Qtpfsgui Notes - The Reinhard '02 TMO

[David]

Qtpfsgui Reinhard '02 Notes

Test Image: Test1.hdr (Hass Burn images 357, 359, 361, April 2009 copyright DARW)

Initial Set-Up: As per Qtpfsgui Drago Notes.

Reinhard '02 Tone Mapping Operator

The default settings presented to the user are a radio button, Use Scales, unchecked, and two sliders:

Key Value – default 0.18, range 0.00 – 1.00; and
Phi – default 1.00, range 0.00 – 50.0.

Checking the Use Scales box activates three further sliders:

Range – default 8, range 2 – 15;
Lower Scale – default 1, range 1 – 100; and
Upper Scale – default 43, range 1 – 100.

The usual Adjust Levels and pre-gamma tools are available.

Using the default settings on the test file gives rise to a realistic and usable image, with good mid and high tones, but somewhat dark on low tonal values. Saturation is generally good. The histogram is well-spread. Reducing pre-gamma to 0.5 brightens the image, increases contrast, and enriches the saturation, but the dark tones appear clipped. However, the histogram suggests not. Further work with post-processing software may be fruitful.

Returning to defaults and now lowering Key Value to 0 results in a blackout. At a value of 1.00, the Key Value gives an image of much lower contrast, but one in which the dark tones have a great deal of detail. From the defaults, increasing Phi to its maximum does not appear to do anything to the image, nor does setting it at zero.

With the Use Scales box checked, the image remains the same with the scale sliders at their defaults and altering any and all of the sliders up or down appears to make no difference. There is a possibility, having perused the original paper, see Technical Notes below, that this TMO is very much image dependant. Thus, the parameters that I have no apparent effect may be useful in images with larger dynamic ranges.

Conclusions:

The Reinhard '02 TMO gives reasonable realistic images on the default settings. Low tonal values can be enhanced by increasing the Key Value, and extra saturation and contrast introduced by lowering pre-gamma. Creative or OTT images are not really an option with this TMO. Processing times are a few seconds for a 1024x1536 image.

Technical Notes:

The Reinhard '02 TMO appears to be based on work described in: “Photographic Tone Reproduction for Digital Images”, E. Reinhard, M. Stark, P. Shirley, and J. Ferwerda,
In ACM Transactions on Graphics, 2002.

Reinhard et al. make reference to techniques developed by Ansel Adams, particularly the Zone System. The Key Value parameter is derived from the idea of high-key, low-key images, the higher the value the higher the key. The value Phi is, apparently, a sharpening function. The TMO algorithm appears to be a logarithmic function, but the authors acknowledge that in very high dynamic range images detail is lost by this method. The scaling sliders may be associated with trying to overcome this problem.

Sources: As per other notes.

[COLOR="Yellow"]This is the last of the TMO's currently implemented in Qtpfsgui. I intend to produce a summary sheet for all these notes along with some notes on the "front-end" of Qtpfsgui. If there are any specific points that anyone wishes to address please let me know through appropriate comments.



Cheers

David