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— HDRI: Introduction —

What is an HDRI?

HDRI stands for High Dynamic Range Image, a picture with a high dynamic range. Here, we understand dynamic as the range from dark to bright and how fine the steps are resolved. A print from a photograph has a dynamic range of about 100:1. Pictures displayed on a computer screen have a dynamic range of 256:1 for each of the primary colours red, green and blue. Values in the range of 0 to 255 can be represented with 8 bits, a pixel therefore needs 24 bits to display all colours. This sums up to 16,777,216 single colours (grandiloquently called «true color»), the dynamic range is only 256:1 nevertheless.

On the other hand, an HDRI needs 96 bits to represent one single dot; 32 bits each for red, green and blue as a real or floating point number. Such a number can represent 4.7 x 1021 brightness values (4.7 sextillions) or 253 Exposure Values (EV).

Of course, such a dynamic range cannot be displayed anymore. One would need as light source nothing less than a full sun in each computer screen. With all due confidence in technical progress, the assumption that we will get such monitors in the next 20 years appears to be a bit overly optimistic to me.

So what is the sense of HDRI photographs anyway, if they cannot be displayed in their full glory? A rather academic reason would be that the picture contains everything, even though it can only be displayed slice by slice. A more practical reason is that the pictures can be recalculated into a range that can be displayed. This is called tone-mapping.

Tone-mapping permits to extract and assemble from a photograph exactly the ranges sought to create a particular impression. Sharp tongued people could argue that this is counterfeiting but then, which picture is not a counterfeit? One could see this definitely a bit more constructive and argue that photographers evolve to artists — many a photographer is an artist already and now they get a powerful tool in their hands.

Tone-mapping is an art. It permits to edit a photograph in such a way that it is not only real but also appears to be real. Viewing is a system where eyes and brain work together and it is often so that a photograph does not represent our impression we had when taking it — however felicitous it may be. The sensation is individual. When we have an HDRI in front of us, we can reconstruct that impression. On the other hand, we can alienate it in such a way that it creates a particular sensation to the viewer. However unreal such an edited picture may appear, it is still not artificial but rather artistic. Look closely at the light in pictures of great painters. We are enthralled, but with any arrangement of lights, the objects depicted can never be lit the way they are in reality. The painter «cheated» und only with that created a piece of art that delights us.

  © 2004 - 2018 by Horo Wernli.