A destructive overlay in the image affects the image data itself.
In CVB it is done by transforming the active image in the display object into an overlay-capable image format.
The transformed image then has only a 7-bit dynamic in each image plane.
The lowest bit of each image plane is reserved for the overlay and preset by 0.
As soon as one overlay bit is set to 1 the overlay becomes visible.
The overlay is shown in blue as here in the example %CVBTutorial\Image Manager\VC\VCDemo :
Interactively it is possible to draw rectangles ( LB_DRAWRECT ), lines (LB_DRAWLINE) or points (LB_POINT) and for sure combinations of this.
In a program every pixel of the overlay could be set as described in the chapter programming destructive Overlays.