Monday, September 1, 2008

XDR, Part IV

High-dynamic-range imaging

32-bit file sizes are twice as large as 16-bit files (four times as large as 8-bit files). HDR uses 32-bit editing spaces to hold all the data contained in multiple 16-bit bracketed exposures. Creating an HDR file from 16-bit (65,536 shades of gray) capture doesn’t create a true 32-bit (4,294,967,296 shades of gray) file. Few capture devices today deliver a full 16 bits of data. Software programs, like Photoshop, use 16-bit editing spaces to hold data over 8 bits (256 values). (As an aside, for complex coding reasons, a 16-bit file in Photoshop actually offers 15 bits of data plus 1 value or 32,768+1.)

New Math
Adjustments made to high-bit data display fewer artifacts, but, whether significant or insignificant, there are artifacts nonetheless. Tweak more, and you continue to lose values and encounter rounding errors. Adjustment layers minimize the total change made in an editing process. RAW files adjust the appearance of a file during conversion to a gamma-encoded color space. Whether 8-bit or 16-bit, these are still LDR solutions—only whole values exist (128 or 129). HDR floating point solutions allow for decimals (128.1 or 128.15)—you can split the bit—providing a near infinite number of values. No gamma is needed because you can get finer levels whenever you need them. Image data can then stay in linear space, unchanged by our perception of them. The upper and lower limits disappear. You can assign any value to any pixel and then adjust the relative relationships of other values to it. Depending on how a tonal structure is mapped, values may stray out of the histogram—what’s being used—but they’re never lost.

No-Bounds Flexible Data Handled Nondestructively
The extra data gives you astonishing fidelity and flexibility. You lock into a solution only when you render the image to LDR. If you preserve the HDR image, you always can return to it and rerender it at another time. HDR essentially isn’t bound to the limits of a capture or output device.

Here’s the fundamental difference between RAW and HDR. A RAW file captures the scene as a sensor sees it. An HDR file captures the full values of the scene. You can “reexpose” this HDR scene digitally as often as you like by taking a snapshot of it and developing an LDR image for output.

File Format & Processing
HDR imaging (sensors, processing and viewing) is the future of photography. Tomorrow, HDR capture and file formats will supersede RAW workflows. Today, HDR extends RAW files.

HDR capture and processing is complex. Capture involves bracketing exposures, which reduces naturalism for moving subjects and favors using a tripod (though images made with fast auto-bracketed motor-drives sometimes suffice). When capture devices offer fast single-shot HDR images, this will disappear. Processing bracketed, aligned, merged exposures is somewhat complex. Rather than applying output-referring gamma curves to an image, you choose how the tonal values are rendered. Tone mapping takes a scene to an output-referred state. Once merged into an HDR image, exposure becomes adjustable; you can select one exposure or a combination of all or selective exposures. Tone mapping can be as simple as selecting an exposure or as sophisticated as simulating local adaptation. You even can go to extremes and produce hyperrealistic renditions. Extreme HDR renditions can reduce tonal contrast selectively (classically in the midtones), can produce distortions in relative luminosity (reversing dark and light values selectively), and can exaggerate contour- and texture-enhancing detail. Many HDR renditions challenge classic photographic aesthetics and offer new ones. There’s no right or wrong answer to how an image appears, only consensus and personal preference. HDR images offer many choices.

Images in print are limited to roughly 6 EV. You can move the smaller window of the printable DR up or down within the larger captured DR; the larger the captured DR, the more flexibility you have. HDR offers more options in print. It doesn’t offer higher-dynamic-range print materials. It’s difficult to create a surface that reflects less than 1% of the light it receives, and it’s impossible to create a surface that reflects 100%—though light-emitting substrates are becoming more and more available. Many new technologies are starting to expand the 6 EV print limit, but there’s a great deal of room for improvement before we catch up to the eye’s 12 EV.

How far do we really want to go? As the brain tries to decode reflected light based on what it sees, extreme ranges of illumination may hinder rather than help perception. The artist who removes excess dynamic range in an image may actually make it easier for you to see the scene than if you had to view and interpret it. To a large extent, images already are fully interpreted. HDR imagery solves certain technical problems and offers greater freedom of interpretation.

Who Needs HDR?
Not everyone needs HDR. And not everyone needs HDR all of the time. If you start with LDR information and end with LDR output, you don’t need HDR. But when you start with HDR information, it’s important to know how to capture, preserve and process it today; and know that both methods and technologies for processing it and viewing it will evolve rapidly tomorrow.

Do you need HDR? No. Do you want HDR? Maybe. When to make the transition is a question of how to invest the most important thing you have—your time. There’s nothing wrong with standard photographic practice. While it’s more limited, throughout the history of photographic practice, people have been and are still working within those limits, and are producing wonderful images. So, when do you want to make HDR images? When the dynamic range of a scene exceeds the ability of a camera to capture, particularly if DR is exceeded along gradated transitions or if the DR is substantially exceeded. Then, if you don’t want to compromise or sacrifice shadow or highlight detail, you need to turn to HDR imaging. (Remember to be mindful of the current limitations with respect to motion.) You may want to turn to it sooner, knowing that indeed, it is the future. Imagine being able to interpret and reinterpret exposure indefinitely. Though they’ll be commonplace tomorrow, today the possibilities are staggering.

John Paul Caponigro is an internationally respected fine artist, author of Adobe Photoshop Master Class and the DVD series R/Evolution, and a member of the Photoshop Hall of Fame. Read more of his column “R/Evolution” online at Get over 100 free downloads and his free enewsletter Insights, at


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