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Don’t crop images.

In the Photosynth guide, it said that no image should be cropped as it will confuse photosynth. What's wrong with removing unnecessary parts of the photos?
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  • I’m curious.
    I would like to know this as well. I assumed that this was the easiest way to keep people from removing the overlapping parts of the photos which are necessary for the synther to most accurately reconstruct the scene.

    I just assumed that when explaining this to the general public it is easier to say "don't crop" than "if you really are careful to preserve overlap in all your photos, then cropping is okay".

    Live Labs, is there some technical reason for this "rule"?
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  • David Gedye (Principal Group Manager) August 29, 2008 05:23
    Ok -- here's the whole story. Photosynth assumes that the center of your photograph was exactly where your lens was pointing when you took the photo. If, for example, you crop away the left half of your photo, photosynth will assume that the middle of the remaining piece was where the lens was pointing -- which is wrong.

    Why does this matter? It matters because the main computer vision magic of photosynth is an algorithm called "bundle adjustment" which works out exactly where each of the photos was taken from and exactly which way the camera was pointing when the photos were taken. Photosynth gets confused if you crop because it gets the wrong idea about what subject was directly in line with central axis of the lens.

    Well does that mean that you can crop as long as you do it symmetrically? The answer is a guarded yes. It's not a complete yes because the other piece of information that Photosynth uses is the focal length as reported in the photo's EXIF tag. If you crop then you are simulating a lens with a different focal length, and this too will confuse Photosynth. However, Photosynth is fairly resilient to focal length errors, so it usually works.

    More than you wanted to know?
    David.
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  • Ken (Official Rep) August 29, 2008 16:29
    If you want to crop but find that the resulting image isn't getting included in the synth try including both the original and the cropped image. The original should fit very well since we normally can compute the Field of View (FOV) from the EXIF. And the cropped photo should match closely to the original. It's not perfect but often works.

    There's one time when cropping is a good idea - if you're trying to match a wide angle shot to a zoomed shot and the synther isn't fitting them. The synther doesn't use the full size image for matching (it's way too slow) so the trick is to crop the wide angle view so it will match to both the wide angle and telephoto shot. I'll post a more detailed description of how to do this on another thread.
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  • I would love to learn how to do this, because I have a situation where I have old photographs which I cannot gather more data. They overlap, and are high resolution, however they are too largely scaled in order to work well for what I want them to do.

    Can you elaborate on symmetrical cropping?
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