How to remove noise in digital photos with the gimp

Digital imaging is a blessing but has  given rise to a number of new problems which need ot be addressed. One of these problems is noise, which appears as random fluctuations in the colour or intensity of an image. It is not generally a problem unless you want large prints in which case the noise may become obtrusive and is not a problem with a high end digital SLR but may be a problem with older  and compact cameras.  High end  image editing programs such as Photoshop automate this process  but inevitably rob the person editing the picture  of  some control over  the process.

Some experimentation  with the free image editing program GIMP (the GNU Image Manipulation Program: GNU stands for GNU is not UNIX) on my Apple powerbook  revealed  a procedure that  worked for most  cases and gives good results.  Abusing this process could result in odd and artistically effective results. 

For those of you impatient to experiment the  process has  three steps.

1.Blur the image
2.Sharpen it again
3.Add noise.

If  blurring and sharpening an image then adding noise seems an odd way to remove noise read on

All the options are on the Filters tab in the main menu. As always, work on a copy of the image

Gaussian blur will not do the business. What is needed is  the selective Gaussian blur option. Selecting this  brings up a  dialogue box with a preview and two parameters, blur radius and max Delta. The larger the blur radius the greater the blurring.  The Max Delta is the interesting parameter.  Two pixels  that differ by  more than the max delta  act as barriers prventing the blurring spreading. The default values are good in most cases.  Experiment with these values. Be patient as the blurring is slow. Keep going back and forth between a highly magnified view of  a selection and of the whole image.   Selecting part of the image should confine  the effects to that selection. When happy save the intermediate result. If the work is critical then  do each stage of this process on a copy of the results of the previous stage.

The next stage involves Filters/Enhance/Unsharp Masking which brings up another dialogue with three parameters radius, amount and  threshold.   The details of unsharp masking are beyond the scope of this note which is   more of a how to cook book. Experiment with these parameters and you will find it is easy to create halos round edges. Less is more and none is not an option when it comes to unsharp masking.  Again save the results of this stage

You now have a picture that may be TOO smooth. Adding noise  restores some texture. As with unsharp masking a little goes a long way. Again experiment with these parameters  and remember  noise will be more obvious and annoying in areas  like sky so you may want to change  only a selection at a time

If this sounds like a time consuming process, it is. If the picture is important enough be ready to do each stage on a different day. Have fun.

You may also like Monochrome-Image Conversion with the GIMP which tells you how to remove colour from your images