Delphi - make a thumb image

procedure makethumb(src,dst:TImage; size:integer);
var tr:TRect;
newwidth,newheight,orgwidth,orgheight:integer;

begin
 orgwidth:=src.width;
 orgheight:=src.height;
 if (orgheight/orgwidth > 1) then begin      // portrait
   newheight:=size;
   newwidth:=round(orgwidth*(newheight/orgheight));
  end
 else begin                                 // landscape
   newwidth:=size;
   newheight:=round(orgheight*(newwidth/orgwidth));
  end;
 dst.autosize:=false;
 dst.stretch:=false;
 dst.width:=size;
 dst.height:=size;
 dst.canvas.pen.color:=clwhite;
 dst.canvas.brush.color:=clwhite;
 tr.left:=0; tr.right:=size;  
 tr.top:=0; tr.bottom:=size; 
 if (newwidth < size) then begin      // portrait
   tr.left:=(size-newwidth)div 2;
   tr.right:=tr.left+newwidth;
   tr.top:=0;tr.bottom:=size;
   dst.canvas.rectangle(0,0,tr.left,size);    // fill white at left
   dst.canvas.rectangle(tr.right,0,size,size);// fill white at right
  end;
 if (newheight < size) then begin     // landscape
   tr.left:=0;tr.right:=size;
   tr.top:=(size-newheight)div 2;
   tr.bottom:=tr.top+newheight;
   dst.canvas.rectangle(0,0,size,tr.top);      // fill white above
   dst.canvas.rectangle(0,tr.bottom,size,size);// fill white below
  end;
 dst.canvas.stretchdraw(tr,src.picture.graphic);
end;

findings

For simplicity, the thumb is assumed square.

Thanks to David Wilbourn, who found an error when the image was square. Corrected now.
Thanks to Bengt Karlström, who found another embarassing error. Also corrected.