problem with logic/math

[Billy Patton <bpatton dal asp ti com>]

I have the beginning of an app.  THis app is meant to display a cell of an
ASIC cell design (Application Specific Integrated Circuit)
THis cell can contain 1 to K's of rectangles, clodes polygons, text and
our own picular ty;pes of figures (center line figures, center point
rectangles, bla,bla,bla)

Inorder to view/edit this data I must have the ability to increase/
decrease the viewing scale.  I do not yet hava a mouse zoom in/out
just "i" for zoom in and "o" for zoom out.  The arrow keys move the data
around.

To control the data that is displayed and any scale I have the following
variables.
scale   -> applied to all coords of the data
shiftX  -> insures scaled data fits on the screen
shiftY  -> insures scaled data fits on the screen

What I need to happen is that when I zoom in/out (with a key) the center
of the screen needs to remain the same.  The shiftX/Y is not being
adjusted correctly.   I don't seem to be able to see the trees for the
forest!

Here's a snippett of my key_press_event
from my key_press_event function
/* scroll the data */
  case GDK_Left:
    {
      shiftX -= 50;
      redraw();
      break;
    }
  case GDK_Up:
    {
      shiftY -= 50;
      redraw();
      break;
    }
  case GDK_Right:
    {
      shiftX += 50;
      redraw();
      break;
    }
  case GDK_Down:
    {
      shiftY += 50;
      redraw();
      break;
    }

/* fit the data to the screen */
  case GDK_f:
    {
      int x;
      set_scale(db,fcellp,windowExt,&scale);
      x = (fcellp->extent.ur.x - fcellp->extent.ll.x) * scale;
      shiftX = ((windowExt.ur.x - windowExt.ll.x) - x) / 2;
      shiftY = 0;
      redraw();
      break;
    }

/* zoom in */

  case GDK_i:
    {
      scale *= 1.40;
      shiftX = (int)(shiftX * 0.60);
      shiftY = (int)(shiftY * 0.60);
      redraw();
      break;
    }

/* zoom out */
  case GDK_o:
    {
      scale *= 0.60;
      shiftX = (int)(shiftX * 1.40);
      shiftY = (int)(shiftY * 1.40);
      redraw();
      break;
    }