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H20T R-JPEG Images
371 4 7-10 05:16
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JonoMcG
lvl.2
South Africa
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Good day All,

I've been working on trying to decode the R-JPEG images captured by the new DJI Zenmuse H20T thermal camera so that one can extract the temperature information from the images without needing to go through DJI Thermal Analysis Tools.

It seems crazy to me that DJI has not provided any way for a user to even export the actual temperature measurements for each pixel from their software into a different format (like a .csv or .tiff). The DJI Thermal Analysis Tool is extremely basic and very limiting for any proffessional inspection reporting using the thermal images.

I've managed to figure out that the raw thermal sensor values are stored within the APP3 segment of the Jpeg image as 16-bit unsigned integer pixels. I can export this out as a 16-bit .tiff image. However, these values are only the raw sensor values and does not give one the actual temperature measurements (the values in this image range from 18073 - 21222). Whereas when the same image is opened in DJI Thermal Analysis the temperatures range from 7.5°C to 27.7°C (depending on settings for emissivity, distance etc.).


Flir uses a formula like this to convert the raw sensor value to temperature: Temperature (in Kelvin) = (B / Log(R1/(R2*(S+O))+F))
S = 16 Bit Raw sensor value
T = object temperature in Kelvins
R1 = Planck R1 constant
R2 = Planck R2 constant
B = Planck B constant
F= Planck F constant
O = Planck O (offset) constant.
The specific calibration data above can be decoded from the Flir R-JPEG file using ExifTool. The guys there managed to reverse engineer the Flir format.


It seems like there could be some calibration data stored within the APP4 segment of the image. I am hoping that if we can decode this we would be able to use the calibration data to correct the raw sensor values to give actual temperature readings. Unfortunately my experience with reverse engineering binary data is next to nothing. Is there anybody else who has experience or any ideas with how to retrieve the calibration data (and subsequently the temperature info) from within the image?


On another note if DJI Support can give any insight into whether they have any planned support for getting the temperature information out of the image any other non-proprietary format?


I've uploaded the original H20T R-JPEG image as well as the converted raw sensor 16-bit .tiff image to the following OneDrive folder for anyone who wants to download and play around with it: https://hawk62aviation-my.sharepoint.com/:f:/g/personal/jonathan_hawk62aviation_onmicrosoft_com/ErzBhLPJjKpLvljPM6k6d4gBCm6gjeZNPjLBF7LS8_0Vaw
7-10 05:16
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rdosdcuatrogmail.com
lvl.1

Spain
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hi JonoMCG unfortunately I have the same problem, DJI's solution is to wait for a terra update.  Have you been able to advance any further on this topic?  Who do you think could help us?
7-10 21:52
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rdosdcuatrogmail.com
lvl.1

Spain
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I cant download the image not show link.
7-10 21:57
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JonoMcG
lvl.2
South Africa
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I've posted the question on ExifTool's forum: https://exiftool.org/forum/index.php?topic=11401.msg61059#msg61059. They are the ones that managed to decode Flir's R-JPEG images so I'm sure they'd be able to do the same for DJI's R-JPEG images. I haven't had any more luck decoding it myself. Hoping somebody with more experience will jump on board.

Find it very frustrating that DJI would limit the use of their professional thermal camera so much. It's also very misleading to people to openly advertise that it is radiometric without letting people know that you cannot get temperature information out from the images except when looking at the images in their very basic and poorly designed DJI Thermal Analysis Tool.

What exactly did they say about Terra? Are they planning to allow users to convert the images from within Terra or only view them similar to the DJI Thermal Analysis Tool?

I've also updated the link in the previous post. It should work now. If not the images can also be downloaded from my post on ExifTool's forum.
7-10 23:22
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rdosdcuatrogmail.com
lvl.1

Spain
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They want to improve DJI thermal analisis tools and add in terra an option to perform orthomosaics.  I count myself as your frustrated.  the camera is awesome and works really well but without being able to export images you cannot get its full potential.
If you want add me to telegram  @SamyNeutron
7-11 10:28
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