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This GIMP (GNU) tutorial demonstrates how to adjust an image’s colours by applying GIMP’s Retinex Filter. To work along, you are welcome to download the Start Image I am working with Here. Unzip the file and open the image onto GIMP’s workspace.
Notes
You can quickly Undo a step at any time by pressing Ctrl then Z. Alternatively, click a previous Undo History snapshot - Windows then Dockable Dialogues then click Undo History. In addition, to Zoom in (or Zoom out) of your image; from the top menu choose View then select a Zoom Tool from the subsequent drop-down list.
Launch GIMP & Organise Its Workspace & Palettes
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Open your choice of Start Image onto GIMP’s workspace - File then Open - Ctrl then O.
Note
Ensure the Layers and the Undo History Palettes are visible, and then drag them into position over your workspace - (Windows then Dockable Dialogues - then click Layers and Undo History).
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Free Select Tool Note
If you prefer, before applying GIMP’s Retinex Filter, you can protect (Mask) the Cat’s Eyes by isolating them with a Selection Marquee using the Free Select Tool. Protecting the eyes will prevent then from becoming washed out.
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Now, from the top menu, choose Colours and then choose Retinex. And from the subsequent Retinex Image Enhancement dialogue box, experiment with the settings, and then click OK.
Retinex Notes
Retinex improves visual rendering of an image when lighting conditions are not good. While our eye can see colours correctly when light is low, cameras and video cams can't manage this well. The MSRCR (MultiScale Retinex with Colour Restoration) algorithm, which is at the root of the Retinex filter, is inspired by the eye biological mechanisms to adapt itself to these conditions. Retinex stands for Retina + cortex. Besides digital photography, Retinex algorithm is used to make the information in astronomical photos visible and detect, in medicine, poorly visible structures in X-rays or scanners.
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Congratulations, your Retinex Image Conversion is complete, and it is ready to save - top menu, then File then Save As.
Sharpen Tip
To create extra impact, apply a Sharpening Filter of your choice - Filters then Enhance.
Wendi E. M. Scarth. Top of Page.
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