Deep Images

HDRI: Faking Film's High Bit Depth

High Dynamic Range Imaging was originally developed for lighting 3-D scenes, as a way to capture the real-world range of luminescence, and has been a boon to realistic 3-D lighting work for many years.

But, it has another use. By tone mapping the image, HDRI's wide contrast range can be represented in 8-bit space to stunning effect—preserving details in the shadows and minimizing clipping in highlights. As this aesthetic became popular, several techniques have developed to create HDRIs from digital snapshots and then convert them for display on monitors or in print.

Creating HDRI images from digital photographs requires Bracketed Exposure—taking a set of photos with different exposure settings to give a wider collective latitude than the camera natively allows (Figure A). Afterward, the bracketed images are combined into a single HDRI. Although this can be in the terminal, it's far easier with CinePaint's self-explanatory Bracket to HDR plugin (included in the package). Once created, the HDRI either can be turned into a light probe (for lighting a 3-D scene) or tone mapped for display and/or printing (Figure B).

Figure A. A Set of Photos with Different Exposures

Figure B. Using the Bracketed Exposures to Create the Perfect Picture

Tone mapping interpolates an HDRI into 8-bit space without clipping the high and low ends—it compresses the image nonlinearly to preserve the details otherwise lost. The result is a much richer image than could normally be captured by 8-bit equipment. At the moment, tone mapping isn't available in CinePaint or Krita (although it is on Krita's to-do list). Instead, pfstools, a command-line suite of algorithms for configuring the interpolation curves, does the job.

Fortunately, for those of us who don't like experimenting blindly in the terminal, a thoughtful soul has written a GUI that offers the full range of options available at the command line, with a preview window. The program, Qpfstools, along with an introduction and tutorial, can be found here: theplaceofdeadroads.blogspot.com/2006/07/qpfstmo-hdr-tone-mapping-gui-for-linux_04.html.