Foreword

Though one could argue that the image is a human invention, high dynamic range is not. Light spans an enormous range, and our eyes can adapt from the dim level of a starlit night to intense daylight, spanning over eight orders of magnitude (100 million to 1). This is comparable to the energy difference between getting going on a bicycle and taking off in a jumbo jet.

With this kind of range in real-world illumination, how have we gotten by so long with image contrast ratios of less than 100 to 1? The answer is deceptively simple. Although the range of light is enormous, our brain cares most about reflectance, and its range is comparatively modest. In fact, it is very difficult to create a surface that reflects less than 1% of the light it receives, and reflectances over 100%% are forbidden by conservation of energy. Since the brain is trying to figure out surface reflectances based on what it sees, the huge range of illumination is often more of a hindrance to perception than a help. Therefore, an artist who skillfully removes the excess dynamic range in a painting is actually doing your brain a favor, making it easier for you to see what he (or she) sees. Skill and practice are required to accomplish this feat, for a painter who is inexperienced and does not know how to create the illusion of range within the limited reflectances available will create something more like a cartoon than a rendition. Realistic painting techniques were perfected by the Dutch masters during the Renaissance and are little practiced today.

Photography poses a similar problem in a different context. Here, we have the ability to record an image equivalent to that which lands on your retina. Starting from a negative, skill is once again required to render a positive that captures what matters to the brain within the limited reflectances of a black-and-white or color print. For scenes that contain little dynamic range to begin with, nothing needs to be done and the whole process can be automated. For more challenging subjects, such as storm clouds over mountains or the shadowed face of a child in the sunshine, the darkroom technique of burn-and-dodge is indispensable. With this method, either a round obstructer on a stick is used to "dodge" overexposed regions or a piece of cardboard with a hole cut out is used to "burn" underexposed regions during enlargement. This is an arduous, manual process that requires considerable talent and experience to achieve good results. The move to digital cameras has made such exposure adjustments even more difficult due to their limited dynamic range, which is one reason professional photography has been so slow to migrate from film.

High dynamic range imaging (HDRI) enables the centuries-old practices of renowned artists in a new, digital arena. HDRI permits photographers to apply virtual burn-and dodge techniques in their image editing process, leaving as little or as much to automation as they wish. Digital artists using HDRI are able to create virtual worlds that are as compelling as the real world since the physics of light can be simulated in their full glory. Using physically based rendering and image-based lighting, HDR photography and virtual scenes and objects may be seamlessly combined. This is common practice for mainstream productions in special effects houses around the world, and even television studios are getting into the act. Enthusiasts, both professional and amateur, are also pushing the envelope, and commercial software, shareware, and freeware are available to assist the transition from a traditional imaging and rendering pipeline to HDRI.

Of course, not everyone needs HDRI. If the starting point is low dynamic range and the ending point is low dynamic range, there is little need for HDR in between. For example, a graphic artist who creates on a digital canvas and prints his work on paper gains no immediate benefit since reflection prints are inherently low dynamic range. It may be easier using existing tools to simply stick to printable colors and create the desired "look" using a conventional WYSIWYG paradigm. In the long run, even such an artist may enjoy features and effects that HDRI enables, such as lens flare and the like, but these will take time to reach the mainstream. Meanwhile, there is much for the rest of us to explore.

In this book, Christian Bloch introduces the topic of high dynamic range imaging and how it relates to the world of computer graphics, with an emphasis on practical applications. Starting with film photography, Mr. Bloch re-examines the digital image, looking at file formats and software for storing and manipulating HDR images and going on to discuss how such images can be captured using conventional digital cameras. This is followed by a description of tone-mapping operators that can transform HDR images into something viewable and printable on conventional devices. Then, a more complete discussion of HDR image editing introduces color in the larger context of illumination rather than simple reflectance. A chapter covering the specialized topic of HDR panorama capture and reconstruction then leads into the final chapter on computer graphics imaging and rendering, describing the techniques that have revolutionized the film industry.

Whether you are an artist, a hobbyist, a technician, or some exotic mixture, these pages offer valuable explanations, methods, and advice to get you started or to take you further on the path of high dynamic range imaging.

Greg Ward of anyhere Software