Big Ben's Panorama Tutorials

Colour Correction

The new colour correction feature of Panorama Tools is a very powerful tool that can save a lot of time if used correctly. Unfortunately colour correction  is one of the least understood areas of digital images and as a result some people do not get the desired result using this feature.  Here are a few points to consider before using colour correction for stitching a panorama.

The colour correction is based on equalising the histograms of the RGB channels of the overlapping regions of the images. Therefore:

  • The method assumes that the overlapping images contain identical content. If they don't then this may throw out the final result (e.g. moving objects in only one image of a pair)
  • More overlap provides a better result.
  • Small overlaps at the edge of the images with radial fall off will produce unwanted results. (e.g. back to back fisheye images)
  • There will most likely still be a small variation in colour between the images. Attempting to correct images with small variations may not reduce the variation between the images.
  • Colour correction works best on images with large differences, but will still leave small variations.

Exposure Correction

This test shows the ability of Panorama Tools to correct for variations in exposure. Each image was first aligned with the reference image and then output as TIFF file with colour correction applied.  From this it can be seen that quite usable results can be achieved with exposures +/- 2 stops from the "normal" exposure. The underexposed images showed increased noise, with the -3 stops image being unusable for a high resolution panorama. At the opposite end of the scale though, the +4 stops image was still quite usable.

Relative 
exposure
(f stops)
Original
Image
Reference
Image
Corrected
Image
-3
-2
-1
0
+1
+2
+3
+4

Colour/Contrast Correction

Another problem regarding colour and contrast occurs when shooting with multiple lenses, e.g. shooting a high resolution zoom image. There are many factors that can create differences in colour, the main one being lens flare.

In this demo I shot the original panorama with an 8mm lens and then took another set of shots with a 135mm lens for the purpose of creating zoom images.  Having stitched the panorama I then extracted a view of the area to be zoomed at a suitable size for inserting the images shot with the l35mm lens.   Even though I used the same exposure for both there was a considerable difference between the negatives.

Two zoom images without colour correction

... and with colour correction.

Note that the result is not perfect. You can still see the edges of the images, particularly towards the right side of the image BUT it's a lot closer than I could get attempting to manually adjust the images.  Very little work is required from this point to make the images seamless.


Failure

There are of course times when using this feature will not work.  Any panorama with parallax errors is also prone to having overlapping regions with different histograms.  The rooftop panorama is an example is one such situation where colour correction cannot be used without careful consideration.

 

 


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This page, its contents and style, are the responsibility of the author and do not represent the views, policies or opinions of The University of Melbourne. All photographs Ben Kreunen 2000

Ben Kreunen <bernardk@unimelb.edu.au>
Department of Pathology
Last modified: February 24, 2003