The Lens fade fixer??

Making Panoramas can be fun – but it can also be disappointing. If like me you use a cheap camera chances are you'll produce some panoramas that looks like this.

Your lens has failed you. The edges of the images can be over an f-stop darker than the centres causing these ugly bands when you stitch it all together. Some advise to take more photos with more overlap - this is not good solution. Ideally you buy a better camera and start again. But the opportunity has passed, you may never see this place again or in such perfect weather. Let us try to salvage what we have, it should be easy enough – I'm a hacker (not cracker) and that's the sort of thing we do when we should be doing something else.

If you have a sharp eye you will see some of the offending negatives on the right of the strip. The centres are darker in the negatives hence lighter on the positives. The negs on the left however look ok - this is because they were taken with the zoom lens in a telephoto mode. The fade varies with focal-length, aperture, contrast and the phase of the moon. If you are unable to lock the aperture setting then the amount of fade may vary within a panorama series. "The internet" says that the fade is related to the fourth power of the cosine of the ray angle, maybe it's true but I don't believe it.
So learned how to access BMP files (and later PNG,JPG and TIF) and wrote some code. Processing every pixel, computing it's distance from the middle and applying a correction is not that hard but you do need to un-gamma correct the bitmap and such. I found a simple x^2 curve worked best for my ricoh-rz115 photo but a x^3 worked better for my older camera (the reef photos).
But there was a snag – color cross-over! In tech terms the d-log-e (density/log exposure) curves are slightly (or not so slightly) different for the 3 color emulsions on the film. In lay terms the dark parts of the photo will have one color and the light parts the compliment. For example sunny bits might have a green cast and shadows magenta. Its fairly common and usually its not to offensive. What this means it that the edge of the photo is not only darker but its also a different color! In my test photos this was mainly a yellow/blue cross-over with the edges tending towards blue. I tried to ignore it but the keyboard called me and I wrote some more code. Not knowing exactly what the film curves looked like I could only do a crude color correction.

This is a stitch of two images. The top one has a dark band in the middle, the bottom one has had "the treatment". I think I've made an improvement - the banding is much reduced and the color x-over not too noticeable. My biggest problem is getting consistent scans from my scanner/software – this might be my next adventure. The left side is has more yellow/green but this is in the original scan.

The program is intended to batch process a series of images in a series but this will only work if images have the same amount of fading etc. Sometime the image will need to be processed one at the time. Before doing any batch processing you load an image and try to make it write. This is done to working out the parameters for the batch job. It is worth noting that the fading is easier to see in a small image and making the preview images larger would actually make it harder to set the adjustments.
The "curve" options let you choose how exposure correction is applied. If only the extreme edges are dark you'd use a x^4 curve. If the fade is very gradual you'd try linear. So far sqr (x^2) and x^3 have worked the best for me but your milage may vary.
Moving the "Fade compensation" sliders should move the graph is real time and also correct the right hand image. In the graph "up" is lighter and the green horizontal line is neutral (no correction). It is a linear scale.
It is nearly impossible to know what the image gamma really is – play with it until it looks the best. Mainly you are looking for uniform contrast across the image. The controls tend to interact somewhat.
As stated earlier the color x-over correction is fairly crude. Avoid moving the controls to far left because you will ruin your blacks. Essentially you are changing the contrast of the three colors independently and the overall contrast will be change to some extent.

Beta 1.1 is here. if you want to play. It is a windows program (sorry about that). It has been run on win2000 and NT4 and I'd expect it will work on 95 and 98 and (heavens forbid) win-XP. It is written in delphi-5. My PC is a 950 athlon with 128K of RAM and 40G of disk. Older machines may struggle. You don't have to installed anything but you need to put the .ddl file where the program can find it (such as in the same directory).

There will be bugs – report them here.
There are some know issues which I will probably fix sometime.
The output filename generation needs to be a bit smarter so it doesn't append a count when a only single file is processed. It could also be taught to skip filenames which are already used.
The only TIF files supported are 24bit uncompressed.

The "check update" feature is a bit experimental. For the (rightly) paranoid all it does is load a small text file from this website onto your drive – if the version number is newer it tries to download the new zip for you. It does not run it. It does not send anything out on the net expect the file requests. However it is a little disturbing how easily something nasty could be hidden inside a program like this – it pays to run a (free) firewall in any case.

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