Image Processing

As time has gone on, I’ve started using a wide range of image processing software. I’m still looking for the “perfect” package which probably doesn’t exist. The issue is, what are you trying to do? The software I use for photos is different from what I use for digital images taken with my digital camera or videocamera.

My digital camera takes images 1280×960, my digital videocamera takes images 740×480 (with non-square pixels!). Those numbers are fixed. With 35mm film, resolution gets measured differently. The typical figure of merit for a photographic lens is a 25 µm spot size—“circle of confusion” in photography lingo. For a 50mm lens, this works out to an equivalent pixel count of about 1440×960. Based on those figures, it would seem there is no good reason to use different software. However….

Digital images typically have limited dynamic range compared to film. A typical frame from my digital camera, videocamera, or webcam is noisy and is quantized to 8-bits/color. Exposure time is limited to what the hardware will support (typically a small fraction of a second) and to get good signal-to-noise one has to “stack” multiple images.

Film, on the other hand, can be scanned at 16-bits/color and the image can be oversampled during the scanning process. My typical photo scan is around 3300×2200 pixels and produces a 40+ MB uncompressed TIFF file. Working with that is considerably different from working with a 1 MB BMP frame from my videocamera. You can use the same software for both, and in practice there is definitely some overlap. Still, you will find different tools useful for different things.

NB: This page is rather dated and I suspect you can get better info via Google.  Look at the review date before jumping to any conclusions.

AIP4WIN

This is a very nice image processing package which works on FITS images and only FITS images. However, it comes with a good importer that will load most image formats. The notable exception is 48-bit TIFF images will not be loaded. It also includes a “Universal Loader” function will will work on binary bitmap-type formats like those of the NetPBM package, which is how I get my 48-bit images loaded. What makes this an especially good value is that it comes with a textbook-like tutorial on everything you ever wanted to know about astronomical image processing, The Handbook of Astronomical Image Processing.

AstroStack

AstroStack is a “simple” program that takes multiple Windows Bitmap (.bmp) files, aligns and averages them. It has the ability to run some filters over the finished results and inclues dark and flat frame normalization as well. The chief limitation is the averaging part; your image is limited to 8-bits per color plane which is fine for planetary images but insufficient for deep-sky. “AstroStack II” has been released and I have not yet used it. It has increased the image size restrictions, but as of this writing, is still limited to black-and-white and is not yet feature-complete when compared to AstroStack 0.9.

AVI2BMP

AVI2BMP does a little bit more than its name would suggest. In addition to converting AVI files into Window bitmap files, it can convert both AVI and BMP files into the more standard FITS files used by most astronomical imaging software. And, while doing this, it can take an automatically crop planetary images to a rectangular area around the planetary disk. This is immensely useful for shrinking a huge, mostly blank image into something that requires a lot less disk space and it will make later processing steps (e.g., by Iris) run much faster as they are manipulating smaller images.

Version 0.50 is only available in French, version 0.49 is availabe in both French and English. If you don’t speak French, that won’t keep you from using version 0.50, especially after you become familiar with version 0.49, and if you stick with version 0.49 you aren’t missing much.

GIMP

The GIMP is the Open Source Movement’s answer to PhotoShop. With the GIMP, you can to pretty much anything you can do with PhotoShop. Although I’m certain that graphics arts professionals may have a different view due to their level of experience, the casual user will be quite happy with how much you can do. My use of the GIMP is largely limited to creating image rollovers to provide labels for my photos. Like PhotoShop, this is best accomplished via layers which allow you to leave your base image untouched and makes it east to group your changes in an easy-to-throw-away fashion. Yes, that sounds goofy, but I have taken and thrown out an entire layer to start over with it.

The GIMP also has the tools you would expect with a graphic arts program, filters to sharpen (or blur) your images, brightness and color curve and histogram adjustments, color correction, image type conversion, etc. I consider the GIMP and PhotoShop useful for final composition but they are probably not what you want to start with. The biggest limitation with the GIMP is that it does not do 16-bits/color. It will import a 48-bit TIFF, but internally it has already converted it 24-bit TIFF. This is to be rectified in version 2, and there is a version called Film GIMP which already has 16-bit/color support, albeit I understand it only works with CMYK images at present. PhotoShop is marginally better, but even there you will have to convert your image to 8-bits/color before you can create any layers. I have on good authority that this is true even in PhotoShop 7.

