Grass Valley XtenDD v.4.2.4 - Manual (Page 104)

2. Panel Operation
2 – 75 Operating Instructions – Rev. 1 / 7.2002
Mask inv The Mask inv button can be used to invert the mask signal.
This means that, for instance, with a square mask, not the inside but the outside
of the square is used for masking.
Mask PVW The Mask PVW button is used to highlight the mask signal on the preview monitor.
This function serves to adjust the mask.
Note: In Mask PVW mode, the mask is only represented as a 1-bit signal.
If you want to exactly position a mask with softness, simply switch on
the mask with the Mask on key in the Keyers panel.