How to get the update region in a TGraphicControl #196

Answer 1

I had similar problems once and did an extra check on Canvas.ClipRect in the Paint procedure. When updating the component when only parts of the component needed to be redrawn, I used:

{ ... }
var
  R: TRect;
begin
  if FValue <> NewValue then
  begin
    FValue := NewValue;
    R := GetClientRect;
    InflateRect(R, - 1, - 1);
    InvalidateRect(Handle, @R, not (csOpaque in ControlStyle));
  end;
end;

Now I could check for this rectangle in the Paint procedure. When the ClipRect was not the same as I expected it to be (eg caused by a form being moved which covered it partly or completely before the move) a full redraw was needed and performed.

{ ... }
with Canvas do
  if (((ClipRect.Right - ClipRect.Left) <> (Width - 2)) and
      ((ClipRect.Bottom - ClipRect.Top) <> (Height - 2)))  then
    DoDrawAll := True
  else
    {only update what needs to be updated.}
{ ... }

When a full redraw was not required the rest of the paint routine knew what to update by other means and only updated these parts.

The component in which I used this was a descendent of TCustomPanel. I'm not sure if ClipRect is setup the same for a TGraphicControl but I guess so.

Tip by Pieter Zijlstra

Answer 2

Thank you very much Pieter, it works! This is how I implemented it for the time being: In WM_WINDOWPOSCHANGED I assign a private field fTestRect like you do, but with

fTestRect := BoundsRect;
InflateRect(fTestRect,- 1,- 1);

Then I can override TControl.Repaint, it's natural for a user to call repaint on an animation step:

{ ... }
  if fUseRegions then
    if assigned(parent) then
      InvalidateRect(parent.handle, @fTestRect, False)
    else
      inherited
      else
        inherited;
{ ... }

fUseRegions is the flag the user can set to use those update regions for speedup. Then in Paint, I do as you do, and only really use the update regions if Canvas.ClipRect is the analogue of fTestRect. I can't see a slowdown. Of course, chances are that a window pops up on the control which has exactly the dimensions of fTestRect, but I think that chance is rather slim.

Tip by Renate Schaaf