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Date Posted: 09:29:47 03/18/00 Sat
Author: Michel
Subject: Flickering on NT


Hi All,

On Windows NT, when scrolling an image in the ImageScrollBox the refresh mayvery slow with a "dragging effect". I've tested without the Envision library, by dropping a TImage on a TScrollBox, and loading the TImage with a TBitmap. When
scrolling, there is flickering.

I have traced it down to TWinControl.ScrollBy which uses the ScrollWindow API call. The API docs recommend calling UpdateWindow which the VCL does not do.

Here are some changes to be applied to EnImgScr.Pas which will reduce greatly this "dragging" effect. Although some flicking still occurs in the TScrollBox, it should be
considerably better with these changes.

1. In the TImageScrollBox protected section, add:

{ MB Jan 30, 2000. Override to call UpdateWindow which reduces flickering
on Windows NT }
procedure WMHScroll(var Message: TWMHScroll); message WM_HSCROLL;

procedure WMVScroll(var Message: TWMVScroll); message WM_VSCROLL;

2. Add in the implementation section of the unit:

procedure TImageScrollBox.WMHScroll(var Message: TWMHScroll);
begin
inherited;
UpdateWindow(Self.Handle);
end;

procedure TImageScrollBox.WMVScroll(var Message: TWMVScroll);
begin
inherited;
UpdateWindow(Self.Handle);
end;

3. Change the TDragMouseHandler.MouseMove method implementation to:

procedure TDragMouseHandler.MouseMove(
const Shift : TShiftState;
const X, Y : Integer );
begin
if FMouseIsDown then
begin
ImageScrollBox.HorzScrollBar.Position := ImageScrollBox.HorzScrollBar.Position + FStartMousePos.X - X;
ImageScrollBox.VertScrollBar.Position := ImageScrollBox.VertScrollBar.Position + FStartMousePos.Y - Y;
UpdateWindow(ImageScrollBox.Handle);
end;
end;

Best regards,

Michel

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