Processing Images

This page documents my amateurish attempts to improve the look of my photos with the GIMP.

Some basics

The first thing I do to make an image look better is to try and correct the colour levels. The histogram is good tool for this. A histogram is a graph where the x-axis consists of intensity values while the y-axis consists of the number of pixels with those intensity values. For an RGB image, you have three histograms, one for each channel. In the GIMP, there's a fourth histogram/channel called "Value", which is effectively what you'd get if you converted the image to greyscale.

Side note: the documentation I've read says that the Value channel is calculated by using max(R,G,B) which would make it, from what little I've seen, the V chanel from the HSV space.

The basic process involves moving the white and black markers on either end until they hist the values as they start to rise.

There are a number of tools that help you do this automatically.

I have not yet found:

Potentially useful:

Another weird thing: I've actually tried adjusting the histogram markers in the Value channel to known values, say 75 and 210. I've then tried manually decomposing and image into HSV channels, adjusted the markers in the V channel to the same values, and recomposed the image. The results we're not the same. What gives?

Unsharp Filter Resources

Smart Sharpening

There are plugins for GIMP that make the whole process of smart sharpening much easier.

Improving Colour Range

Decent sites here:

The first thing you want to do is improve the colour range and remove what haze you can from the photo. One technique involves fiddling with the colour levels. In the GIMP, choose levels and move the sliders so that they start where the histogram begins.

Another similar trick involves running an unsharp filter with a large Radius value (50-100) and a small Amount value (0.25-0.75).

ProcessingImages (last edited 2012-07-26 21:13:18 by DesmondRivet)