setviewport function sets the current viewport for graphics output.

Declaration :- void setviewport(int left, int top, int right, int bottom, int clip);

setviewport function is used to restrict drawing to a particular portion on the screen. For example setviewport(100 , 100, 200, 200, 1);
will restrict our drawing activity inside the rectangle(100,100, 200, 200).
left, top, right, bottom are the coordinates of main diagonal of rectangle in which we wish to restrict our drawing. Also note that the point (left, top) becomes the new origin.

C programming source code for setviewport

#include<graphics.h>
#include<conio.h>
 
main()
{
   int gd = DETECT, gm, midx, midy;
 
   initgraph(&gd, &gm, "C:\\TC\\BGI");
 
   midx = getmaxx()/2;
   midy = getmaxy()/2;
 
   setviewport(midx - 50, midy - 50, midx + 50, midy + 50, 1);
   circle(50, 50, 55);
 
   getch();
   closegraph();
   return 0;
}