Draw outline/border around a shape on TCanvas in Delphi

Thanks, but I don't think the solution quite fits.

To explain further, multiple shapes (without a border colour) are drawn on to the canvas (similar to the blue area in the attached example)

What I want to be able to do is draw a 1-pixel black border over the outer-most blue pixels from the resulting shape.

umm... if you are drawing the shapes, then I don't see why you can't draw a border around it, especially if you succeeded in doing other shapes

Here's an example of drawing a line on a canvas:

procedure TForm3.Button1Click(Sender: TObject);
begin
  self.Canvas.PenPos := point(50,50);
  self.Canvas.LineTo(250,50);
end;

                                              

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Attached image 2: the blue area shows each of the shapes drawn on to the canvas with a black border,
Attached image 3: shows the result that I'm trying to achieve

Please explain how you are drawing the shapes Give me an example to work with. You are probably drawing shapes on a canvas and setting a 'border'. What you want to do in this case is draw partial borders - to say "only draw the top, left and bottom" etc.

What I'm ideally looking for is either:
- an algorithm that will trace around the outside edge of the shape on the canvas and replace (in this example) the outer-most blue pixel with a black one, or
- a floodfill-type alogrithm that hits the boundary and sets it to an alternative colour

it all depends on how you are drawing the shapes ...
if it is a completely closed shape then are you using a polygon ?

with that, you can do all at once:
The Polygon function draws a polygon consisting of two or more vertices connected by straight lines. The polygon is outlined by using the current pen and filled by using the current brush and polygon fill mode.

BOOL Polygon(

    HDC hdc,      // handle to device context
    CONST POINT *lpPoints,      // pointer to polygon's vertices
    int nCount       // count of polygon's vertices
   );

Question, must this be dynamic? In other words, right now you have a 'T' shape made up of little rectangles, and if the user rearranges this to say, an 'L' shape you want it to have the intelligence to draw a border?
In that case the algorithm I came up with is to iterate through all shapes in that object, and detect if they have any shapes to their north, west, south and east, and draw lines according to that.

actually if you would have a algorithm to create polygons from your shapes (i assume they are all rectangles)
you could also use PolyPolygon as well

even not adjacent ones ...

Apologies, I should provide some more background information...

The problem is dynamic. The overall shape is based on a grid made up of individual (cells) rectangles (either selected or not)

I agree that you could use your approach to detect if an individual rectangle has any other rectangles to it's north, west, south or east... but it starts getting complicated if you end up with a shape something like the one attached below!

That's why I was hoping to draw the rectangles without borders initially and retrospectively draw the border around the final shape.

i'm actually trying to come up with an algoritm to convert adjacent rectangles into polygons

from there it would be simple ...

The problem with the polygon approach is that each of the rectangle's co-ordinates on the grid are being retrieved from a database. It would be a huge task to retrieve the rectangles' co-ordinates in the correct order to be able to draw the whole shape as a single polygon.

but ... you are drawing the shapes individually ...
and for that you need ... those coordinates too
or am i missing something obvious ?

I guess that polygon approach is the most feasible...

"It would be a huge task to retrieve the rectangles' co-ordinates in the correct order to be able to draw the whole shape as a single polygon."
- If your program has the intelligence to know what rectangles constitute 1 'shape', then you could iterate through each one and put them 'in order' working left to right, bottom to top.
..and if you can do that, then you can also use the algorithm I described, but something makes me wonder if the polygon approach isn't better

Thanks rfwoolf... I think we're getting somewhere with your solution.

The rectangles have an associated X and Y position on the grid (which is stored in the database), so I could use the North-East-South-West method to determine a rectangle's relationship to all others.

Final question, how do I draw the border (say to black) for just one of the rectangles edges?

i'm off elsewhere ... home ...
i'll look into such an algorithm tonight ...

He can also set the brush style to bsClear and use the Canvas.Rectangle function to draw the rect around the
shape at the same correds of the shape, he also needs to set the pen.width and pen.color to his desired values.

uhm, i gave it a start for finding a set of polygons for the adjacent rectangles
i allready had to make the assumption of rectangles and ortogonal grid

looking at where this is going NESW seems indeed like your best valid option
unless you find a algorithm for finding the polygon
i bumped into dff looking for a solution myself
it seems some people are hooked on delphi:
http://www.delphiforfun.org/default.html

just a idea that springs to mind:
add a layer of shapes behind the ones you see, 1 pixel bigger (like GrowRect function)
draw those shapes first
then draw your normal visible layer

and that would be by far the most simple solution !!!
attached is screenshot

type
  TForm4 = class(TForm)
    Panel1: TPanel;
    Shape1: TShape;
    Shape2: TShape;
    Shape3: TShape;
    Shape4: TShape;
    Shape5: TShape;
    Shape6: TShape;
    Shape7: TShape;
    Shape8: TShape;
    Shape9: TShape;
    Shape10: TShape;
    Button2: TButton;
    procedure Button2Click(Sender: TObject);
  end;
 
var
  Form4: TForm4;
 
procedure TForm4.Button2Click(Sender: TObject);
var
  I: Integer;
  s, X: TShape;
 
begin
  for I := 1 to 10 do
  begin
    s := TShape(FindComponent(Format('Shape%d', [I])));
    if Assigned(s) then
    begin
      x := TShape.Create(Self);
      x.Parent := Panel1;
      x.Left := s.Left - 1;
      x.Top := s.Top -1;
      x.Width := s.Width + 2;
      x.Height := s.Height + 2;
      x.Pen.Color := clGreen;
      x.Pen.Width := 3;
      x.Brush.Color := clBlack;
      x.Brush.Style := bsClear;
      x.Visible := True;
      x.SendToBack;
    end;
  end;
end;

                                              

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with a pen width of 3 it had to be :

      x.Left := s.Left - 2;
      x.Top := s.Top -2;
      x.Width := s.Width + 3;
      x.Height := s.Height + 3;

Hmm I haven't read through Geert's code, but it occured to me at the time I suggested the NESW approach that if some shapes are say, 1 pixel off where they're supposed to be the algorithm might not work - so just keep that in mind. Fortunately colani's software might ensure that this doesn't happen because it's based on a grid system, but if it does, the algorithm might file. That's why this polygon idea seems good in that (from what I understand) the shapes are 'connected'.

I see Colani is closing the question. I assume he has found a solution. Hopefully you will share the outcome.