When symbolizing a multiband color raster layer, in QGIS there is a field called contrast enhancement. In this field I have four options to choose from: 'no enhancement' 'Stretch to MinMax' 'Stretch an Clip to MinMax' 'Clip to MinMax'. What is contrast enhancement good for and what is the difference between the options?
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1 Answer
Stretching a raster image is a simple but effective visualisation technique which can help highlight features in an image.
If you consider an image in it's histogram form then it is most commonly observed with a Gaussian distribution. Now if you have the colour gradient running linearly from the Min and Max of the data set then you end up with a much shallower gradient. This means features often get lost in the image that share a relatively similar value. Now to get around this you can set your Min and Max values to remove "outliers" which gives you a larger colour gradient, thus features with relatively similar values stand out more.
For definitions I refer to Menke et al. (2016) which states the following:
No enhancement: No enhancement is applied. The color gradient is stretched across all values in the entire gray band. While this may be desired sometimes, it may tend to make the raster look overly gray.
Stretch to MinMax: This method stretches the color gradient across the gray band between the Min and Max values. It generally produces a higher contrast, a darker rendering than No enhancement. All cell values below the Min value are assigned the lowest gradient color and all cell values above the Max value are assigned the highest gradient color.
Stretch and clip to MinMax: This method stretches the color gradient across the gray band between the Min and Max values. It produces the same rendering as the Stretch to MinMax method, except that all cell values below the Min value and all values above the Max value are assigned no color (and they are transparent).
Clip to MinMax: This method stretches the color gradient across all values in the gray band, which is the same result as No enhancement, except that all cell values below the Min value and all values above the Max value are assigned no color (and they are transparent).
References
Kurt Menke, G.I.S.P., Smith Jr, R., Pirelli, L. and John Van Hoesen, G.I.S.P., 2016. Mastering QGIS. Packt Publishing Ltd.
