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Simple vignette correction

clolsonus

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Most people of course upload their images to one of the major commercial software tools to produce orthomosaics and vignetting is corrected automatically. But for the crazy people here that are writing their own mosaicing software (and I may be the only one) I wanted to experiment with some simple vignette correction to help the final fit/mosaic presentation look more even. Here is what I came up with:

Simple Vignette Correction – Gallinazo

Fitted images before vignette correction:

vignette4.jpg

After vignetting correction:

vignette5.jpg
 
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Not sure what question you ask specifically. However, isn't the problem that some of the images are exposed differently from the rest?
Personally I would use manual exposure setting.
I have used OpenDroneMap a few times, which to my knowledge does some moderation of the exposure. Maybe you could use part of that software in connection with your own software.
 

clolsonus

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I'm learning as I go here ... but I think there are two separate things happening. One is the vignetting (which is a consistent variation across each individual image.) And then the other is the overall exposure (or white balance or gamma?) There is probably some super smart way to do a global optimization to match all of this up in one step? My little experiment only focused on the vignetting bit. I've also seen people do histogram matching (i.e. force/interpolate the histogram of one image to match the histogram of another image.) In a set of 2000 images with widely varying content, which image histogram should be the master? Also, histogram matching wouldn't correct for vignetting.

What I have been doing after the vignetting step is an adaptive histogram equalization (on an 8x8 grid). I like this because it makes the shading details of the image really pop out and it levels out the exposure differences (but still doesn't account for white balance differences.)

The dataset from the original post was captured about an hour before sunset on a thick/cloudy/dreary winter afternoon, so the lighting was about worst case. For this data set the auto white balance (or gamma?) of the camera really started showing through. For typical well lit sunny days the differences are much less noticeable. But maybe some sort of histogram matching could account for all of that?

For what it's worth, I've flown with both full manual mode and shutter priority mode. I flew an area of steep terrain in January with full manual setup and once side of the canyon was perfectly exposed and the shaded side was very under exposed. So then I started experimenting with shutter priority so I could at least control exposure time, but let the camera pick iso and aperture. I'm usually out imaging wooded areas with terrain so it has been really tricky to get the manual settings exactly right. (I do fly with fixed manual focus of course, but even managed to screw that up on a flight or two.)

I was wasn't so much asking a question here, but more sharing my experiments. But always enjoy feedback and new perspectives. I just see one tiny slice of the world from one tiny perspective. I haven't ventured over to the open-drone-map forum much because I figure they prefer to talk about open-drone-map there and not other people's projects.
 

Lensupthere

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Vignetting usually occurs when the aperture is stopped down (narrow aperture). From your experiments, at what aperture do your images suffer from vignetting?

Do you know the aperture range that your lens is sharpest ? (most/many are sharpest between f/5.6 and f/11).

Perhaps you might want to use Aperture priority instead of shutter priority, or go full manual. And shoot RAW if your sensor/camera allows for it.

Capturing RAW allows for better pre-processing flexibility to correct things like shadows and peaked whites.

We’ve mapped wooded varying terrain parcels (Yosemite, trees in the Sierra Nevada’s for dead tree studies, etc.). Changing your ISO to 200 or 320 or 400 may help with the darker areas. Adjust the other settings accordingly. Note: Increasing ISO may also increase grain in your images. Experiment to figure out where your sensor lives best. If you need to introduce ISO grain, then learn how to pre-process it out (batching via photoshop) using Noise Reduction functions.

Edit: Forgot to add a comment about solar noon. Sometimes it’s best to avoid flying solar noon depending on reflectivity of the objects on the ground. Solar noon will make your shadows between the trees darker, for example, maybe before 11:15 a.m. and after 1:00 p.m. might result in better (less contrasty) images.
 
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clolsonus

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Thanks for the feedback @Lensupthere. I hadn't thought about a connection between aperture and vignetting. I'll have to experiment with that on my camera. There is a lot more details in your message that my brain is struggling to process this early in the AM! I am going to make another coffee and then come back and read through it again ... maybe something will sink in.
 

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