I've posted an article on my blog on a technique I found useful in dealing with low contrast shots due to smoke-induced haze. I hope you find it useful: Getting rid of the haze
Note: pasting the article here wasn't practical due to the number of images that would need URL format conversion for a posting in this forum.
Getting rid of the haze (technique)
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30th January 2010, 12:43 PMeNoGetting rid of the haze (technique)
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30th January 2010, 04:09 PMDonaldRe: Getting rid of the haze (technique)
eNo
Good stuff. Thank you. Copied into the ever growing list of 'Important Notes'. -
30th January 2010, 05:40 PMrpcroweRe: Getting rid of the haze (technique)
Thank you for posting the link to this tutorial. I am traveling to China this April and with the generally terrible air quality present in many areas of that country, this technique might help save some shots for me!
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30th January 2010, 10:02 PMPopsPhotosRe: Getting rid of the haze (technique)
Thank you.
Pops -
31st January 2010, 01:02 AMwillgossRe: Getting rid of the haze (technique)
I used a similar Approach...except step one (curves) which I use (Un-Sharp Mask with High Radius (50-0.5-0 Gimp) and then a second Pass using High Pass Sharpen Set low (3)......now I duplicate and set Multiply Mode (Opacity to taste).......
This does work well on a hazy shot or on a flatly lit shot......I did use it on some China Pictures where it is very hazy...... -
5th February 2010, 08:41 PMarithRe: Getting rid of the haze (technique)
You learn something new everyday here. I've just tried it on an old image and it is by far a superior method of haze removal and the sharpening suggested by willgoss much better than ones I used previously. Why is the filter called 'high pass'? :) I read about it somewhere but can't remember a thing. :(
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9th February 2010, 08:31 PMrick55Re: Getting rid of the haze (technique)It refers to letting higher frequencies through in the frequency domain. If you transform an image to the frequency domain (fourier transform), the transitions will be higher frequencies. If you let those frequencies through (pass the high frequencies = high-pass), you're enhancing the edges, sharpening the image. Contrariwise, a low-pass filter smooths things by only letting the low frequencies through.Quote:
Originally Posted by arith
As far as I'm aware, the actual processing in real-world tools doesn't happen in the frequency domain, because the transforms to and from are expensive, but operations on a window of pixels can be shown mathematically to be equivalent.
Cheers,
Rick -
9th February 2010, 10:22 PMarithRe: Getting rid of the haze (technique)
I was going to let this go sort of embarrased you know; don't want to look too stupid. But I'm not and I know I'm going to regret it. So firstly , what is frequency in an image?
What is a frequency domain? Fourier transform is that really important or just a function that will do. I mean Fourier transform is not any more something I'm familiar with. Actually I don't think it ever was but sort of crept in somewhere.
I am very grateful for your answer Rick, I can of course find the answers now just from what you have said. Thank you -
9th February 2010, 11:41 PMrick55Re: Getting rid of the haze (technique)As I said, I don't think the real-world tools actually process things in the frequency domain. If you want to experiment with it, ImageJ is a good platform. I'm sure The GIMP provides it, as well, but it's been some years since I've used that.Quote:
Originally Posted by arith
Fourier transform, or specifically for our images, Digital Fourier Transform (DFT) is the mathematical technique that does the transform. The theory of image processing, especially for things like sharpening and blurring, is helped tremendously by the fact that an image, which is a spatial representation of intensity, can be mathematically transformed to a frequency representation. A frequency representation contains the rate of change of intensity over the distance (x,y pixel coordinates) of the image. So just as a "brighter" button can raise pixel intensities in the spatial domain, a "smooth" slider can conceptually decrease the rates of change in the frequency domain. Similarly, edge-detection is the process of leaving in only the greatest rate of change.
The best thing I could find on-line about image processing is this. Most references seem to assume that you already understand 1-D transforms (like for audio processing), and don't seem especially useful.
Cheers,
Rick -
10th February 2010, 01:37 AMarithRe: Getting rid of the haze (technique)
Cheers for that; I'll have a look at it and the program looks useful. I will get some old books out and look up Fourier since I'm not familiar with this anymore. It will give me something to do. :)
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10th February 2010, 10:20 AMarithRe: Getting rid of the haze (technique)
OK had a quick look and it is a bit like Laplace Transform but with an infinite series of periodic frequencies or harmonic analysis. It is 40 years almost since I did anything remotely similar. I don't know if I want to go down that road but just use the applications and get on with my photography.cheers :)
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10th February 2010, 04:14 PMrick55Re: Getting rid of the haze (technique)You're exactly right that the Laplace Transform and Fourier Transform are similar, used to work in the frequency domain. But you don't need to necessarily study the ins and outs to work on the processing: it was my fault for bring the terms in.Quote:
Originally Posted by arith
The original question was "high pass" and "low pass," and the point is that it's valid to consider an image as being made up of frequency components, where areas with intensity jumps contain high frequency, and areas of similar intensity are low frequency. If you only "pass" some of those frequencies through, you'll either sharpen or smooth the image.
Cheers,
Rick -
10th February 2010, 05:06 PMarithRe: Getting rid of the haze (technique)
I can understand that one. cheers Rick :)