Public Lab Research note

Channel swapping

by warren | June 21, 2013 15:15 21 Jun 15:15 | #8353 | #8353

What I want to do

Due to the issues with blue channel infrared leakage, I was interested to see if it was possible to use the green channel data in place of the blue channel data to create useful NDVI alternatives. (I suggested this approach in this comment.)

My attempt and results

As you can see in the lead image, the green channel does seem to be detecting reflected infrared (as this graph suggests it will), but much less than the neighboring R and B channels. Unfortunately it also seems to detect less reflected light on non-vegetation, so maybe the approach is doomed from the start...

Anyhow, here is the image with the B and G channels swapped (which I did in order to use the Infrapix prototype web utility), plus the "NDVI" alternative and the infrared channel:


(see the full data here:

And here is a larger version of the full alt-NDVI:


Questions and next steps

Basically it seems not to work, from what I can tell -- vegetation is not highlighted vs. the non-vegetation background.

Perhaps by looking at these histograms of the raw image (working from Chris Fastie's post, we can see if there really is enough differentiation to understand differences in health:



What do you think?

Update: I did one more where I ran auto-levels on the green channel to boost the range, which may not be "scientifically justified" but it did produce better contrast in the resulting alt-NDVI. It still did not, however, seem to differentiate plants:



I played with this image quite a bit, and could get very little plant health information from it. You recognized one of the problems -- there is no channel in which plants are the brightest thing. The histogram of just the tree tops illustrates another problem. The red and blue channels are nearly identical, apparently because they are both dominated by infrared light. The green channel is darker. Swapping the blue and green channels does not accomplish much. Using the original green channel for the visible channel in NDVI calculations maybe fails because the green channel has so much IR in it even though it is darker. Maybe your only hope is matrix math.


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