PDFView - Zoom to Rect

Q:
I am using PDFTron's PDFViewer control and want to call SetZoom factor
to implement 'zoom to rectangle' functionality.

What is the meaning of x,y paramater in SetZoom method?
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A:
SetZoom(x, y, zoom) sets the zoom factor to a new value using the given
pixel coordinate (x,y) as a zoom center.

The x,y coordinate is represented in the 'screen' coordinate system.

The zoom point (x,y) is represented in the screen coordinate system,
which starts in the upper-left corner of the client window.

Calling this function is similar to the behavior of zooming on a given
point (x, y) by mouse clicking. The new view will be centered on the
clicked point at the given zoom factor.