ACQUIRING GRID IMAGES
Before acquiring calibration images in Vic-Snap, select a name for the images by clicking Edit Project in the menu or toolbar.
A consistent suffix such as “cal” will make future reference easier. In this case, we’ll be keeping both the calibration and
test images together in a folder called “bending-test”.
Click Ok to accept. Then, for each grid position, capture an image in Vic-Snap by using the space bar, or the remote.
At least four calibration images must be acquired. More calibration images will give a more accurate result; in addition,
acquiring redundant images leaves more room to discard poor images (images that contain highlights, defocus,
obstructions, or other issues that makes them unsuitable for use). For a typical setup, 15-20 images will be a good number.
• To acquire calibration images, capture several images of the grid in various poses. Include significant rotations
about all 3 axes.
• To accurately estimate perspective information, the grid should be tilted off-axis and/or moved closer/farther from
the cameras for some images.
• To estimate aspect ratio accurately, the grid should be also rotated in-plane in some images.
• To estimate distortion accurately, grid points should cover the corners of the image field in some images. If a small
grid is used, this will require specifically moving the grid to each corner and acquiring images. If the grid nearly fills
the field, it will naturally fill in the corners.
• For each image pair, the grid should be visible in both images. If calibration is performed in roughly the same plane
as the specimen, this will happen naturally.
• Calibration images should be fairly well focused across the width and height of the target.
• When using a grid which doesn’t fill the field of view, take more images of the grid in all regions of the field, as well
as moving it closer and farther from the camera. (For a small grid, tilt alone will not provide the necessary
perspective information).
• If a grid dot is partially off the edge of the image, it will be discarded. However, if it is partially blocked by, i.e., a
thumb, Vic-3D may estimate the center incorrectly. This should be visible as a high error for that particular image.
• Some overdrive/saturation is acceptable, as long as black grid dots don’t appear shiny and white.
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