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The camera’s optic axis is the point about which the image expands or contracts as objects move closer or further away. Lens distortion is also centered about this point. By convention of SynthEyes and most animation and compositing software, this point must fall at the exact center of the image.
Usually, the exact optic center location in the image does not greatly affect the 3- D solving results, and for this reason, the optic center location is notoriously difficult to determine from tracking data without a laboratory-grade camera and lens calibration.
Assuming that the optic axis falls in the center is good enough.
There are two primary exceptions: when an image has been cropped off-center, or when the shot contains a lot of camera roll. If the camera rolls a lot, it would be wise to make sure the optic axis is centered.
Images can be cropped off-center during the first stages of the editorial process (when a 4:3 image is cropped to a usable 16:9 window), or if a film camera is used that places the optic axis allowing for a sound channel, and there is none, or vice versa (none is allowed for, but there is one).
Image stabilization or pan/scan-type operations can also destroy image centering, which is why SynthEyes provides the tools to perform them itself, so they can be done correctly.
Of course, shots will arrive that have been seriously cropped already. For this reason, the image preprocessor stage allows images to be padded up to their original size, putting the optic axis back at the correct location. Note that the shot must be padded to correct it, rather than cropping the image even more! It will be important to identify the degree of earlier cropping, to enable it to be corrected.
The Fix Cropping (Pad) controls have two sets of three spinners, three each for horizontal and for vertical. Both directions operate the same way.
Suppose you have a film scan such that the original image, with the optic axis centered, was 33 mm wide, but the left 3 mm were a sound track that has been cropped. You would enter 3 mm into the Left Crop spinner, 30 mm into the Width Used spinner, and 0 mm into the Right Crop spinner. The image will be padded back up to compensate for the imagery lost during cropping.
The Width Used spinner is actually only a calculation convenience; if you later reentered the image preprocessor dialog you would see that the Left Crop was 0.1 and the Width Used 1.0, ie that 10% of the final width was cropped from the left.
The Fix Cropping (Pad) controls change the image aspect ratio (see below) and image resolution values on the Open Shot dialog, since the image now includes the padded regions. The padding region will not use extra RAM, however.
It is often simpler to fix the image centering in a way that does not change the image aspect ratio, so that you can stay with the official original aspect ratio throughout your workflow. For example, if the original image is 16:9 HD, it is easiest to stay with 16:9 throughout, rather than having the ratio change to 1.927 due to a particular camera’s decentering. The Maintain original aspect checkbox will permit you to update the image center coordinates, automatically creating new padding values that keep the aspect ratio the same.
©2024 Boris FX, Inc. — UNOFFICIAL — Converted from original PDF.