Removing blue haze in airborne pics

[peterh337]

I take a lot of pics from a light aircraft, at fairly high altitudes up to 20,000 feet, and more often than not the result has a blue tint.

This one
Pyrenees, FL140 picture by peterh337 - Photobucket
came out OK

but this one
http://www.peter2000.co.uk/aviation/...a/pyr3-big.jpg
I have not managed to do anything with.

I have Photoshop 7 and the "auto colour" feature is much too primitive. It does make it better but it goes way over the top.

The haze itself one can reduce significantly by increasing both brightness and contrast by say 20%, but what can one do about the blue tint?

[jimH]

I dealt with some photos of the Grand Canyon taken while the forest service was doing some prescribed burns in the woods surrounding the canyon and there was a blue smoke haze hanging throughout the canyon. It obscured much of the color so I tried increasing the contrast during PP and that seemed to help somewhat. What you are dealing with is atmospheric water vapor haze and possibly playing with the contrast might help there too.

[Damn Brit]

A polariser might help.

[flyer]

A "Skylight 1A" filter is your best friend for that type of pictures. It won't remove haze, dust and vapor, but it will reduce the bluish cast.

[Spock]

Originally Posted by flyer

A "Skylight 1A" filter is your best friend for that type of pictures. It won't remove haze, dust and vapor, but it will reduce the bluish cast.

Won't a (digital) camera's auto white balance compensate for the skylight filter?

[pbo]

Hm... Try using levels. Here's what a simple auto levels, a bit noise reduction and sharpening would do - not good enough yet, but much better than before, and you can make it even better with more pp.

[peterh337]

Indeed; I use Auto Levels, but very often that results in a very obviously over-enhanced image. The one you did (above) is what I got using auto levels but I consider it too dark. It also IMHO looks over-enhanced.

Unfortunately I don't have a good understanding of how to compensate for tints.

I have a Hoya UV(0) filter already.

[peterh337]

I should add that - perhaps stupidly - I have been using Auto White Balance. I will try Daylight from now on.

[flyer]

Originally Posted by Spock

Won't a (digital) camera's auto white balance compensate for the skylight filter?

It will balance for the slight tint, but it can't add the UV light that's being removed.

[newarts]

There's a LOT of haze or flare in the photo. Haze or some kinds of flare give a uniform light overlay to the image. As a simple example of a correction technique, I subtracted a uniform intensity from the photo then adjusted the contrast so that what was once full exposed was again fully exposed.

Note that this is not a trick, but is really a correction - there really was a uniform light overlaying the image that can be analytically removed. It amounts to at least 40% of the total image intensity.

A better job would be done if a similar adjustment would be made to each color channel (there's more blue haze than green or red haze because of the source of the haze...scattering from atmospheric "particles" smaller than the wavelength of light; ie. Rayleigh Scattering. This implies that it is possible to figure out what fractions of blue, green, and red should be removed from the image....maybe I'll try it!)

I think it is particularly interesting that this is not so much"image enhancement" as it is "image extraction" firmly based in physics.

After the haze is removed it would be fair to add some noise reduction and focus correction.

Dave in Iowa

[newarts]

Physics is SO COOL! I removed the haze from each channel of your photo, increased that channel's contrast to bring the highlights back to where they should be, then re-merged the channels. Here's the result:


Here's the split channels & the brightness distribution for each.

I assumed each histogram's leftmost peak was due to haze, so I subtracted that peak intensity level (154, 130, 86, for B,G,R) and renormalized (255/(255-154), 255/(255-130), 255/(255-86).) - I really should have used the foot of the leftmost peak rather than its maximum I think.

I'd say that your first approach should be to get a good UV filter to take the original which will knock a lot of the blue haze out, then apply BGR corrections if necessary.

It is gratifying that these corrections could be done automatically, without peeking! Fundamentally this is no different from PBO's approach above, except for being analytic in nature. I did not apply any noise reduction or sharpening, which would greatly improve the result.

Dave in Iowa

PS a good strategy might be to assume that there should be black in each color channel, then subtract sufficient light from each channel to enforce that assumption followed by assuming there should be a white spot somewhere and multiplying by appropriate factors to assure that result.

[peterh337]

Thank you everyone! This is fantastic. I need to learn a lot more about Photoshop...

And yes I will try the polarising filter too. Many years ago, when I went through the "must try every possible filter" stage I did use polarised filters a lot, and got stunning results of scenes like Bryce Canyon, where the sky (already pretty dark due to the altitude) ended up very dark blue, and the rocks had good contrast, e.g.
bryce1.jpg picture by peterh337 - Photobucket

[Ben_Edict]

Apart from atmospheric haze, water vapour etc. you should take into account, that most airplane windows are tinted. You could do a manual white balance before take-off shooting a white paper through the window, to compensate that.

A polarizer will help reduze the haze a bit, in so far as it can reduce the light scatter in the water vapour - but it won't get rid off the whole blueish cast. Also, the blueish cast is neccessarily a gradient, most severe near the horizon and less pronounced when viewing straight down (because the light path to the horizon is much longer, than down to the ground) und normale conditions (forrest fires are something special). So applying a subtractive gradient will help, because it will affect the foreground far less, than the distant background.

Ben

[graphicgr8s]

This isn't as heavy handed and I used correction layers so the original file is never damaged. Only had a couple of minutes literally to work on it and most of that time was spent looking for a shot I have that needed cast removed althought the cast in that one is red.

[peterh337]

I think the above pic has had the haze removed in the bottom (foreground) portion only.

It looks very good.

I had another go at mine, and while it isn't as good it is a lot better than its previous version.