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HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 1 / 16
HDR Images(High Dynamic Range)
© 1995-2016 Josef Pelikán & Alexander Wilkie
CGG MFF UK Praha
http://cgg.mff.cuni.cz/~pepca/
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 2 / 16
Dynamic Range of Images
bright part(short exposure)
dark part(long exposure)
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 3 / 16
Dynamic Range of Images
bright area (short exposure)
dark area (long exposure)
10-6 10-3 1 103 106
reality
10-6 10-3 1 103 106
image(0 .. 255)
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 4 / 16
HDR Graphics
High Dynamic Range– As opposed to Low Dynamic Range – standard images– Pixels – floating point numbers– e.g. float[3] for RGB (96bpp)
Capture of HDR data– Synthetic / via computation (rendering)– Photographic (multi-exposure, specialised cameras)
Display on LDR output devices– Transfer to reduced range („tone-mapping“)
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 5 / 16
RGBE Pixel Format (Radiance)
Format of .hdr files (Radiance)– Reduced size (only 4 bytes per pixel)– Individual mantissa [RGB], shared exponent [E]
Mantissa [RGB]– Type float, normalised to between ½ a 1
Exponent [E]– Binary exponent in two's complement (8 bit number)
Example: [ 0.3, 0.02, 0.1 ] = [ 0.6, 0.04, 0.2 ] · 2-1 → [ 153, 10, 51, 127 ]
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 6 / 16
Other HDR Formats
OpenEXR (.exr)– Industrial Light & Magic (G. Lucas 1975, Star Wars etc.)– Completely open, libraries are open source– Several compression algorithms (ZIP, wavelets), type half– User-extensible pixel format
Portable Float Map (PFM)– Analogous to PPM / PGM / PBM– Three float per pixel– No compression
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 7 / 16
Multiple exposure– Static scene– Constant aperture, varying exposure time– Sequence e.g. from 1/1000s to 2s– Built-in „bracketing“ (–2 EV, 0, +2 EV)– „Super-bracketing“ (e.g. 7 photos in rapid succession)
Processing a sequence of images to HDR– Conventional photo applications (PhotoShop, GiMP..)– HDR Shop (http://www.hdrshop.com/)– Functions: auto-calibration, registration
HDR Photography
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 8 / 16
kvalitativně známá funkce– konkrétní konstanty je třeba nastavit (kalibrace)– „auto-kalibrace“ při skládání (předpoklad stejné předlohy)
Křivka citlivosti senzoru (CCD)
0
255
hodnotapixelu log
10 Exp = log
10 ( Radiance·Dt )
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 9 / 16
HDR Acquisition Example
15 exposures between 1/2000s and 8s (1 EV steps)Assembly: HDR shop
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 10 / 16
Reproducing HDR Images
Simple trimming of image range– Overflow → flare (white or „glare“ effects)
„tone mapping“– Standard: transformation of entire HDR range to LDR– Global vs. local techniques– Local contrast preservation etc.
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 11 / 16
„Tone Mapping“
Transformation of HDR to LDR– Goal: maintaining of contrast & details in dark & light areas
global vs. local conversion
10-6 10-3 1 103 106
HDR
10-6 10-3 1 103 106
LDR(0 .. 255)
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 12 / 16
Logarithmic curves, sigmoids, ..– Implemented as LUT and/or GPU shader
Global Operators
0
255
pixelvalue L
d
2 4 6 8 10
Ld=log(Lw+ 1)
log (Lw ,max+ 1)
HDR input Lw
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 13 / 16
Sigmoid
0
255
pixelvalue L
d
2 4 6 8 10
Ld=Lw
Lw+ 1
HDR input Lw
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 14 / 16
Gamma Compression
Existing mechanism– Disadvantage: discoloration
Gamma compression of intensity– Colour information is retained– Intensity is compressed as above
Ld=Lwγ
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 15 / 16
Local Operators
Goal: locally retain contrast– Edge detection– Local analysis of brightness
Bilateral filtering (1998+)– Non-linear filtering
according tointensitydifference
HDRLDR
filter
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 16 / 16
Applications for HDR
Photography– Improved reproduction of natural scenes– Powerful „tone mapping“, natural looking results– HDR panoramas (sun in the frame, sky vs. landscape)
CGI (computer generated images)– Standard use for „environment mapping“ (light maps
of the surround)– All internal results and calculations are „HDR“– Realistic looking glossy reflections, motion blur, etc.
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 17 / 16
Examples – „tone mapping“
LDR Tone-mapped HDR
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 18 / 16
Examples – „tone mapping“
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 19 / 16
Examples – „tone mapping“
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 20 / 16
Examples - „tone mapping“
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 21 / 16
Example
Environment-map (latitude/longitude mapping)
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 22 / 16
Light Reflection Example
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 23 / 16
Environment („cube-map“)
HDR 2013 © Josef Pelikán, http://cgg.mff.cuni.cz/~pepca 24 / 16
End
Further information:
http://www.debevec.org/
http://cgg.mff.cuni.cz/~pepca/hdr/
http://www.hdrshop.com/
http://www.mpi-inf.mpg.de/resources/hdr/