Tuesday, February 14, 2012

High Pass Sharpening in GIMP


GIMP (GNU Image Manipulation Program) is a powerful image editor that is open source & cross platform, working on Linux, Mac OS and Windows operating systems.

High pass sharpening is a method of sharpening an image without incurring too many undesirable artifacts such as noise.  It also operates by creating a layer which can be adjusted to increase/decrease the effect or turned off to remove the effect completely without affecting the underlying image.  A layer mask can also be added to limit the sharpening effect to desired areas.

This method is very well explained in episode 164 of Meet the GIMP.  If you find this process interesting, then I recommend you watch this video.  The Meet the GIMP videos and web-site are an excellent resource for using this application.

For this walk-through of the process I shall use an image of the Palacio Real in Madrid.




 In GIMP I create a duplicate layer where I adjust the colour curves to enhance contrast



The initial image that we will begin sharpening looks like this:



And so we begin with the High Pass Sharpening.


1    Duplicate the layer twice



2    Invert the colours of the top layer

    Color > Invert



3    Set layer mode to Grain Merge



4    Apply a gaussian blur, typical blur value of 20.0 seems to work quite well, but experiment to find a value that works best for your image.

    Filter  > Blur  > Gaussian Blur



The image outline will then appear "etched" in the grey background.


5    Merge the top layer down

    Layer  >  Merge down



 6    Set the layer mode to Overlay



7    The effect can be adjusted with the opacity of the sharpening layer

This is the final image with 100% sharpening applied