Thursday 11 December 2014

Forensics : Gaussian Analysis - Artefacts

Scope and Objective
In An Introduction to Gaussian Analysis (HERE) I outlined how most image quality parameters exhibit a Gaussian distribution around an optimum quality standard.  This presents an opportunity to look for 'Gaussian Signatures' left behind in digital images.  Here I am analysing the Gaussian signature for the fifth of our five Image Quality Parameters, namely Artefacts.

First off, I would recommend having a read of the page HERE to become reacquainted with the various common artefacts, including my individual take on what is and what isn't an artefact.

The Gaussian Signature for Artefacts
A number of artefacts are directly associated with our other image quality parameters.  Image Quality itself becomes the signature for these artefacts as a whole and the Image Quality Tool can be used to provide a rough measure for these artefacts.  I.e., the poorer the quality, the greater the artefacts.  There probably isn't a whole lot more to be said here, other than to look at those artefacts which we can associate with our image quality parameters, as compared with those which are unrelated.

As we can see, the frequency or extent of certain image artefacts is linked to image quality.  On one side of the graph we have MoirĂ©, Defocus (inside), Camera Shake, Underexposure, Noise (frequency of).  On the other, we have Defocus (outside), Camera Shake, Overexposure and Blooming.  All of these have one interesting thing in common - they are all, to an extent, within the control of the photographer.  By concentrating on obtaining a detailed, sharp and optimally-exposed image, quality-related image artefacts should take care of themselves.  On the other hand, where image quality is compromised, this issue will be further compounded by the introduction of artefacts.

When one lists and considers all of the remaining common artefacts we can see that most of those are uncontrolled by the photographer and indeed they are also difficult to rectify at the photo-finishing stage.  There isn't a whole lot we can do about these other artefacts, and we don't have any useful way to measure or estimate their likely impact on image quality.

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