Making HDR Pictures (Part IIA)


posted on 21st of august, 2008

When you load pictures, in RAW format, to Photomatix for HDR processing you specify what algorithms the processor is to use to correct for camera movements and objects motions in the scene. There are two algorithms for each.

In camera motion correction I usually take the more compute intensive option, ie to align the picture by matching the features in the scene instead of the simpler vertical and horizontal corrections. However sometimes this complex algorithm fails. Then I revert to the simpler one.

In object motion, the algorithms are for the presence of a moving objects, and something Photomatix called "background movement". The latter includes things such as ripples, leaves on trees, etc. Again I usually make Photomatix work harder to treat leaves, clouds and so on as "moving objects". Sometimes the moving object algorithm fails for moving objects like people, boats, birds, etc leaving artefacts in these objects. Then I assess if I can correct/eliminate these artefacts later on in Photoshop. If not I would abandon further processing altogether.

Then there is the white balance setting. White balance is an important setting. Some minor adjustment can be made after HDR generation, but the dominant WB setting, eg Cloudy or Sunlight will be made here. I mostly leave it at "As Shot" as my out of camera WB is generally good.

Then Photomatix will churn for some time, converting the RAWs, aligning them, and then generating the HDR. And then comes the crucial step of Tone Mapping.

There are two sets of parameters for Tone Mapping, one set above the Histogram and the other below it. I will cover the former firstly, for which there are four parameters: Strength, Saturation, Light Smoothing and Luminosity.

The most important parameter, to me, is Luminosity. This determines how much of the shadows are deemed details. A low luminosity should be used if shadows were not well exposed in your input images, or else there will be lots of noise and artefacts n the resultant tone mapped image. On the other hand maximal luminosity can be used if your shadows have been well exposed at capture.

Next is the Strength parameter, for which I see a close correlation with Luminosity: a high luminosity value allows a high strength value, and likewise a low luminosity, a low strength.

Most of my HDRs are at maximal luminosity and 100% strength. If I cannot set these two parameters at very high values it is usually due to poorly exposed shadows, and the resultant image usually not suitable for stock photgraphy. Also a low strength HDR image can be generated more efficiently without use of such complex and resource intensive HDR processing.

The paramater giving HDR its characteristic look and feel is Light Smoothing: the highest value gives the most natural look, and the lowest the most surreal. Less light smoothing gives more local contrast and lots of details. There will be an aesthetic optimal somewhere between the most realistic and the most detailed.

Finally there is Saturation and it can be a matter of taste here. Further it may be adjusted iteratively with the Color Tabbed Panel to be discussed in Part IIB.

(This is a multi part article. The other parts are here: Part I, Part II)

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Photo credits: Lawrence Wee.

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