High Dynamic Range Imaging

Advancements in digital compositing have led a formerly complex process and technology to become mainstream. High Dynamic Range Imaging (HDRI) is now a one-click process with Photoshop CS5. HDRI  is a series of multiple exposures combined into a single image providing a greater dynamic range of luminance between the lightest and darkest areas of an image. HDRI allows for a more accurate representation of real scenes, capable of displaying a greater dynamic range than what can be captured with film, digital and even the human eye.

The dynamic range (AKA Contrast Ratio) is the number of shades a medium is capable of displaying. In photography, dynamic range is measured in Exposure Value (EV) differences or stops. Each EV value is half or twice the light of the preceding/following number. HDRI uses several photographs of the same scene, each with a different EV value under- and over-exposing the scene to register the details of the highlights and shadows.

The dynamic range test below illustrates how a display device can render a wide range of shades in a color spectrum. The more shades of color you see before they blend together shows you the dynamic range capabilities of the monitor you’re using.

Dynamic Range Test

The Human Eye

The human eye has an equivalent of registering approximately 576 megapixels with a dynamic range of about 10,000:1 in a single viewing (12 stops). Our eyes continually adjust to compensate for highlights and shadows, the brain then processes the information into a single image. Our eyes have a theoretical dynamic range of 10,000,000:1 (12,272 stops) however due to glare, our eyes are incapable of registering more than 16 stops in any given scene.

Film & Digital

The most advanced 35mm film is equivalent to approximately 200 megapixels with a dynamic range of 11,500:1 (14 stops). The most advanced digital cameras are 50 megapixels with a dynamic range of 11,000:1 (13.5 stops). With today’s current technology we can only capture (without manipulation) about 35% of the resolution and 87% of the detail our eyes can. There are still lots of details that our eyes are incapable of capturing in very complex environments.

History

The first multi-exposure image was created in 1857, printed from two negatives by Gustave Le Gray. Gustave exposed two negatives one exposed for the sky and one for the sea. He then combined the images in the darkroom to make one properly exposed image.

by Gustave Le Gray

Nearly a hundred years would pass before multi-exposure images would take the next great leap. The first HDRI was created in 1954 by Charles Wyckoff. It was a combination of several high-speed images of the detonation of an atomic bomb. It is famously depicted here on the cover of TIME magazine.

By Charles Wyckoff

Computer Generated HDRI 

In 1993, Steve Mann developed an algorithm called Comparametric Equations, giving birth to modern-day HDRI. It would take nearly 5 years to make Steve Mann’s theory a reality. In 1997 Paul Debevec produced the first computer-generated HDRI image using Steve Mann’s algorithm.

Several images of the same subject are taken at multiple exposures. Generally, each exposure is plus or minus one EV.

4 under exposed and 4 over exposed images

Here we can see in detail the normally exposed image, notice how the white marble and sky are over exposed and the celling between the columns is underexposed.

The normally exposed image

The HDRI allows us to see detail in both the shadows and highlights that was otherwise lost.

The HDRI with all 9 images blended

In this example we can see the difference in the histogram for the range of exposures. In the HDRI histogram the levels of each color is represented from the shadows to the highlights without any signifiant spikes.

Tone Mapping - This shows the exposure range in the under- and over-exposed images

Surrealism

HDRI images often look surreal even when they are naturally processed. The scenes rendered (below) have a higher dynamic range than the human eye is capable of in the natural environment. An HDRI image can have a dynamic range of 17,000:1 (20 stops).

Low Dynamic Range Rendering

HDRI images that have their dynamic range overcompressed and/or saturation multiplied give the image an artistic painting-like effect.

If you want to composite some of your own HDRI I reccomend Photomatix (avaiable at http://www.hdrsoft.com/) Adobe PhotoShop CS5 also has some great built in HRDI tools.