High Dynamic Range Images (HDR)


posted on 3rd of february, 2009

Since the last camera upgrade last year (details here), I've been experimenting with the HDR techniques. My previous cameras did not have the exposure auto-bracketing function, and this function was one of the reasons why I have decided to buy a Canon 450D.

HDR comes from Higy Dynamic Range images and refers to images having a greater dynamic range of luminance between the light and darker part of a scene than normal photography techniques. Its intention is to more accurately render the wide range of light intensity levels found in real scenes (from direct sunlight to shadows for example). Although most of the time over-processing that looks fake is associated with this technique, its scope is to render an image closer to how it is perceived by our eyes.

This technique was first developed around 1930s and 1940s, but the first applications in digital photography appeared in 1993. A HDR image can be considered even the raw file of the most recent digital cameras, as one pixel is defined in 24 bits (having thus a wider range of luminance), but on its display on the screens (camera and computer screens) it looses information as the pixel-depth is only 8 bits. Tone-mapping techniques have been introduced to convert a 24 bits image to 8 bits, in an attempt to differentially map the luminance levels of each pixel. Different algorithms are available on the market to make this conversion, the most notable being the one included in the most recent Photoshop releases or from the specialized software for HDR - Photomatix.

The tone-mapping technique can be apply to a raw image or at least 2 jpeg images of the same scene taken with different exposures, with comparable results. Many parameters of this technique influence the final result. The following results can be noted: details in both shadows and direct sunlight or both the sky and subject and enhanced color rendering.

To create my own HDR images I take 3 shots of the same scene with different exposures (bracketing +/-(1+1/4) usually). I use the tone-mapping algorithm from Photomatix and set the parameters so that the final results to look as real as possible. However, the images obtained this way look different than normal exposed photographs.

Here are some examples with comments:

© Bogdan (Help)
This image of Bern skyline (Switzerland) was taken after sunset, as my first try-out of this technique. A normally exposed photo would not preserve the same level of details in the sky.

© Bogdan (Help)
Just before Christmas, the first serious snow provided an opportunity for a photo trip in the surroundings. We have reached the Castle of Aigle a little too early in the morning and half of the Castle was still in shadow. The normal approach to photograph it would render the shadow part almost completely dark, given the bright sunshine and the white caps of the mountains in the background. By using tone-mapping technique, detail of the dry vineyards in the foreground were able to be preserved, generating an image similar with the one perceived by my eyes.

© Bogdan (Help)
This HDR render of the Rathaus (city-hall) in Vienna did not bring almost anything extra to the image taken with normal exposure other than slightly increased color saturation without altering the quality.

© Bogdan (Help)
The image on the right taken in the Unirii Square in central Timisoara, Romania is however totally different than the normally exposed one. The thin layer of clouds are causing a uniform light which the normal capturing technique cannot render properly. The sky ended up almost completely white, while the details and colors of the buildings were washed out by under-exposure. I couldn't believe my eyes when the algorithm generated this image - beautiful details in the sky; warm colors of the buildings and even a pure and bright green of the meadow... It was one of the best surprised that HDR has yet offered me...



© Bogdan (Help)
Another magnificent example of the HDR technique with tone-mapping is this picture taken in Lausanne, Switzerland. Amazing details have been obtain in the area under the bridge and rooftops in the foreground, while the Notre-Dame of Lausanne cathedral shines as a crown over the city in front of a gloomy dark sky.

© Bogdan (Help)
This last picture of Ouchy Castle in the port of Lausanne on Lake Geneva, Switzerland is not exactly what I was hopping for. From my previous experience with overcast skies I was hopping to obtain more colors in this composition. However the gloomy result has its own charm...

I am continuing to experiment with HDR techniques. Some more images are even on the pending line... Let me direct you to a personal gallery of HDR images, where my further experiments will be found.

-Bogdan

Comments (14)

Comment by Kittycat on September 26, 2009