FDRTools and the Compressor Algorithm: An approach

In one of the recent posts I promised to explore FDRTools' Compressor Algorithm.  The 'Compressor' is, according to FDRTools, a local toning operator.  That means that it covers the surface point by point and it tries to adjust each pixel according to the brightnesses of neighboring pixels within a certain radius.  The FDRT manual says this "The Compressor algorithm works as follows: the intensity of each pixel is regulated individually and depends on the intensities of neighbouring pixels. If neighbouring pixels differ strongly in their respective intensity, e.g. in the surroundings of a strong light source, then these differences are strongly attenuated. Where the differences are small, e.g. in shadowy areas, there is little or no attenuation, evtl. the difference is even increased. As a result the dynamic range of the HDR image is strongly reduced while local tonal value differences are preserved. Local contrast and thus the perceptibility of details are considerably improved compared to global tone mapping methods like Simplex or Receptor."


The authors of this manual are, I think, Germans and I'd like to rephrase this as follows: "Compressor examines each individual pixel and changes it depending on what it finds in neighboring pixels (within a certain radius).  If the immediate neighborhood has a big dynamic range then it is reduced.  If the neighborhood has a relatively narrow dynamic range then it is increased.  The result is that the dynamic range of the entire image is significantly reduced but in such a way that local relative differences are preserved and local contrast is even improved relative to Simplex or Receptor."


Good enough.  But is it true and how do we use it?  Let's concentrate on a small number of details just at first.  We start with the gross image of the column drum that I used last time.    Here it is:

Yow.  


What we see with defaults is this:

Now this is another low-contrast image.  All the brightnesses are squeezed into the center of the histogram as we see.  We know that we need to draw in the left and right sides (black point and white point) to more nearly enclose the actual values.  The result is this:

Now it's no longer grossly low-contrast.  In fact the gravel looks pretty good.  But the brights have lost all the definition even of the default settings and somehow that needs to be restored.  Another way of saying this is that the brights are mapped into too narrow a range and to spread out that range you steepen the curve on the bright/right side, like this:

You may have to experiment but, at least in this case, the right side (RHS) of the curve had to be steepened so that there would be more output grays in a very small horizontal brightness distance.  That's a very clumsy way of putting it but it's the best way that I can think to explain it.  Let's pretend that the brights are o.k. and examine the darks.    In this particular case I really couldn't find any better setting for the darks (the gravel) and so I left it alone.    There are several more important controls and let's look at the Compressor slider.  This slider, according to the tool tips,  regulates the strength of tonal length compression.  I confess that I don't know what that means in practical terms so we're going to have to approach that empirically.  When I moved it all the way to the left (0.0) the histogram shrunk symmetrically around its center.  When I moved it all the way to 10.0 the histogram expanded around the center.  The effect was, at 0.0, to lower contrast, at 10.0 to heighten contrast.  The tool tips say that you should use values from 0.0 to 5.0 for a single raw (which this is) and 5.0 to 10.0 for multiple images.  I found that this all looked best when I moved it up to 8.0 or even 10.0.  Here it is:

HDR gets details out of burned-out or from under-exposed areas.  Once you get that it's always in low contrast.  The whole problem of HDR, as I'm sure you're aware, is to increase the contrast (that is, to increase the range of grays) on the result of HDR processing without going all the way back to the original picture.  That was our challenge with the simplex method.  But FDRTools Compressor seems to do the trick.  What do you think?  Let me know in the comments section if I've grossly mishandled this or if you have better ideas.  I'm going to finish up by processing this result and then post-processing in Photoshop.  I gave it a lick with Curves to increase the contrast, sharpened the L layer,  and here's the final result:

It'll never be a great picture but it's a perfectly good illustration of a column drum.  If I say so myself there's nothing at all wrong now.  It's a clear and adequate picture with a full and reasonably distributed range of grays.   Perfect for squinchpix.com.  The sunlit wedge in the right foreground is a potential source of trouble (just after the top of the drum itself) but here it gave no problem.  


Also the purple grid marks are what FDRTools puts over pictures when you don't have a license. 


So:
1. bring in the curve ends so that they enclose the histogram.
2. Evaluate the gray range in the brights.  If it's too narrow (which it was) then increase the range there by steepening the right end of the curve.  Steeper means more grays.  Shallower means fewer grays.
3. Evaluate the gray range in the darks and increase the gray range if you need to by making the left end of the curve steeper.
4. Experiment with an optimal setting of the 'Compressor' slider.  By this stage you should have an reasonably distributed tonal range of grays.  That's your real goal for HDR processing.  If you get to that point you're successful.
5. Finish up in Photoshop to get complete control over the result in the way you're used to.  You can adjust contrast, correct color casts, etc.


I'm mindful that color is a distraction and what I'd like to do next is to use FDRTools on just the L-layer of a Lab version of this picture.  I've done things like this in the past and it's time to do that again.  In the next day or two.


And there's something that I now realize that I assumed throughout but never actually stated:
With HDR Always think in terms of brightness - Never think in terms of color.  HDR is not an idea about color.  It's an idea about brightness.  Always, always, always!!!


 Till next time...

Bob