2012-12-25

Astronomy HDR still in its infancy

It was a nice evening, clear sky and new 10cm refractor was ready for me at Vyškov's observatory. I've always been fascinated by earthshine (Earth's reflected light) and the evening has been ideal for my project: acquire a high dynamic range (HDR) image of Moon including earthshine. Unfortunately, the earthshine is very difficult to record and to display due to enormous intensity range between light and dark parts (which is about one thousand).

Softening contrast

 I've currently experience with using global mapping methods which non-linearly compress high to low ranges. When, we defines contrast as ratio of intensity of a detail against to a local mean, the contrast is compressed together with intensity. As result of the compression, pictures has characteristically softened details. 

Perhaps our perceive prefers the contrast over global luminance, the main problem of HDR photography is compress of the global luminance while preserving the local contrast.

Luminance HDR

During last summer, I've spend some time with algorithms for rendering HDR images. The project which I've been studied deeply is Luminance HDR. Advanced algorithms included in the package works well (at least acceptable) on both normal (landscapes, interiors or erotic photographs, etc) and astronomical scenes.

Earthshine

The algorithm for taking photos has been:
  • I taken set of exposures with different exposure times to cover full range of intensities. I'm preferring division by two sequence: 16,8,4,..  1/32,1/64... For long-exposure night scenes and high ISOs, darks are necessary.
The processing has parts:

  • ll images are unified on same exposure time. Images are spatially (angular) arranged and composed by weighting summation. Weights are choose as a function with maximum in half saturation range and zero outside the range (pfscalibration).
  • The tone mapping maps HDR to a limited range. The package offers many methods, but for my requirements, the just "A Perceptual Framework for Contrast Processing of High Dynamic Range Images" by Rafał Mantiuk, Karol Myszkowski, Hans-Peter Seidel is acceptable (source).



Images

Images of Moon and earthshine has been taken on Oct 20 2012, about 17h UT by Canon EOS 60D (ISO 400). 15 exposures from 32 to 1/64 sec per factor  two. Near horizon, in primary focus of 10cm refractor, Medium seeing, light mist. Approximate ratio of dark and light 1:1000.


Near MonteBoo observatory, superior night in Brno. 32,16,8...1/512,1/1024 s, fish-eye, Canon EOS 60D (ISO 640), ratio 1:2000.

My office. Canon Powershot S90 (ISO 80): 2, 1/2, 1/8, 1/32, 1/128, 1/512 s. Ratio 1:1000.

Český ráj, sandstone cliffs. Canon Powershot S90 (ISO 80):  1/25, 1/60, 1/200, 1/1600 s. Ratio 1:60.


Conclusions

Although, the images at day-light conditions are very nice,
there are some open problems:
  • Some aureole, worse contrast and lower resolutions are visible. I'm meaning ones are products of too-rough  arranging (needs better subpixels). Note that all images are published in two- or tree-times binning.
  • I've worry about correct showing of really full range, single exposures shows even more details.
  • Also many defect (black areas in clouds, purple in dark exposures) are corrupting visually images.
  • I've no ideas about better intensity profile (just gamma is used).
  • The best rendering algorithm is Mantinuk's. Acceptable results gives  also Fattal (both removes gradients). All others, including Gaussian blurring, practically works on base of global methods and gives poor results in contrast.
  • The night exposures are even worse, probably growing noticed defects.
  • As the best rendering, I'm supposing images which are visually indistinguishable against to original scene (by my eye). Various strongly-coloring pictures looks cool, but ones are out of my interest.

Why, an astronomical CCD image is missing? Because there is no converter FITS -> EXR (HDR). :)

Using of the HDR algorithms on astronomical pictures is my future goal.

Many thanks for Fantom, who focused me on qtpfsgui (as  the start point).
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