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In this method, the lens parameters are automatically adjusted to cause specific features present in the image to form a straight line, when they are known to be straight in reality. This method can be used live shots that contain a number of suitable long straight features, ie separate calibration images may not be needed. Of course, the accuracy of the process is limited by the lines present in the image. This is a relatively weak method, as "straightness" doesn't constrain the possible settings very strongly. It is especially poor at finding the lens center if there is little distortion.
Here's an image where the edges of the buildings, sides of the windows, and roof lines can all be used for line straightening.
The image is an example of a full-circle fisheye lens. It requires some additional steps for setup because the optical image doesn't extend to the edge of the digital image, discussed later.
The line-straightening method can also be used with calibration images of a grid of dots or checkerboard as input. If the grid/checkerboard is perfectly regular, straight lines can be automatically constructed from the grid. See the Array of Dots and Checkerboards sections (below) for information about how to set up for that. If the dots or checkerboard are irregular, you'll have to set up lines manually, just as if you were working from an actual scene.
Here we describe how to set up lines for straightening. Line-based calibrations use SynthEyes's "Flex" and "Curve" subsystems to store the line data. Those subsystems proved less useful in practice than anticipated, so the requisite control panel isn't shown by default.
To display the Flex panel, right-click in the right of the room bar at the top of the user interface (Summary, Roto Masking, Features,...) and select Add Room.
Enter Flex as the Room Name, and select Flex in the Panel dropdown. Click OK, then select the new Flex room. (You can save this configuration as your Preference if you want, via the right-click menu.)
Click the New Curve button to start adding a new (curving) line. Click repeatedly along the line in the camera view to create vertices. Right-click to stop adding vertices. Rough the line in initially, then zoom in and adjust the control points so that the curve falls along the line in the image along its entire length. Use shift-click to add additional vertices where needed.
NOTE : Do not allow lines to cross boundaries between pieces of paper, if the test image was assembled from multiple pieces of paper. (Please, always use a large-format printer to produce a single monolithic test pattern.)
Continue adding additional lines on straight features throughout the image. Note that when lines are used, only a single frame is examined for calibration (even though curves can be animated during the shot).
Whatever the source of the lines being straightened, there is no point adding nearly- radial lines, only (bent) circumferential lines supply information. However, the lines should not be exactly circumferential: diagonal lines are better!
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