Saturday 4 April 2015

Colour - The Effects of Post-Processing of Images

Brightness, Contrast, Saturation and Sharpening Tools
I came across an excellent posting by Mike Chaney looking at the interaction between these four simple image modification/analysis tools HERE.  Proving that there is a subtle relationship between all forms of image post-processing modification, it is interesting to consider what happens to image colours when an image undergoes some basic post-processing modification using any one of a range of simple tools.

Difference between RAW files and 8-bit images
It is important to state from the outset that these parameters interact differently during the original creation of an image for viewing from the RAW image data file.  When we are working with an actual image file we are post-processing the image. Because RAW images contain far more data than a final JPEG, PNG or other typical 8-bit file there is a lot more latitude to adjust these parameters individually within a RAW-to-image workflow.  If you use Camera RAW or a similar RAW workflow package you will find that these and various other subtle modifications appear to work quite independently of one another.   However when working with a far more compressed, 8-bit JPEG or similar file we see that modifications have greater reach. 

King Penguin (Aptenodytes patagonicus) Falkland (Malvinas) Islands, makes a nice subject for a comparative analysis.  It is hard to resist increasing contrast and/or saturation to enhance the beautifully rich colours and markings of these birds.  But sometimes we need to resist such temptation to aid careful colour analysis.  The image above was created from RAW.  I boosted colour saturation a little bit but also was careful to keep the fine shadow detail in the white areas of the breast.  As stated above, there is latitude for this while working in RAW but not when working with a JPEG or other 8-bit file.

Post-processing of JPEG or other 8-bit images
The images below were made by post-processing a JPEG image file.  We can compare the impact of each type of adjustment on the JPEG and see how each adjustment has an affect on all four parameters simultaneously.  An adjustment of brightness for example changes colour saturation and, where it clips blacks or highlights it also therefore clips colours and reduces overall contrast.  Contrast is closely related to saturation and both impact on the saturation of colours.  The key difference between contrast and saturation is that an increase in contrast compresses all tones whereas an increase in saturation only compresses colours, and does not alter luminance levels at all.  Extreme high contrast and high saturation adjustment results in a loss of mid-tones while contrast reduction can clip blacks and whites.  Image sharpness is largely about acutance or edge contrast and this experiment neatly shows how the effects of brightness, saturation and contrast adjustment all impact on acutance.  For it's own part, the over use of sharpening tools like unsharp mask can introduce sharpening halos which in turn alter the colour and tones around the edges of colour patches.  This can drastically alter the colour of small or narrow markings. 

From left to right above we have darkened, normal and brightened images.  Next we have a contrast-reduced, normal and contrast-increased image.  This is followed by a saturation-reduced, normal and saturation-increased image.  Lastly we have an artificially blurred image, followed by a normal and lastly an artificially sharpened image.  These were all created from the same JPEG image.

All of these adjustment tools are part of the normal processing of a RAW digital file into an image for viewing.  But, as there is greater latitude to work within RAW these corrections can be made far more independently of one another at that stage.  The extent to which these tools or parameters are applied automatically from RAW depends on in-camera settings and manufacturers preferences.  However, alternatively we can work with the original RAW file using Camera RAW or a similar package, where these can all be adjusted manually to suit our needs.  If we decide to leave our image processing to the end and work with a JPEG file instead of a RAW data file we find that our modifications have greater consequences.

So the key message once again as regards the accurate capture of colour for sampling and analysis is to work in RAW, work towards maximizing tonal range and calibrate colours properly using a DNG profile and grey card.  Be careful with the post-processing use of brightness, contrast, saturation and sharpening tools.  Keep these adjustment within the RAW editing workflow to minimize loss of tonal range and, above all to prevent clipping.  Lastly, be judicious with the use of these key adjustments.

See also HERE and HERE.

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