Use system variables to get lower left and upper right coordinates of the model space viewport

By Gopinath Taget

You can use the system variables VIEWCTR, VIEWSIZE and SCREENSIZE to calculate the lower left and upper right coordinates of the current viewport in the ModelSpace . The VIEWCTR system variable has the view center coordinates reported in current UCS. So steps to follow to get the required coordinates are:

1) Get the VIEWCENTER in current UCS and transform the point to DCS (Display Coordinate System)

2) Get the VIEWSIZE, which is the height of the view.

3) Get the screen aspect ratio using SCREENSIZE system variable. Using the value returned by SCREENSIZE, get the screen width and screen height. SCREENSIZE returns a list of two reals, first one is the screen width and second one is screen height.

4) Calculate the width of the view using the formula: width = height * (screenwidth/screenheight)

5) Calculate the lower left corner as:

LL(x) = center(x) – (width / 2)
LL(y) = center(y) – (height / 2)

6) Calculate the Upper right corner as:

UR(x) = center(x) + (width / 2)
UR(y) = center(y) + (height / 2)

7) Transform the LL and UR coordinates from DCS to WCS. Note that this will return the points that lie in the DCS XY plane, reported in WCS.

The following code shows how to do this using the ActiveX API in VB/A.  It also draws a line joining both the points.