How to create a slowing scroll effect on a scrollbox?

Question

I like to create a smooth slowing scroll effect after panning an image in a scrollbox. Just like panning the map in maps.google.com. I'm not sure what type it is, but exactly same behaviour: when dragging the map around with a fast move, it doesn't stop immediately when you release the mouse, but it starts slowing down.

Any ideas, components, links or samples?

Answer 1

The idea:

As per your comment, it should feel like Google Maps and thus while dragging the image, the image should stick to the mouse pointer; no special effects required so far. But at releasing the mouse button, the image needs to move (the scroll box needs to pan) further in the same direction and with a gradually easing speed, starting with the dragging velocity at the moment the mouse button was released.

So we need:

• a drag handler for when the mouse is pressed: OnMouseMove will work,
• the panning speed at the moment the mouse is released: during the drag operation, we will track the latest speed with a timer,
• something that still moves the image after the mouse release: we use the same timer,
• a way to update the GUI: updating the image position, scrolling the scroll box and updating the scroll bar positions. Luckily, setting the position of the scroll bars of the scroll box will do all that,
• a function to gradually decrease the speed after mouse release. I chose for a simple linear factor, but you can experiment with that.

Setup:

• Drop a TScrollBox on your form, create event handlers for OnMouseDown, OnMouseMove and OnMouseUp and set the DoubleBuffered property to True (this needs to be done runtime),
• Drop a TTimer on your form, set its interval to 15 milliseconds (~ 67 Hz refresh rate) and create an event handler for OnTimer,
• Drop a TImage on the scroll box, load a picture, set the size to something big (e.g. 3200 x 3200), set Stretch to True and set Enabled to False to let the mouse events through to the scroll box.

Code (for scroll box):

unit Unit1;

interface

uses
  Windows, SysUtils, Classes, Controls, Forms, JPEG, ExtCtrls, StdCtrls;

type
  TForm1 = class(TForm)
    ScrollBox: TScrollBox;
    Image: TImage;
    TrackingTimer: TTimer;
    procedure ScrollBoxMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure ScrollBoxMouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure ScrollBoxMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure TrackingTimerTimer(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    FDragging: Boolean;
    FPrevScrollPos: TPoint;
    FPrevTick: Cardinal;
    FSpeedX: Single;
    FSpeedY: Single;
    FStartPos: TPoint;
    function GetScrollPos: TPoint;
    procedure SetScrollPos(const Value: TPoint);
  public
    property ScrollPos: TPoint read GetScrollPos write SetScrollPos;
  end;

implementation

{$R *.dfm}

procedure TForm1.FormCreate(Sender: TObject);
begin
  ScrollBox.DoubleBuffered := True;
end;

function TForm1.GetScrollPos: TPoint;
begin
  with ScrollBox do
    Result := Point(HorzScrollBar.Position, VertScrollBar.Position);
end;

procedure TForm1.ScrollBoxMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FDragging := True;
  FPrevTick := GetTickCount;
  FPrevScrollPos := ScrollPos;
  TrackingTimer.Enabled := True;
  FStartPos := Point(ScrollPos.X + X, ScrollPos.Y + Y);
  Screen.Cursor := crHandPoint;
end;

procedure TForm1.ScrollBoxMouseMove(Sender: TObject; Shift: TShiftState;
  X, Y: Integer);
begin
  if FDragging then
    ScrollPos := Point(FStartPos.X - X, FStartPos.Y - Y);
end;

procedure TForm1.ScrollBoxMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FDragging := False;
  Screen.Cursor := crDefault;
end;

procedure TForm1.SetScrollPos(const Value: TPoint);
begin
  ScrollBox.HorzScrollBar.Position := Value.X;
  ScrollBox.VertScrollBar.Position := Value.Y;
end;

procedure TForm1.TrackingTimerTimer(Sender: TObject);
var
  Delay: Cardinal;
begin
  Delay := GetTickCount - FPrevTick;
  if FDragging then
  begin
    if Delay = 0 then
      Delay := 1;
    FSpeedX := (ScrollPos.X - FPrevScrollPos.X) / Delay;
    FSpeedY := (ScrollPos.Y - FPrevScrollPos.Y) / Delay;
  end
  else
  begin
    if (Abs(FSpeedX) < 0.005) and (Abs(FSpeedY) < 0.005) then
      TrackingTimer.Enabled := False
    else
    begin
      ScrollPos := Point(FPrevScrollPos.X + Round(Delay * FSpeedX),
        FPrevScrollPos.Y + Round(Delay * FSpeedY));
      FSpeedX := 0.83 * FSpeedX;
      FSpeedY := 0.83 * FSpeedY;
    end;
  end;
  FPrevScrollPos := ScrollPos;
  FPrevTick := GetTickCount;
end;

end.

