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Sky Based Light Metering for High Dynamic Range Images
@ 22nd Pacific Conference on Computer Graphics and Applications (Pacific Graphics 2014)


Yulia Gryaditskaya1      Tania Pouli2      Erik Reinhard2      Hans-Peter Seidel1     
1MPI Informatik      2Technicolor R&D      

2014
Our algorithm employs sky models to estimate a scaling that can convert images to absolute luminance values. (a) We detect and segment sky pixels, which are (b) analysed to determine properties of the sky dome and camera parameters. (c) By scaling the image with our estimation, we obtain absolute radiometric values, allowing images to be processed by appearance models such as Reinhard et al. 2012. (d) The same processing was applied to the image after scaling it with the ground truth scale factor. (e) The CIE ΔE94 differences between (c) and (d) indicate good correspondence between our estimate and the ground truth.


Abstract

Image calibration requires both linearization of pixel values and scaling so that values in the image correspond to real-world luminances. In this paper we focus on the latter and rather than rely on camera characterization, we calibrate images by analysing their content and metadata, obviating the need for expensive measuring devices or modeling of lens and camera combinations. Our analysis correlates sky pixel values to luminances that would be expected based on geographical metadata. Combined with high dynamic range (HDR) imaging, which gives us linear pixel data, our algorithm allows us to find absolute luminance values for each pixel - effectively turning digital cameras into absolute light meters. To validate our algorithm we have collected and annotated a calibrated set of HDR images and compared our estimation with several other approaches, showing that our approach is able to more accurately recover absolute luminance. We discuss various applications and demonstrate the utility of our method in the context of calibrated color appearance reproduction and lighting design.

Downloads


2014 Paper
(9 MB)
2014 Supplemental
(3 MB)
2014 Calibrated HDR Images
(399 MB)

Calibrated HDR Images


Our HDR dataset was photographed with a tripod-mounted Nikon D2h digital SLR camera, which acquires up to 9 images each spaced 1 EV apart, using autobracketing. The white point was set to a fixed 6700K (nearest to D65 that this camera supports) and the exposures were saved in the sRGB color space. Images were assembled into linear HDRs using Greg Ward's Photosphere application, which we have also used to derive the response curve of the Nikon D2h. We placed both an 18% grey card and a GretagMacbeth color checker in each scene for calibration. To scale the HDR images to absolute values, measurements of Yxy components of the grey card and several patches from the ColorChecker were taken with a Minolta CS100 color and luminance meter. We also transformed the images to the D65 white point, as per the sRGB standard.

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Citation

2014 Yulia Gryaditskaya, Tania Pouli, Erik Reinhard, Hans-Peter Seidel
Sky Based Light Metering for High Dynamic Range Images
Computer Graphics Forum (Proc. Pacific Graphics), 2014
@ARTICLE{CGF:Gryad:14,
  author = {Gryaditskaya, Yulia and Pouli, Tania and Reinhard, Erik and Seidel,
	Hans-Peter},
  title = {Sky Based Light Metering for High Dynamic Range Images},
  journal = {Computer Graphics Forum (Proc. Pacific Graphics)},
  year = {2014},
  volume = {33},
  pages = {61--69},
  number = {7},
  address = {Oxford, UK},
  booktitle = {22nd Pacific Conference on Computer Graphics and Applications (Pacific
	Graphics 2014)},
  date = {2014},
  doi = {10.1111/cgf.12474},
  issn = {1467-8659},
  publisher = {Wiley-Blackwell},
  url = {http://people.mpi-inf.mpg.de/~jgryadit/Papers/LightMetering/}
}