Northern Eclipse Help Reference

 Acquire Loop

This function repeatedly captures frames in a loop and optionally performs averaging and/or background subtraction.  This function will work with analog or digital cameras since it uses the Snap function of the device.  The function loops until Stop has been selected from the dialog box.

Background Subtraction

In order to perform background subtraction, the image to be used as the background must be loaded beforehand and must be the current window.  In addition, this image must match the incoming image in terms of dimensions and image type.  Click on the “Subtr” checkbox to turn on background subtraction. You can optionally turn on the “Offset” checkbox in order to have a gray background instead of a black background.  Offset also allows for negative as well as positive differences.  Without offset, only positive differences are shown with negative differences truncated to 0.


Averaging can also be done on the incoming image.  Acquire Loop performs a weighted average.  For N averages, the new average is calculated as the previous average (which is weighted (N-1)/N) plus the incoming frame (weighted (1/N)).  Over time, old image frames contribute less to the overall average.  This also means that you get a new average after every frame instead of after N frames (as would be the case with a simple average).

When Averaging and Background Subtraction are both performed, the Averaging is performed first and then the background is subtracted.

Chroma Keying

If the background image has a threshold applied to it and the image is a monochrome image, Acquire Loop will only update the pixels that have been thresheld and no background subtraction will be performed.  This allows you to chroma key a new image into an existing background.

See Also

Live, Freeze, Snap, Integrate, AutoExpose, Digital Focus


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