In short, you probably want something like the GIMP for final image composition, especially if you want to create overlays, but it is not a strict requirement.

ImagesPLUS

I’ve had an opportunity to use ImagesPlus in an somewhat limited fashion.  My primary motivation for using it was to control my Canon Digital Rebel XT (350D) cameras.  Yes, plural.  Alas, that proved to be only partially possible.  It should be completely possible and Mike Unsold has it working with multiple dissimilar camera, but I have two 350Ds with identical firmware and while it sees both camera and even fires their shutters independently but when the software attempts to download the images, it hangs.  Not catastrophically (i.e., I don’t have to reboot Windows), but enough that it’s not fully functional for my needs.

However, ImagesPlus does a lot more than just control digital cameras.  It can also do file format conversions and serves as a fairly nice image processing tool in it’s own right.  Again, I’ve used it almost exclusively in the DSLR control mode and for converting raw images to 48-bit linear TIFF.  Despite the problem with controlling my dual Canon 350Ds, the package is worth a good look and Mike Unsold makes his living from this so he’s quite responsive to issues in the package.  Oh, and while the software itself fits on a single CD,  that CD has several video tutorials of how to use the software and the total package includes another four (yes, a total of five) CDs with more video tutorials.

Iris

Iris is a fairly complete piece of software which is not hard to use but not exatly intuitive either. Some features are easier to use from the Iris console window, others from the menu. It has the feel of something pieced together over time by people who were doing it for themselves rather than as a project for other people. But the more professional AIP4WIN doesn’t have some of the features included in Iris, e.g., maximum entropy deconvolution and Iris is easier to use for batches of AVI frames. My chief complaint is that all of the commands for reducing multiple images into a single registered and normalized image take both a filename prefix and a count but won’t let you select a starting point in the file sequence. That is, you can process files jup1.fit through jup100.fit but not jup51.fit through jup100.fit. You’ll also probably want to take a look at AVI2BMP for preprocessing.

Christian Buil does a great job of adding features to Iris and you will want to check back at his web site every couple of months to see if he’s added something you want. He’s also responsive to email. Iris is the only package I have which will do Maximum Entropy Deconvolution. However, like many other features, this one is not available from the menus. That is probably the only real weakness of Iris; many functions are not on the menus and you must use the command-line tool to get at them. This is not a big deal to me, but it does mean that if you are accustomed to learning by poking at the menus you won’t begin to exploit all of the features. The HTML documentation is available via the Iris Home Page and is kept up-to-date.

PictureWindow

Picture Window Pro has been a wonderful find for me. I saw it recommended by several members of the APML group. I’ve found that for most of my image processing, this is the tool of choice. First, it does all of its operations in 16-bit mode. There are only a handful of functions you can do in 16-bit mode with PhotoShop and none (yet, anyway) in the GIMP. It’s mode of operation is a bit different that PhotoShop or the GIMP, in particular, there are no layers. When PWP operates on an image, it creates a copy of the modified image. The “Tools” menu is an exception, e.g., the clone tool to remove blemishes, then the current image is modified in place.

There are certainly a handful of things which PWP doesn’t do: there are no “advanced” filters specific to astronomy, e.g., Richardson-Lucy deconvolution. But it does have a very good alignment/registration tool. While RegiStar is better for deep-sky images probably in quality and definitely in ease-of-use, PWP has been invaluable for registering my lunar images. I highly recommend this program.

RegiStar

RegiStar is the easiest to use image registration program I have ever used. Iris has more-or-less automatic registration for planetary images that works with only a little setup, but it only operates on FITS. RegiStar works on whatever you load in but it only works on non-planetary images which, as the name implies, have stars in them. Its registration process is about one step short of magic. Load your images, tell it to find the stars, then pick a pair and ask it to register the images. Repeat this using the same base image for each of your images and finally ask it to combine the set. You have a choice of several different ways of merging the images (mean, median, add, …) and of cropping (union, intersection, crop to base image).

RegiStar will work with up to 16-bits/color so if your images are already 16-bits/color you will not be able to add them, you will have to average them. This is perhaps the only weakness, and its pretty minor. Yes, it would be nice to be able to take half-a-dozen 48-bit TIFF deep-sky images, add them, convert to FITS and write out for another round of processing. But