(With Delphi 3) I've loaded a bitmap into TImage. The bitmap (actually it is
a real map!)has a scale line on it somewhere representing a certain
distance. I need to do three things:

1. Click mouse at two points on the scale line to record x pixels

2. Click a "draw" button (for example) to draw a grid over the entire
bitmap, the length of the sides of its squares being the number of pixels
previously recorded. I assume I can use the LineTo method of TCanvas to draw
the first side of a grid square, then an algorythm of some sort to construct
the rest of the grid. Now I'm lost.

3. Use mouse to position grid over map before setting its position with a
button or right click etc.

The gridded (is there a better word) bitmap can then be saved/printed.

I have absolutely no idea how to proceed and any advice would be most
welcome.
Thanks. Paul.

In article <397d2...@nap-ns1.netconnect.net.au>,
  "Paul" <holdf...@netconnect.com.au> wrote:

    First you probably need some sort of global state variable
to keep track of what the next click on the grid "means". Like

type TNextClickMeans = (ncmFirstX, ncmSecondX, ncmGridPoint);

var nextClickMeans: TNextClickMeans;

   firstX, secondX: integer;

   gridPoint: TPoint;

Before starting all this you say

nextClickMeans:= ncmFirstX;

Now the image's OnMouseDown handler inspects the value of
nextClickMeans to decide what to do, something like

procedure whatever.MouseDown(...);
begin
case nextClickMeans of
 ncmFirstX:
  begin
   firstX:= X;
   nextClickMeans:= ncmSecondX;
  end;
 ncmSecondX:
  begin
   secondX:= X;
   nextClickMeans:= ncmGridPoint;
  end;
end;
  ncmmGridPoint:
    begin
      gridPoint:= Point(X,Y);
      nextClickMeans:= whatever seems appropriate;
      DrawTheGrid;
    end;
end;

Now the DrawTheGrid method does something like this:

procedure DrawTheGrid;
var  j, delta, XMin, YMin, XNum, YNum: integer;
begin
  delta:= abs(firstX - secondX);
  XMin:= GridPoint.X - (1 + GridPoint.X div delta)*delta;
  YMin:= GridPoint.Y - (1 + GridPoint.Y div delta)*delta;
  XNum:= 2 + image.Width div delta;
  YNum:= 2 + image.Height div delta;

  for j:= 0 to XNum do
    begin
     image.Canvas.MoveTo(XMin + j*delta, 0);
     image.Canvas.LineTo(XMin + j*delta, image.Height;
    end;
  for j:=0 to YNum do
    begin
      image.Canvas.MoveTo(0, YMin + j*delta);
      image.Canvas.LineTo(image.Width, YMin + j*delta);
    end;

end;
end;

    That's off the top of the head, may need a little
tweaking. (Off the top of the head I suspect that it
draws all the lines you want, possibly also drawing
one or two invisible off-screen lines. Except for
typos.)

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