Delphi - mapping world and screen

Mapping world and screen coordinate systems to each other is
done by scaling with respect to an offset.

be the world window be defined as:
  
 [xLow..xHigh,yLow..yHigh], the coordinates are of type float

be the screen window be defined as:

 [tlx..brx,tly..bry], the coordinates are of type integer

the following function do the conversion :

   function mapW2SxLin(xf:float):integer;
   begin
    result:=round(tlx+(xf-xlow)*(brx-tlx)/(xhigh-xlow));
   end;

   function mapW2SyLin(yf:float):integer;
   begin
    result:=round(bry-(yf-ylow)*(bry-tly)/(yhigh-ylow));
   end;

   function mapS2WxLin(xs:integer):float;
   begin
    result:=xlow+(xs-tlx)*(xhigh-xlow)/(brx-tlx);
   end;

   function mapS2WyLin(ys:integer):float;
   begin
    result:=yhigh-(ys-tly)*(yhigh-ylow)/(bry-tly);
   end;

Logarithmic axis may used to display signal amplitudes amongst
other things. For a logarithmic scale use :

   function mapW2SxLog(xf:float):integer;
   begin
    result:=round(tlx+(ln(xf/xlow)/ln(xhigh/xlow))*(brx-tlx));
   end;

   function mapW2SyLog(yf:float):integer;
   begin
    result:=round(bry-round((ln(yf/ylow)/ln(yhigh/ylow))*(bry-tly)));
   end;

   function mapS2WxLog(xs:integer):float;
   begin
    result:=xlow*exp(((xs-tlx)/(brx-tlx))*ln(xhigh/xlow));
   end;

   function mapS2WyLog(ys:integer):float;
   begin
    result:=yhigh/exp(((ys-tly)/(bry-tly)*ln(yhigh/ylow)));
   end;

Improvements

• Snap
  The procedures above are the raw mapping procedures.
  However when using the mouse one would wish to display
  sensible numbers. Assume the screen is 100 pixels wide,
  the cooresponding world 0..1, making 0.01 per pixel.
  When the screen is only 99 pixels wide, the numbers become
  ugly. One should define a snap of 0.01 and the world is reduced
  to 0..0.99.
  Some thoughts about implementing snap are here
• Grid
  A grid should be drawn at sensible nubers. In a world of 0..1
  on 100 pixels, I'd want the grid at 0.0 0.2 0.4 0.6 0.8 1.0 and
  certainly not at 0.03 0.23 0.43 0.63 0.83.
  Some thoughts about implementing a grid are here