Originally Posted by mavlo77
Some interesting progress here...
I found a way to remove the purple center spot by using a VirtualDub filter. And probaly the anti-vignetting is going to work as well.
Then I used the Layer Filter by Brian Allen Vanderburg II to substract this correction layer file from the original frames.
The result is quite nice! See samples below! It's not optimal yes: just 6 images from a ceiling.... But the effect is already quite descent.
I also experimented with removing the vignetting using this filter.
I remember an anti-vignetting has been discussed in this thread, so probably I have to try this.
If I found an optimum filter set, it's easy to process all movies in a batch.
Pretty nice correction of the hot spot! I've not tried to use the levels filter, not really knowing what it could do.
The vignetting needs a stronger filter setting. There's also a third-party Vdub filter just for this called hotspot.vdf by Donald Graft. I've used it with pretty good results, but I found that the strength of the vignetting is not a constant. I.e., there is much less noticeable vignetting when the exposure level is very high (e.g. bright sun) and much stronger in low light. The hot spot effect also varies with light level. So one filter to fit all videos is not likely to produce as good results as having maybe three, one for low, medium, and high light levels. If a video has a lot of varying light levels, you might have to compromise with one filter anyway.
All-in-all, good sleuthing with freeware solutions!
Having said that, it's still a lot of work. For tasks like that, I found the MAGIX MEP17+ commercial editor a very good solution. I can use one filter (same .bmp I created for use with the Vdub Hotspot), and that editor can selectively vary not only brightness with the filters, but also the RGB colors.
And slicing the video where there's a big variation in brightness allows the filter to be reselected and adjusted for just the selected slice. Very powerful and fairly easy to use and adjust in real time preview once the control toggles are understood. But, it's a $65 solution with a learning curve attached, not free!