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7th June 2009, 10:37 AM
Qtpfsgui Notes: Summary of TMO's
Qtpfsgui Notes: Summary of TMO's
I have reviewed in a number of recent posts the set of tone-mapping operators, TMO's, available via the Qtpfsgui platform for HDR imaging. Those posts describe the parameters associated with each TMO, provide an empirical “walk-through” for mapping a test file (Test1.hdr) describing as far as possible the effect of changing each parameter, discuss technical points of potential interest, and note some conclusions. This post summarises the findings in terms of the nature of the TMO's, and their ease of use.
Several general points should be borne in mind:
first, the use of the pre-gamma and Adjust Levels tools is important, all of the TMO's can be tweaked and enhanced using these functions;
second, study the histograms shown via the Adjust Levels tool, these provide valuable information about the general nature of the image and about what needs to be done to improve matters;
third, note that the results of tone mapping can be dependant on the size of the image being processed;
fourth, remember that if you wish to save an image for further work you will have to set the appropriate size before tone mapping.
Ashikhmin TMO – Using any of the options, but with Local Contrast Threshold set to low values can give realistic images. Simple to use and short processing times. Not of use for creative imaging.
Drago TMO - The simplest of the TMO's to use with only one parameter, Bias, to adjust. Very useful for preliminary study of an image particularly if pre-gamma is set to lower values, say 0.5. Saving a Drago image and blending it with an image from another TMO, e.g. Fattal, is popular in the Qtpfsgui community. Fast processing times.
Durand TMO - This TMO did not work correctly using the latest version of Qtpfsgui, 1.9.3. Using version 1.8.12 realistic images can be obtained provided the Base Control parameter is set to low values and pre-gamma to about 0.5. However, processing times become substantial with large images and the TMO exhibits size dependency. Overall, this TMO would not be one of immediate choice.
Fattal TMO - A complex TMO with several parameters, the Fattal algorithm nevertheless can produce both realistic and creative images. It is not the easiest TMO to master, but it is worth the effort. A key point is to set a value for the Noise Reduction parameter. Thereafter, realistic images are obtained using default or close to default settings and creative images by reducing particularly the Beta parameter. Processing times are moderate, but the TMO shows some size dependency. This would be one of my two choices (the other is the Mantiuk TMO) for creative work.
Mantiuk TMO - This is another complex TMO that can generate both realistic and creative images, although the authors note that it is not designed to produce natural looking images. The current version has three parameters, and careful manipulation is needed, particularly of the Contrast Factor and pre-gamma, to get good results. In my hands, the Mantiuk TMO gives good dark tone variation, possibly the best of all the TMO's. Processing times for large images are long, thus carrying out parameter variation on small image sizes is recommended, followed by scaling up when the parameter choice is settled. I have not investigated whether this algorithm exhibits size dependency. Overall, this TMO would not be first choice if a photo-realistic result was required, but is, along with the Fattal TMO, best for creative work.
Pattanaik TMO - This is a very complex TMO to use, with several parameters. In my hands, there seems to be a problem with saturation as well as getting the parameters to give quality images. To me, this is a TMO to be avoided.
Reinhard '02 TMO - This TMO is probably one of the most important in the history of tone mapping. The authors relate it to Ansel Adam's Zone System and one of the two primary parameters, Key Value, takes its name from high key, low key photographic images. The TMO is easy to use and would be, like the Drago algorithm, one of choice for quick work, giving reasonable realistic images on default settings. Saturation is enhanced by lowering pre-gamma, and low tonal values enhanced by increasing the Key Value. Processing times are fast. Creative images are not really an option with this TMO.
Reinhard '05 TMO - This is another reasonably easy TMO to use, but default values need to be substantially modified to give realistic images. (In my hands: Brightness -2.00, Chromatic Adaptation -0.60 and Light Adaptation 0.60.) Creative images are not really an option. Processing times are fast. However, my personal preference would be the Reinhard '02 TMO over this one.
Summary of Summary:
For creative work - Mantiuk and Fattal TMO's.
For realistic images – Reinhard '02, Drago and Ashikhmin TMO's.
[COLOR="Yellow"]Comments, amendments, additions welcome.
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