25 Idea is simple* Don t use a visible flash, instead use a non-visible flash Perform some type of fusion similar to the previous paper to filter the ambient image to remove noise Since overall idea is from Petschnigg s 2004 paper, this paper is written to explain details of building a dark-flash camera * Simple to state, building the device is not obvious. 25

26 Light Their flash was modified to significantly attenuate the visible light to only allow IR and UV to pass through (see paper s Appendix). 26

27 Hardware properties UV Visible Range IR This plot shows camera RGB response to spectrum Shows flashes ability to produce Irradiance on a while surface. Camera response to the irradiance above. Fuji IS Pro (Camera) + Nikon SB-14UV (Flash) This camera is known for its response to UV and IR light. Flash was modified to remove UV absorber, they also placed a filter on the flash to remove visible light (see previous slide) 27

28 Reconstruction/Filtering This paper does not use the bi-lateral filter method by Petschnigg Instead, they fuse the images in the gradient domain Somewhat similar to Poisson Image Editing, but a bit different A simplified version of what they are solving for*: R 2 2 Find a reconstructed image R that minimizes the equation R should look like the ambient image The gradient of R should be similar to the gradient of the dark flash image D * Approach in the paper is more complicated and tunes the equations based on noise and spectral response properties. Refer to the paper. 28

29 Dark Flash Summary Very cool idea and more practical than the original flash/no flash Suspect you may see something like this available in consumer cameras in the future 29

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42 Helmholtz reciprocity light camera scene Note from Levoy (

43 Helmholtz reciprocity camera light scene

44 Measuring transport along a set of paths projector photocell scene

45 camera Reversing the paths point light scene

46 Forming a dual photograph dual camera projector dual photocell light scene

47 dual camera Forming a dual photograph dual light image of scene scene

48 Physical demonstration light replaced with projector camera replaced with photocell projector scanned across the scene conventional photograph, with light coming from right dual photograph, as seen from projector s position and as illuminated from photocell s position

49 2005 Marc Levoy Related imaging methods time-of-flight scanner if they return reflectance as well as range but their light source and sensor are typically coaxial scanning electron microscope Velcro at 35x magnification, Museum of Science, Boston

50 The 4D transport matrix projector photocell camera scene

51 The 4D transport matrix projector camera P C pq x 1 mn x 1 mn x pq T scene

52 The 4D transport matrix mn x pq C = T P mn x 1 pq x 1

53 The 4D transport matrix mn x pq C = mn x 1 T pq x 1

54 The 4D transport matrix mn x pq C = mn x 1 T pq x 1

55 The 4D transport matrix mn x pq C = mn x 1 T pq x 1

56 The 4D transport matrix mn x pq C = T P mn x 1 pq x 1

57 The 4D transport matrix mn x pq C = T P mn x 1 pq x 1 applying Helmholtz reciprocity... pq x mn C = T T P pq x 1 mn x 1

58 2005 Marc Levoy Example conventional photograph with light coming from right dual photograph as seen from projector s position

59 2005 Marc Levoy Dual photography from diffuse reflections the camera s view

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