What Lines Do I Need?

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‌What Lines Do I Need?

The line alignment solver can be used after a shot has been solved and a lens field of view (FOV) determined; it might be used without a solve, with a known FOV; or it might be used to determine the lens FOV. In each case it will determine the camera placement as well.

If the FOV is known, either from a solve or an on-set measurement, you will need to set up at least two lines, which must be parallel to two different coordinate axes in 3- D (X, Y, or Z). This means they must not be parallel to each other (because then they would be parallel to the same axis). You may have any number of additional lines.

When the FOV is not known, you must define at least three lines. Two of them must be parallel to each other and to a coordinate-system axis. The third line must be parallel to a different coordinate system axis. You may have additional lines parallel to any of the three coordinate system axes.

Note: SynthEyes permits unoriented lines to be used to help find the lens distortion. Unoriented lines do not have to be aligned with any of the desired coordinate system axes—but do not count at all towards the count of lines required for alignment.

Whether the FOV is known to start or not, two of the lines on different axes must be labeled as on-axis, meaning that the scene will be moved around until those lines fall along the respective axis. For example, you might label one line as On X Axis and another as On Y Axis. If you do not have enough on-axis lines, SynthEyes will assign some automatically, though you should review those choices.

The intersection of the on-axis lines will be the origin of the coordinate system. In the example above, the origin will be at the bottom-right corner of the left-most of the two horizontal windows above the door. As with tracker-based coordinate system setup, there is no “correct” assignment—the choice is up to you to suit the task at hand.

To maximize accuracy, parallel lines should be spread out from one another: two parallel lines that are right next to each other do not add much independent information. If you bunch all the lines on a small object in a corner of the image, you are unlikely to get any usable results. We can not save you from a bad plan!

It is better if the lines are spread out, with parallel lines on opposing sides of the image, and even better if they are not parallel to one another in the image. For example, the classis image of railroad tracks converging at the horizon provides plenty of information.

Also, be alert for situations where lines appear to be parallel or perpendicular, but really are not. For example, wooden sets may not really be geometrically accurate, as that is not normally a concern (they might even have forced perspective by design!).

Skyscrapers may have slight tapers in them for structural reasons. The ground is usually not perfectly flat. Resist the temptation to “eyeball” some lines into a shot whenever possible. Though plenty of things are pretty parallel or perpendicular, keep in mind that SynthEyes is using exact geometry to determine camera placement, so if the lines are not truly right, the camera will come out in a different location because of it.

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