This filter will not normally be required except when trying to improve images acquired from pause mode. The basic idea is that timing errors in the video signal in puase mode can lead to a horizontal relative shift of the two video fields, so that a nominally vertical line may end up as a square wave, the amplitude of which may vary over the height of the image.
Normally either the Average or Duplicate interlace information filter would be used in preference. However, those two filters discard half the information contained in the signal, whereas this corrective filter does not destroy any information.
To utilise this filter the image must contain at least one and preferably two sharply defined vertical lines. DigImage uses these lines to detect the positional mismatch between the even and odd lines. The lines, satisfying a user-specified threshold, are located automatically within user-specified windows.
Buffer to correct interlace on? Any valid image buffer may be corrected. The buffer filtered will be made the current output buffer. If the buffer specified is in extended memory, then the buffer is first copied to buffer 0; after the filtering is finished, the result will be copied back to the extended memory buffer.
Vertical line interlace filter window 1 In order to locate the vertical lines used to determine the mismatch between the two video fields, two windows must be specified such that the vertical line may be described uniquely in terms of a threshold (and transition direction) within each window. If the mismatch between the two video frames has a vertical structure, then it is highly desirable to maximise the vertical extent of the line and window. The first and second windows are treated in an identical manner; if the image contains only one line, then the two windows may be set to reference the same line.
The normal method and considerations for specifying a window are relevant. Help with window specification is available through the H help option in the windows submenu.
Threshold for this window ? The threshold, together with the direction of intensity change (see next entry) defines the vertical line within the window as a transition of intensity from below (above) to above (below) the threshold.
Look for step Up or Down for this window ? The searching for an intensity transition takes place from left to right. If a step Up (Down) is specified, then the line is defined as the transition from an intensity below (above) to above (below) the threshold specified above.
Vertical line interlace filter window 2 Considerations for this window are identical to window 1.
Threshold for this window ? Threshold considerations are identical to those for window 1.
Look for step Up or Down for this window ? As with window 1, this specifies the sign of the transition.
Fit how many terms: 1, 2 or 3 ? Once the mismatch in the vertical line between the two fields has been established, DigImage attempts to fit a constant (1 term), linear (2 terms) or quadratic (3 terms) to this information. The fit is used at the basis of the correction.