Clear timage.canvas in delphi 7 - delphi

Question

How to clear timage canvas to avoid duplicate image when changing input size? Why nil command doesn't work?
This is my code
begin
image1.Canvas := nil;
image1.Canvas.Pen.Color := clRed;
image1.Canvas.Brush.Color := clBlue;
image1.canvas.rectangle(10,10,vwpj,vwlb);
end;

Answer 1

You can't assign Nil or any value to Canvas, Canvas is a property for read only, so you need to remove the first line and then draw on the TImage canvas:
Image1.Canvas.Pen.Color := clRed;
Image1.Canvas.Brush.Color := clBlue;
Image1.canvas.rectangle(0,0,Image1.Height,Image1.Width);
Edit:
You have to set the image to default every time you draw on it's canvas:
Procedure:
Procedure TForm1.Default(Image: TImage);
begin
Image.Canvas.Pen.Color := clBtnFace;
Image.Canvas.Brush.Color := clBtnFace;
Image.Canvas.FillRect(Rect(0,0,Image.Height,Image.Width));
end;
Then call it as:
procedure TForm1.Button1Click(Sender: TObject);
begin
Default(Image1);
Image1.Canvas.Pen.Color := clRed;
Image1.Canvas.Brush.Color := clBlue;
Image1.canvas.rectangle(0,0,Image1.Height,Image1.Width);
end;

Answer 2

From what you have written and tried to explain in your many previous edits.
This is a possible solution to your problem.
Requirements:
Add four TEdit components to your form.
Add one TButton.
And one TImage.
Code:
var
Xorigin,Yorigin,vwpj,vwlb:integer;
....
begin
vwpj := strtoint(vwpjEdit.text);
vwlb := strtoint(vwlbEdit.text);
Xorigin := strtoint(XoriginEdit.Text);
Yorigin := strtoint(YoriginEdit.Text);
// You have to wipe the canvas with a base color,
image1.Canvas.Brush.Color := clwhite;
image1.Canvas.FillRect(rect(0,0,image1.Width,image1.height));
image1.Canvas.Pen.Color := clRed;
image1.Canvas.Brush.Color := clBlue;
image1.Canvas.rectangle(Xorigin,Yorigin,vwlb,vwpj);
end;
Explanation: I understand that you want to draw a rectangle on the Canvas property of a TImage. With the condition of each time you resize the rectangle you want to clear the Canvas (you implied this by assigning nil to canvas which is wrong considering that Canvas is a read only property).
Now the above code does this by filling the canvas with a base color (I chose clwhite) by using the Fillrect() method.
From this you need to understand that there is no such thing as clearing the image, either you delete it (using the free command as you say) and it will be gone and if you want to draw on it again you will need to create it.
the second option is that you fill it with a background color (the base clwhite I chose) or as a third option, resize the image as well.
All what maters is that as long as that image is still there the canvas and what you have drawn on it will remain.
Results of the code above