FDR Tools. The Simplex Algorithm

I've been intrigued by the performance of FDRTools and I put up a small post two days ago to show how it handled the color sheet.  Now I want to consider the Simplex algorithm which, according to FDR, is a simple tone-mapping operator.  Let's start with a black and white test sheet and see how it performs.  Straight out of the box with default settings it produces this:

This has good separation in the blacks and not so good separation in the whites.  No problems with the lines or their intersections and - until it reaches the brights - realistic rendition of the gradient with some banding.

I looked for a simple manipulation of the contrast curve that would improve things and so I steepened the slope on the left side hoping to increase separation among blacks.  That worked pretty well so I did the same thing on the light side.  That resulted, of course, in the classic low-contrast reverse S-curve and, well, see what it did:

This simple change produced low contrast (see previous) but a huge improvement overall in terms of rendition and separation.  Now that I felt that I was on the right track I tried to optimize it:

I brought in the outer bounds so that they more tightly bounded the observable values on the histogram and I steepened the sides of the curves in the darkest and brightest areas.  I also brought the middle of the curve way down so that it produced the darker band in the center of the gradient - in so doing it helped to intensify the separation in the lights (you probably couldn't do this with a real photo).  I think the result looks really good but, at least, by knowing that we're searching for some reverse S-curve to improve the scene we have a clue as to how to proceed.

  Now keep this reverse S-curve in mind because the next thing I wanted to do was to see if I could apply just this one thing to a real-life scene.  I chose the following awful picture that I took in Argos, in Greece.

Here's a column-drum lying on the ground.  The lights are on the edge of being burnt-out but everything else, almost, is in inky blackness.  I'm going to use FDR tools on this and just take the default settings first.  What does Simplex do to this without my doing anything?

Lots of detail here and overall very low contrast.  Here's something to work with.  Let's apply the reverse S-curve and see where we are but, it's clear, we have to move in the upper and lower curve bounds on the histogram itself.  Here are the settings that I found and the resulting image.

I found that it's not just a matter of applying any old reverse S curve.  You have to experiment with the shape on each end until it looks as good as possible.  I moved in the upper and lower bounds (do that first) on the curve until they just touched the values on the highest and lowest end and that makes a crucial difference.  So the thought for today is, when using FDR tools, the Simplex processor will do a reasonable job and very quickly.  Once you've applied it inspect the histogram and move in the upper and lower bounds appropriately.  After that you need to find the right reverse S-curve to bring out the shadow detail with adequate contrast and then the same on the lights in a way that doesn't burn them out.  In most circumstances you should be able to find a reasonable compromise.  The resulting image won't be perfect.  It will probably be low-contrast relative to the ideal and you'll need to correct that in Photoshop.

But that should get you started with FDR tools.  Next time I'll look at the other two tone-mapping algorithms, Receptor and Compressor.

Till next time,

Bob