FCam - low light / HDR photos

Question

I am using FCam to take pictures and right now without modification, the pictures are par for a smartphone camera. FCam advertises HDR and low-light performance, but I don't see any examples of how to use that when taking pictures.

How do I take HDR pictures? From my experience with SLRs, you normally take 3 pictures, 1 under, 1 over, and 1 exposed properly for the scene.

Since I will be taking many pictures, how should I blend those pixels together? An average?

Walter

Answer 1

The FCam project page includes a complete camera app - FCamera, check the FCam Download Page, last item, which for "HDR Viewfinder" simply averages a long/short exposure image together, and for "HDR Capture" automatically records a burst of suitably-exposed shots. See src/CameraThread.cpp in the sources, I'm not sure how appropriate it is to quote from that but you'll find both pieces in CameraThread::run().

It doesn't average the HDR images for you, it records them as sequence. I guess that's by intent - much of the "HDR" appeal you achieve by carefully tuning the tone mapping after the averaging process, i.e. adjust how exactly the dynamic range compression back to 8bit is performed. If you'd do that in a hardcoded way on camera, you'll restrict the photographer's options with respect to achieving the optimal output. The MPI has a research group on HDR imaging techniques that provides sourcecode for this purpose.

In short, a "poor man's HDR" would just be an average. A "proper HDR" will never be 8-bit JPEG because that throws away the "high" bit in "high dynamic range" - for that reason, the conversion from HDR (which will have 16bit/color or even more) to e.g. JPEG is usually done as postprocessing (off-camera) step, from the HDR image sequence.

Note on HDR video

For HDR video, if you're recording with a single sensor on a hand-held you'll normally introduce motion between the images that form the "HDR sequence" (your total exposure time equals the sum of all subexposures, plus latency from sensor data reads and camera controller reprogramming).
That means image registration should be attempted before the actual overlay and final tone mapping operation as well, unless you're ok with the blur. Registration is somewhat compute intensive and another good reason to record the image stream first and perform the HDR video creation later (with some manual adjustment offered). The OpenCV library provides registration / matching functions.

The abovementioned MPI software is PFSTools, particularly the Tone Mapping operators (PFStmo) library. The research papers by one of the authors provide a good starting point; as to your question on how to perform the postprocessing, PFSTools are command-line utilities that interoperate/pass data via UNIX pipes; on Maemo / the N900, their use is straightforward thanks to the full Linux environment; just spawn a shell script via system().