How do I stitch two star trails photos taken by different cameras?

[neverhood311]

Also posted here

Last night I took some star trails photos. I set the shutter speed to 30", put the camera in burst mode and used my remote shutter release (with shutter button lock) to hold down the shutter button. I actually did this with two cameras at once, a Canon 7D and a Canon Rebel XTi. The 7D was mounted on my tripod, pointing up toward the sky, but low enough to include some trees in the frame. The Rebel XTi was sitting on the ground, pointing straight up into the sky.

I set both cameras to the exact same settings and had them running for the same amount of time. The focal length of the 7D was 17mm and that of the Rebel XTi was 18mm. I used a star trails program to stack the exposures together from each camera to produce two separate star trails photos. I'd like to stitch the two photos together (hopefully automatically) so that the resulting image covers a much larger portion of the sky than is possible with a 17mm focal length. However, when I run the two photos through either AutoStitch or Photoshop's Photomerge option, they fail to match the images, presumably since most of the image is a solid color with a few white streaks across it.

Is there a way to stitch these two photos together automatically? If not, is there a way to do it manually but also compensate for perspective distortion?

[GrahamH]

Hi Justin,
Assuming you have access to PP software. I would suggest the following manual method if the automatic doesn't figure it out. Create a new file with a canvas size large enough to play around with both files.
If there is sufficient overlap between the two images, set the transparency of one low enough you can see the other through it. Select one of the distortion modes (e.g. Skew) and stretch and compress the images until you can see they match. Overlay and erase (non-destructively of course, typically using layers).

Alternately, set the canvas background colour to black. Set the blending mode of one of the images to lighten (so the star trail shows through preferentially). You can then skew one image to fit with the other, or use to give the effect of more stars in the sky than there really are.

I don't know if the two cameras have the same number of pixels, which will cause other issues of course. In that case, lay the smaller image over the larger an increase it's size until they match (or reduce size of larger image). Do this before trying to adjust for difference in view.
Hope this helps. Never done it myself, but it's what I would try first.
Graham

[ajohnw]

There is an open source program called Hugin that will do this for any mix of lenses. There are lots of tutorials on it's web site. Making panoramas is it's main purpose in life but it will do a number of other things as well. Distortion correction and CA parameter extraction for instance. I've not used it but many have. I will be trying it for just distortion correction soon including perspective problems.

John

[stuck]

Is there a way to stitch these two photos together automatically? If not, is there a way to do it manually but also compensate for perspective distortion?

Have a look at Microsoft Image Composite Editor.

It is completely free and is the best image stitcher I've found at any price.

Ken

[neverhood311]

@ajohnw Thanks for the tip. I played with Hugin a bit last night and it looks promising. It also helped me realize that my images don't have nearly as much overlap as I previously thought. I'm gonna play around with Hugin some more.

@stuck Thanks for the tip. The 'Stitch directly from a video' function looks like fun. I myself have become stuck trying to install the 64-bit Windows version. For some reason, the installation gets interrupted and doesn't install. I tried twice, restarted my computer, then tried once again. No luck. Did you have any trouble installing it?

[ajohnw]

I tried it for distortion and perspective correction yesterday. Where is asks for angle of view and focal length as it powers up. I just cancelled that, marked the verticals and let it have a go, It was a 14mm shot on 4/3 were the camera had to be pointed up a bit. It complained about not being able to read the focal length from the 16bit tiff but made a fairly good job of it anyway. All but one vertical was perfect. Easily cured with linear distortion on that region of the shot. I suspect this feature is worth playing with as well.

People who work on open source software often don't keep the documentation up to date so there is usually a need to play around with the software for a while. Understandable really as generally they do the work for fun.

John

[stuck]

@stuck Thanks for the tip. The 'Stitch directly from a video' function looks like fun. I myself have become stuck trying to install the 64-bit Windows version. For some reason, the installation gets interrupted and doesn't install. I tried twice, restarted my computer, then tried once again. No luck. Did you have any trouble installing it?

I've only got a 32 bit system but no, no problem with installing ICE. Having said that, there were two false starts when the installer didn't find a couple of prerequisites. I had to download and install them first. This shouldn't have been a surprise to me as the requirements for ICE say:

ICE requires the Microsoft Visual C++ 2010 Redistributable as well as version 4 of the .NET Framework. If either of these components are missing, the ICE installer will prompt you to download them from the internet.

but hey, who reads instructions before they start on something?

If you still have problems I suggest you ask / search the ICE forum, here:
http://social.microsoft.com/Forums/en-US/ice/threads

Persevere, ICE is fun and painlessly simple to use.

Ken