Revision history [back]

EDIT:

So, thinking about this, there are actually three separate problems.

1. The Surface is not flat. - Use Structured Light
2. The Projector lens has distortion - Use Camera Calibration in reverse.
3. The Projector is not projecting square - Use Homography

You can probably safely ignore the first unless you're trying to project on a round building or something.

For the second the procedure is as follows:

• Project a chessboard on the wall with a calibrated camera viewing it.
• Undistort captured images according to camera calibration.
• Find corners of projected image and warp into a (X by Y) projector resolution image so the corners are the corners of the image.
• Use that to run a normal camera calibration

For the third, it's pretty simple.

• Pick the shape the projection should look like in aspect ratio. (IE: Find the corners of the projection screen)
• Display a totally white image and find the corners.
• Find the largest rectangle of screen aspect ratio that fits within the white. OR within the screen, if the projection is bigger than the display screen.

EDIT #2:

How to adjust the shape of the projection to make it square.

I'm assuming you have moved the projector into the best position you can manage, and already adjusted the keystone to get it as close as possible to correct. But now you need to project some pattern in a uniform manner but the image isn't uniform.

The first step is to get your calibrated camera (since every little bit helps) and take a picture. I'm going to fake the photos in GIMP, it's quicker and you'll get the idea.

Then you need to get your tape measure and measure the projection screen. You need to do this to be sure of the aspect ratio so you can warp it as if you were facing it directly on. Measure it in the X and Y directions. Take your undistorted image and pick the corners of your screen (or surface) and warp them to a Mat of the same aspect ratio of the screen, which I'll call ScreenMat. It should be a larger resolution than your projection.

(See that? That's why you calibrate your camera and undistort first. Your lines should be straight, not bent, I think. If they aren't, you will need do a Camera Calibration for your projector lens. After this though, this comes first.)

This warp is a simple homography that you can calculate by using this function. Your inputs should be the four corner points you picked out of the image, in pixel (x,y). The destination points should be the four corners of your ScreenMat.

For the last step pick the four corners of the actual projection. You want to use as much of your projector as possible, so create a rectangle within the projected area that is the same aspect ratio as your projector. Then do the findHomography again from the actual projection corners to the new corners you pick. Then warp your images with this homography before you project them, and they should show up within the rectangle you picked.

I hope this makes sense. If anything isn't clear, just ask.