Code (for panel):

And in case you do not want the scroll bars then use the following code. The example uses a panel as container, but that could be any windowed control or the form itself.

unit Unit2;

interface

uses
  Windows, SysUtils, Classes, Controls, Forms, JPEG, ExtCtrls, Math;

type
  TForm2 = class(TForm)
    Panel: TPanel;
    Image: TImage;
    TrackingTimer: TTimer;
    procedure PanelMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure PanelMouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure PanelMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure TrackingTimerTimer(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    FDragging: Boolean;
    FPrevImagePos: TPoint;
    FPrevTick: Cardinal;
    FSpeedX: Single;
    FSpeedY: Single;
    FStartPos: TPoint;
    function GetImagePos: TPoint;
    procedure SetImagePos(Value: TPoint);
  public
    property ImagePos: TPoint read GetImagePos write SetImagePos;
  end;

implementation

{$R *.dfm}

procedure TForm2.FormCreate(Sender: TObject);
begin
  Panel.DoubleBuffered := True;
end;

function TForm2.GetImagePos: TPoint;
begin
  Result.X := Image.Left;
  Result.Y := Image.Top;
end;

procedure TForm2.PanelMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FDragging := True;
  FPrevTick := GetTickCount;
  FPrevImagePos := ImagePos;
  TrackingTimer.Enabled := True;
  FStartPos := Point(X - Image.Left, Y - Image.Top);
  Screen.Cursor := crHandPoint;
end;

procedure TForm2.PanelMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if FDragging then
    ImagePos := Point(X - FStartPos.X, Y - FStartPos.Y);
end;

procedure TForm2.PanelMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FDragging := False;
  Screen.Cursor := crDefault;
end;

procedure TForm2.SetImagePos(Value: TPoint);
begin
  Value.X := Max(Panel.ClientWidth - Image.Width, Min(0, Value.X));
  Value.Y := Max(Panel.ClientHeight - Image.Height, Min(0, Value.Y));
  Image.SetBounds(Value.X, Value.Y, Image.Width, Image.Height);
end;

procedure TForm2.TrackingTimerTimer(Sender: TObject);
var
  Delay: Cardinal;
begin
  Delay := GetTickCount - FPrevTick;
  if FDragging then
  begin
    if Delay = 0 then
      Delay := 1;
    FSpeedX := (ImagePos.X - FPrevImagePos.X) / Delay;
    FSpeedY := (ImagePos.Y - FPrevImagePos.Y) / Delay;
  end
  else
  begin
    if (Abs(FSpeedX) < 0.005) and (Abs(FSpeedY) < 0.005) then
      TrackingTimer.Enabled := False
    else
    begin
      ImagePos := Point(FPrevImagePos.X + Round(Delay * FSpeedX),
        FPrevImagePos.Y + Round(Delay * FSpeedY));
      FSpeedX := 0.83 * FSpeedX;
      FSpeedY := 0.83 * FSpeedY;
    end;
  end;
  FPrevImagePos := ImagePos;
  FPrevTick := GetTickCount;
end;

end.

Code (for paint box):

And when the image's dimensions are limitless (e.g. a globe), you can use a paint box to glue the image's ends together.

unit Unit3;

interface

uses
  Windows, SysUtils, Classes, Graphics, Controls, Forms, ExtCtrls, JPEG;

type
  TForm3 = class(TForm)
    Painter: TPaintBox;
    Tracker: TTimer;
    procedure FormCreate(Sender: TObject);
    procedure FormDestroy(Sender: TObject);
    procedure PainterMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure PainterMouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure PainterMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure PainterPaint(Sender: TObject);
    procedure TrackerTimer(Sender: TObject);
  private
    FDragging: Boolean;
    FGraphic: TGraphic;
    FOffset: Integer;
    FPrevOffset: Integer;
    FPrevTick: Cardinal;
    FSpeed: Single;
    FStart: Integer;
    procedure SetOffset(Value: Integer);
  public
    property Offset: Integer read FOffset write SetOffset;
  end;

implementation

{$R *.dfm}

procedure TForm3.FormCreate(Sender: TObject);
begin
  DoubleBuffered := True;
  FGraphic := TJPEGImage.Create;
  FGraphic.LoadFromFile('gda_world_map_small.jpg');
  Constraints.MaxWidth := FGraphic.Width + 30;
end;

procedure TForm3.FormDestroy(Sender: TObject);
begin
  FGraphic.Free;
end;

procedure TForm3.PainterMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FDragging := True;
  FPrevTick := GetTickCount;
  FPrevOffset := Offset;
  Tracker.Enabled := True;
  FStart := X - FOffset;
  Screen.Cursor := crHandPoint;
end;

procedure TForm3.PainterMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if FDragging then
    Offset := X - FStart;
end;

procedure TForm3.PainterMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FDragging := False;
  Screen.Cursor := crDefault;
end;

procedure TForm3.PainterPaint(Sender: TObject);
begin
  Painter.Canvas.Draw(FOffset, 0, FGraphic);
  Painter.Canvas.Draw(FOffset + FGraphic.Width, 0, FGraphic);
end;

procedure TForm3.SetOffset(Value: Integer);
begin
  FOffset := Value;
  if FOffset < -FGraphic.Width then
  begin
    Inc(FOffset, FGraphic.Width);
    Dec(FStart, FGraphic.Width);
  end
  else if FOffset > 0 then
  begin
    Dec(FOffset, FGraphic.Width);
    Inc(FStart, FGraphic.Width);
  end;
  Painter.Invalidate;
end;

procedure TForm3.TrackerTimer(Sender: TObject);
var
  Delay: Cardinal;
begin
  Delay := GetTickCount - FPrevTick;
  if FDragging then
  begin
    if Delay = 0 then
      Delay := 1;
    FSpeed := (Offset - FPrevOffset) / Delay;
  end
  else
  begin
    if Abs(FSpeed) < 0.005 then
      Tracker.Enabled := False
    else
    begin
      Offset := FPrevOffset + Round(Delay * FSpeed);
      FSpeed := 0.83 * FSpeed;
    end;
  end;
  FPrevOffset := Offset;
  FPrevTick := GetTickCount;
end;

end.