8.1 Exploring high dynamic range imaging: §1 introduction to hdri

The advent of digital photography has been marked by numerous and significant technical gains in terms ofresolution, noise, and convenience in post processing. However, one area in which digital media still trails its analogue counterpartis dynamic range. A capturing media’s dynamic range is its ability to maintain a wide range of information, from dark to brightvalues; traditional silver halide camera film excels in its ability to capture a wide range of brightness levels over even that of thepriciest digital cameras.

Almost anyone with photography experience may have noticed that, in certain conditions, something they hadintended to capture was either too bright or too dark to see clearly in the actual print, often becoming completely white orblack and devoid of any useful information. To propose a couple of scenarios in which this phenomenon may occur: One may imaginehimself taking a picture of a subject adorned with jewelry on a sunny day; while the camera meters for the subject’s face, theluster of the silver necklace may cause it adopt a white glow against the skin of the subject, particularly if the subject hasdarker skin (since the camera “thinks” that the average brightness is lower than you or I would perceive it to be). Likewise, taking apicture of a subject with a sunset in the background may cause the subject to lose any three-dimensionality and become a silhouette.The capturing media, in these cases, cannot cope with the extreme contrast of the scene, which, in real-life cases, may often exceeda 50,000:1ratio.

Traditionally, this problem was dealt with in the darkroom by techniques such as dodging and burning. Althoughimage editing software such as Adobe’s Photoshop offer similar tools, the limited color depth of 8-bit file formats and evenproprietary 12-bit RAW formats constrains their efficacy. Along with the myriad performance gains of digital photography in mostother technical areas over traditional media and the relative ease in processing of digital images, there has been great impetus toincrease the amount of captured information digitally. This is usually done by taking several images at different exposure levelsand then combining them to form a high color depth result, usually 32-bits. However, a major problem surfaces with this approach; witha few exceptions, current technology in display technology, printing devices and print media simply cannot handle 32-bit files,severely limiting the usefulness of these formats. One solution is to compress the high color-depth file to one with lower colordepth, such as an 8 bit JPEG or TIFF format, while maintaining the visual perception of increased color depth. In other words, one mayattempt to map the extreme highlight and shadow detail present in the high color depth image into one with lower depth. This type oftransformation is often termed “tone mapping”, and the conversion of multiple images into a high color depth image and “tone mapping”forms the two major bodies of “high dynamic range” imaging (HDRI).