Reducing color balance errors across multiple cameras

Question

I'm an electrical engineer, but I thought I'd ask this in the photography section to get some more insight from experienced photographers.

I am designing a system which will stitch images taken by multiple cameras to create landscape imagery in real time. In order to remove the seam between two adjacent images, I have taken the following steps for each camera:

1. Pointed each camera into a flat field source (like an integrating sphere or a white panel with diffused light) and averaged 20 light images to produce a single average light image.

2. Calculated the average pixel value for each color in the light image. This gives me three values - one for red, green and blue since the camera sensor is a Bayer array. The average value should be slighly greater than corner pixels and slightly less than center pixels due to vignetting.

3. Repeat steps 1 and 2 again with the lens cap on to produce dark image values and create a flat field gain table for each camera.

4. I then used the average light and dark colors to calculate color gains so that I can reference an image taken by one camera to another camera. (light color Cam2 - Dark color Cam2) / (light color Cam1 - dark color Cam1).

5. Then take two adjacent images, flat field them, apply the color gain calculated above to camera 1 and the seam should disappear!

But it doesn't and I am unsure why. When removing seams, is there something else that should be done? All of my algorithms are very basic, such as averaging the green color adds all of the green pixels up and then divides by half the total number of pixels (since this is Bayer, half are green, quarter are red, quarter are blue). Anyone with any experience doing this before?

Thanks!

Answer 1

The seams will only match exactly if the seams are located in areas that correspond to the average values you have used in creating a profile for each camera. If the seams are located closer to the edges of the frame, then perhaps you should sample the values from the same areas to create your camera profiles instead. This problem is compounded by the fact the seams cross areas of varying amounts of vignetting that may vary by different amounts in each individual camera.

You should also do your samples with the same lens set to the same aperture (and focal length if it is a zoom) as you will use in the shots you want to stitch. These variables will also affect the exact values from one camera to the next. If, for example, you take one photo at f/2 and 1/250 second and another at f/4 and 1/60 sec they will be very close, but not exactly the same exposure value. This is because of the minor imprecision between stated and actual apertures and shutter speeds.