The Place of Dead Roads

I have written a GUI interface to Rafał Mantiuk and Grzegorz Krawczyk's collection of tone mapping operators, pfstmo. They in turn extend the resources in pfstools. My previous post, High Dynamic Range Photography with Linux, outlines methods I use for generating HDRI.

Why Tone Mapping

The first two photos below are the shortest and longest exposures used for constructing the final HDRI. In the upper left image, to capture the sunlit details, the back fence and red chair are underexposed. The upper right image captures faithfully the color information for the chair and fence, along with the displayed image on the computer. But this is at the expense of the brightest parts in direct sunlight completely saturating the camera's sensor. As my earlier post explained, using six images spanning the dynamic range of the scene, a HDR image can be constructed.

Input image f/8 at 1/400 sec	Input image f/8 at 1/13 sec
Linear mapping from HDR to LDR	Logarithmically mapped

The reason for Tone Mapping arises when we want to display the resulting HDR image. Most display devices, including monitors and printers are Low Dynamic Range (LDR) devices. We need methods for compressing the range of an HDRI to fit range of our display. The bottom two images show what happens when we try simple methods.

The lower left image tries to linearly map to the 8bits/channel availible in the standard JPG format. Eventhough it maintains strong color information in the midrange, both the high and low end details are completely lost.

The bottom right on the other hand uses a logarithmic mapping, which does a fine job of maintaining the shadow detail. The sunlit parts of the scene also preserve their detail. But this comes at the expense of the color information; the resulting image appears flat.

Tone mapping operators provide more sophisticated methods for acheiving the needed range compression. pfstmo provides seven such operators; along with references to the orginal papers for each operator. For further discussion, I recommend High Dynamic Range Imaging; Acquisition, Display, and Image-Based Lighting by Reinhard, Ward, Pattanaik and Debevec.

qpfstmo

qpfstmo provides a Qt-based GUI for using the pfstmo binaries. Before proceeding, make sure you have a working installation of pfstools and pfstmo. Given that, get the source code via:

• Source tarball: qpfstmo-1.0.tar.gz
  The download is from my public Comcast space, which is not always availible. Let me know if problems persist.
• CVS from pfstools' sourceforge pages:
  cvs -z3 -d:pserver:anonymous@pfstools.cvs.sourceforge.net:/cvsroot/pfstools co -P qpfstmo
  Web interface to CVS

Assuming you got the source code, cd into the source directory and qmake;make to generate the executable qpfstmo. Assuminig you have qt-devel installed, there shouldn't be a problem. The binary doesn't directly link against pfstools or pfstmo; they are used via QProcess.

There is nothing about this code that makes it tied to Linux. If you have Qt and pfstools/pfstmo installed successfully, you should be able to get this program to work.

Overview of using qpfstmo

Assuming you have loaded an HDR image, the first thing to appear is the Main Window.


The across the top, there are Tabs for each of the seven operators included in pfstmo. Before any can be used, a "Work Size" must be selected. By starting with a small size, it is quick to see the effect of adjusting a parameter. To apply the selected tone mapping operator with the specified parameters, just press the "Apply", , button. You can choose to have the new mapped version replace the current image or have the new mapping open in a new viewer window.


The "Viewer" window. If you like what you see, you can save it. For this version, the only save option is as PNG. I am having issues with Qt's QString and file extentions. Gamma can be adjusted, and reset if needed.


Using the "New Window" option in Main window, each tone mapping will appear in its own window. This makes it easy to see what effect adjusting a parameter will have on the tone mapped image.

My favorite tone mapping operators:

• drago03: Simple and fast global operator. Good for getting a quick preview;
• fattal02: A gradiant-based local operator. Can do a nice job compressing large dynamic ranges. But also easy to generate the dreaded "halo" effect.
• reinhard02: Based on Ansel Adams work with the zone system. Automates the idea of "dodging" and "burning". It can be a little hard at first to understand the effect of the different parameters.
• durand02: Slow but powerful operator. Works by splitting the image into the slowing varying high dynamic range content and the low range detailed component. The main control is "Base Contrast".

The top image of this article was tone mapped via the fattal02 operator.