Friday, 29 May 2015

Forensics : Fringe artefacts while working in RAW

Working in RAW is essential for getting the most out of images as discussed HERE.  While working in RAW one is immediately struck by the strong influence of digital noise and the requirement to use sharpening tools with a lot of care, so as not to exaggerate noise in the final image.  It turns out we may have to contend with other digital artefacts introduced while working in the extraordinarily versatile RAW format.

While recently working with some of my images in Camera RAW I have noticed a tendency to introduce an edge artefact using the highlights (recovery) slider.  This looks a bit like purple fringing but after a bit of research I finally came across the explanation for it online.  I noticed this problem while adjusting parameters to extract as much detail as possible from within the highlights of my images.  This has been occurring around high contrast edges and this is apparently where the problem normally arises.  There also seems to be an association here with bright sunlight, which obviously pushes highlights (particularly white plumage) close to the point of being blown.  The original JPEG from the camera doesn't present with these fringes.

In the video below I have presented some examples of this fringe artefact.

Soft-edge Mask Halos
So, what is going on here?  Well, according to this link HERE the explanation for this is most likely down to the use of soft-edge masks in some Camera Raw tools.  These masks are offset from the original image and therefore appear as halos.  So this would make this phenomenon an artefact specifically introduced and left behind by a RAW image processing tool.

Purple Fringing - Chromatic Aberration
The term purple fringing has an etymology going back to early colour photography where it's origin would have been in chromatic aberration due to the limitations of lens design and for instance the sensitivity of film to infrared light.  Chromatic aberration is caused by the varying refractive indices of different wavelengths of light, causing different colours to focus at different points on the film/sensor plane.  The same effect allows us to split white light into it's spectral colours using a prism.  While it may be possible through clever lens design and lens coatings to deal with the range of refractive index within the visible light spectrum, IR light, which encompasses a massive spectral range falls well outside that scope, so a sensitivity to IR light is always likely to result in chromatic aberration.  The solution in this case is not to try and design lenses to cope but instead simply to block IR (and UV) and stop those wavelengths reaching the sensor or film.

While better lens design, coatings and UV/IR blocking filters have all largely resolved chromatic aberration, purple fringing still continues to plague photography.

Purple Fringing - Demosaicing Artefacts
The term purple fringing is also used today to describe a wider range of phenomena in modern photography.  Another big potential cause is the interpolation step between a Bayer raw image and the creation of a full colour image.  This process step is called demosaicing.  I have covered this topic in some detail already HERE where I used the illustration below to show what a Bayer RAW image looks like before it is demosaiced to create the final colour image.

The relationship between demosaicing and fringe artefacts is neatly illustrated by the excellent posting by Adam Hooper HERE.

This posting serves to draw attention to the fact that, even when working in RAW there may be a risk of unmasking or introducing unwanted artefacts.  So, enticing and all as it might be to extract every last detail from RAW files, sometimes it may be necessary to trade off some detail and avoid unmasking or introducing some nasty artefacts.

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