Revision as of 07:30, 23 January 2012

The purpose of this project is to get HDR images using an Elphel camera.

For informations about HDR imaging see: Wikipedia.org. For informations about HDR imaging algorithms see: Debevec.org and Columbia.edu.

There are several ways how to increase the dynamic range and to achieve HDR images:

Exposure Bracketing

Autoexposure Daemon HDR Functions

The autoexposure daemon running in the camera can rapidly alternate between multiple exposure times, allowing a single video stream with over / under exposed sequential frames. In post, these frames can be separated into individual streams and finally combined into a single HDR sequence. This HDR-video can be recorded just like any other normal video with the harddisk-recorder, camogm or captured over the network.

Due to limitations in how fast the exposure time change can be propagated to the image sensor, HDR mode works best in Triggered mode([1]); And by altering the http://*CameraIP*/tuneseq.php?mode=latency- page, setting the EXPOS value to "0". ([2]).

Below is a parsedit url that provides quick access to the required parameters for frame sequential HDR in triggered mode: (replace the IP address with that of your camera)

http://192.168.0.8/parsedit.php?embed=0.15&title=Parameters+for+groups:+autoexposure+whiteBalance+trigger+&WB_EN&AUTOEXP_ON&TRIG&TRIG_PERIOD&EXPOS&HDR_DUR&HDR_VEXPOS

Manual Bracketing

You can do exposure bracketing manually with a PHP script. Just set exposure time -> capture a single frame -> change exposure time -> capture another image, etc. This will be slower than using the daemon but you have finer control over how many brackets you want and how big the EV steps between the brackets are, etc. This is best suited for still image recording where speed is not so much of an issue.

Gain Bracketing

JP4 HDR is a special raw color format that can capture a higher dynamic range in an image without introducing any artifacts as result from combining images captured at different times (exposure bracketing). Though the post processing code to develop the images is still experimental.