Saturday, 19 May 2018

ZWO Gamma Gone! but not quite!

Gamma has been removed from the ZWO SDK!

This means that ZWO cameras will no longer provide a control for gamma.
At first sight this might seem like a strange thing to do, but in reality, it is not.
There is a powerful argument that image processing should be done after the data have been captured.

In general, image processing involves the selective discarding of information in the image data in the effort to recover as much information about the underlying structure of the image, such as the bands on Jupiter, or the arms of a spiral galaxy. If any of the information is discarded at capture time, it is lost forever, whilst if it is discarded post capture, it is still available in the original image data.

Gamma is such a function that involves the selective changing of the mid-range values of the brightness of an image, whilst leaving the darkest and the lightest values untouched. Many cameras allow the control of gamma at capture time, as it can produce a more pleasing view of the subject being captured. However, it must never be forgotten that the brightness values that are changed (and thus lost) will not be available in the captured data.

Experienced planetary imagers such as Christopher Go, eschew the control of gamma at capture time. What is important is to get the exposure, including gain, right at capture time. Some advanced imaging cameras only allow the control of exposure and these are often true 16 bit devices.

In order to mitigate this loss of control over gamma, Nicola has incorporated gamma control in software, in AstroDMx Capture for Linux. In addition to gamma, she has also implemented contrast and brightness in software. This allows the user to control and see a pleasing view of the subject being imaged as usual. However, the changes are not propagated through to the saved image. This means that no information is discarded at capture time and is therefore available for extraction during post processing in software such as the Gimp 2.10.

For many users, this is a paradigm shift that will require a little getting used to. However, it is likely to lead to better results in the long run.

These changes are implemented in the next release of AstroDMx Capture and have been responsible for the slight delay in release. However, the gamma camera control will be retained in addition to the implementation of the software, non destructive controls.