Question: NDVI image issues from GoPro 3+ Silver

by jeffa_plain |

NDVI with Blue Filter

NDVI with Red Filter

Hello, I'm very new to NDVI stuff and I've recently created an infragram camera from a Gopro 3 using the public lab guide and color filters. Everything seems to work as intended (except for photos being a little over-exposed) however when I feed the images into infragram.org, the NDVI spectrum is inverted, (plants are blue while non-photosynthetic material is green through red). When I try and use the opposing preset on these images, the result is just a solid color. is there a way I can get the -1 to 0 and the 0 to 1 color map to be flipped on infragram.org?

Thank you

Hmm, interesting, can you share your original images? I think we might just be able to do (1-R) instead of (R) or something, for some of the values.

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From the research ive done since making this post, I think it may be because I cannot adjust the white balance on a GoPro, both of the images are red/blue tinted because of the filter

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I'm not sure where I would use the (1-R), I just tried it and I didn't get anything that was useable. all that needs to happen it seems is the results need to be flipped from negative to positive and vise versa however I don't know how I would do that in infragram.org

Hi, I think you're right that you need to adjust the white balance, and you can learn more about how that works here: https://publiclab.org/notes/warren/10-23-2017/change-the-white-balance-or-other-configuration-on-a-mobius-camera

However, you can do some to adjust the channels you receive to simulate white balance adjustments using Infragram.org. Since your blue channel was hugely overexposed, I tried just dimming that channel to see what we get, using this expression:

Grey: (R-(B-0.75))/(R+B)

That resulted in this image (starting with your blueish photo above):

There are some issues with it - the shadows are overemphasized, for one. I think if you wanted more confidence, you could compare it to a really well white-balanced version, even from another camera, but once you establish the GoPro's limitations, you could then use it with those caveats.

It also looks like /some/ gopros do have white balance -- https://community.gopro.com/t5/en/White-Balance-Setting-Information/ta-p/390203 -- in case that's helpful.

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I was kinda hoping that this would be as simple as inverting the color scale at the bottom but nothing that simple with stuff like this haha. From the result that you got with (R-(B-0.75))/(R+B), it seems that the trees have lost their distinction from the rest of the materials in the image which is interesting. I have tried white balancing in photoshop but it really didn't get useable results, ending up purple or just white. I will continue to experiment with R and B levels to see if I can get anything that works. I hope that one day we can have a more in-depth and interactive infragram site too.

Additionally, from what I've seen, Gopro's that DO have white balance aren't able to balance to the extremes that the NDVI modifications create although they get closer. I may have to switch to a point-and-shoot camera at this rate, which is disappointing because the results I got with the GoPro seem good, the color scaling is just inverted ̶a̶n̶d̶ ̶i̶ ̶h̶a̶v̶e̶ ̶b̶o̶u̶g̶h̶t̶ ̶a̶ ̶l̶o̶t̶ ̶o̶f̶ ̶t̶h̶e̶m̶...

I think if you got closer in white balance in pre-configuring the GoPros, you can probably un-boost the blue channel some more and get better distinction in the tree pixels. But yeah, not every camera is ideal for this technique, depending on a lot of factors!

I'm curious, did you remove the IR filter from this?

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I did indeed remove the IR lens from this! Thats why I bought the GoPro 3+ Silver because that's the only GoPro 3 that has an easily removable lens with do deconstruction needed. Originally I just broke the filter out of the back of the replacement lenses; later I found some replacement lenses for FOXEER FPV camera lenses that came without an IR filter and were much cheaper, although the FOV is much higher at 170 degrees, so you can see the lens housing around the edge of the screen.

As far as the White Balance issue goes, you cannot adjust the white balance on the GoPro 3+ Silver at all from what I've seen, so I think this may be the end of the road for these cameras. I'm gonna see about doing some post processing magic to get workable results because in the blue-tinted image you can clearly see that the trees and organic material are correctly reflecting IR light and appear beige-red, it's just a matter of processing and colorizing that data.

Yes, i agree there is some infrared data showing in the trees, and I do think it's possible to get useful info out. Just be aware that the dynamic range of the data after post-processing may be quite low compared to a pre-calibrated image. And that it may be worth it to try checking against an image from another camera with better conversion just to give you confidence about what you're seeing with the GoPro.

Good luck!

Yeah, I plan to use a point-and-shoot camera next (preferably one with optical zoom) so I can get proper NDVI imaging done. other than that, thank you for your help so far, it's been a pleasure!