Camera Active Window versus Grabbing Window
The expert-level channel parameters belonging to the Camera Timing category uniquely define the Camera Active Window.
For area-scan cameras, this is a rectangular array of adjacent pixels. For line-scan cameras, this is a linear set of contiguous pixel.
Because of the definition of the camera active window, it is guaranteed that no luminance information appears on the camera video signal outside this window. However, it may happen that some pixels inside the camera active window are not bearing significant luminance information.
Here are two examples:
- The classical interlaced video standard incorporates starting and ending half-lines. For such video signals, the first and the last half-line within the active camera window are black.
- Camera may be specified by the manufacturer for a given effective width, but can exhibit a poor video response for the pixels located on the boundaries.
The frame grabber goal is to acquire a set of significant pixels from the camera active window. However, the frame grabber can impose some rules on the acquisition process. For example, a given frame grabber can restrict the number of pixels constituting a grabbed line to a multiple of four. This may be required to comply with data alignment issues inside the host computer.
Following the above considerations, it is necessary to define a Grabbing Window, which is a rectangular or linear array of pixel having a size and a position matching the camera active window, but in general not totally identical.
The GrabWindow parameter provides several pre-established methods to derive the grabbing window from the camera active window definition.