Drawing on a paintbox - How to keep up with mouse movements without delay?

Question

I decided to have a little go myself at making a map editor for a simple RPG game. The map will allow drawing tiles at 32x32 into the map, nothing too fancy, but to give an idea:

I am using Lazarus again but this applies to Delphi as well.

Now the problem I am facing is when drawing tiles, if the mouse is moved rather quickly then tiles are not been drawn and I think this is something to do with not been able to process the Mouse X,Y coordinates quick enough.

To give an idea, look at the image below:

What I did was starting from the left painted tiles to the right of the paintbox in a speedy manner, hence the gaps between. What I need is to be able to paint into any of those cells regardless as to how quick the mouse was moved.

Just a note, I am using a TTimer with Interval := 1. Inside the OnTimer method I store a record of which tiles should be drawn in which cell. The TPaintbox OnPaint method reads the records and draws the tiles accordingly.

I can post some code if required but I believe the solution could be something that is not related to my code as I notice this behaviour in simple paint programs when drawing brush strokes on a canvas.

Basically when moving the mouse too fast it seems the application does not seem to be able to keep up with the mouse movements and so parts that should be drawn are skipped. Moving the mouse at a slow/normal pace works perfectly, but if moving fast then it does not seem to keep up with it.

So, when drawing on a Canvas/Paintbox for example, how do I keep up with the mouse movements, especially when the mouse is moved very fast as it seems there is some kind of application/system delay?

I have added mostly the full source code below. This by no means represents final code or anything, I only just started this yesterday while messing around to see what I could do on my own so I am aware certain things could be done more efficiently, but that does not mean I would appreciate any tips or input you may have that I possibly am not be aware of.

main.pas

unit main;

{$mode objfpc}{$H+}

interface

uses
  Windows, Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs,
  ExtCtrls, ComCtrls, StdCtrls, ActnList;

type
  TMainForm = class(TForm)
    ActionList: TActionList;
    imgTileset: TImage;
    imgTilesetCursor: TImage;
    lblTiles: TLabel;
    lvwRecords: TListView;
    MapEditor: TPaintBox;
    MapViewer: TScrollBox;
    LeftSidePanel: TPanel;
    RightSidePanel: TPanel;
    ProjectManagerSplitter: TSplitter;
    StatusBar: TStatusBar;
    ProjectManagerTree: TTreeView;
    MouseTimer: TTimer;
    TilesetViewer: TScrollBox;
    ToolBar1: TToolBar;
    Image1: TImage;

    procedure FormCreate(Sender: TObject);

    procedure imgTilesetMouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
    procedure imgTilesetMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure imgTilesetMouseUp(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
    procedure MapEditorMouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
    procedure MapEditorMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
    procedure MapEditorMouseUp(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
    procedure MapEditorPaint(Sender: TObject);
    procedure MouseTimerTimer(Sender: TObject);
  private
    procedure DoDrawTile(X, Y: Integer);
    procedure FinishedDrawing;
  public
    { public declarations }
  end;

var
  MainForm: TMainForm;

implementation

uses
  generalutils,
  maputils,
  optionsdlg,
  systemutils;

{$R *.lfm}

{ ---------------------------------------------------------------------------- }

procedure TMainForm.DoDrawTile(X, Y: Integer);
begin
  if GetKeyPressed(VK_LBUTTON) then
  begin
    DeleteTileAtPosition(FMapTilePos.X, FMapTilePos.Y, lvwRecords);

    with lvwRecords.Items.Add do
    begin
      Caption := IntToStr(FMapTilePos.X);
      SubItems.Add(IntToStr(FMapTilePos.Y));
      SubItems.Add(IntToStr(FTilesetPos.X));
      SubItems.Add(IntToStr(FTilesetPos.Y));
    end;

    lblTiles.Caption := 'Tiles: ' + IntToStr(lvwRecords.Items.Count);
  end;
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.FinishedDrawing;
begin
  CleanObsoleteMapTiles(lvwRecords);
  lblTiles.Caption := 'Tiles: ' + IntToStr(lvwRecords.Items.Count);
  FIsDrawing := False;
  FIsDeleting := False;
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.FormCreate(Sender: TObject);
begin
  DoubleBuffered := True;
  TilesetViewer.DoubleBuffered := True;
  MapViewer.DoubleBuffered := True;
  MapEditor.Height := FMapHeight;
  MapEditor.Width := FMapWidth;
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.imgTilesetMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if GetKeyPressed(VK_LBUTTON) then
  begin
    PositionTilesetCursor(imgTileset, imgTilesetCursor, X, Y);
    ConvertToSnapPosition(X, Y, FSnapX, FSnapY, FTilesetPos);
  end;
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.imgTilesetMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if GetKeyPressed(VK_LBUTTON) then
  begin
    PositionTilesetCursor(imgTileset, imgTilesetCursor, X, Y);
    ConvertToSnapPosition(X, Y, FSnapX, FSnapY, FTilesetPos);
  end;
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.imgTilesetMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  ConvertToSnapPosition(X, Y, FSnapX, FSnapY, FTilesetPos);
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.MapEditorMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FIsDrawing := GetKeyPressed(VK_LBUTTON);
  FIsDeleting := GetKeyPressed(VK_RBUTTON);
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.MapEditorMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  FIsDrawing := GetKeyPressed(VK_LBUTTON);
  FIsDeleting := GetKeyPressed(VK_RBUTTON);
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.MapEditorMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FinishedDrawing();
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.MapEditorPaint(Sender: TObject);
var
  I, J: Integer;
  TileX, TileY: Integer;
  MapX, MapY: Integer;
begin
  // draw empty/water tiles << NEEDS OPTIMIZATION >>
  {for I := 0 to GetMapTilesColumnCount(FMapWidth) do
  begin
    for J := 0 to GetMapTilesRowCount(FMapHeight) do
    begin
      DrawTileOnMap(Image1, 0, 0, I * FTileWidth, J * FTileHeight, MapEditor.Canvas);
    end;
  end;}

  // draw tiles
  with lvwRecords do
  begin
    for I := 0 to Items.Count -1 do
    begin
      MapX := StrToInt(Items[I].Caption);
      MapY := StrToInt(Items[I].SubItems[0]);
      TileX := StrToInt(Items[I].SubItems[1]);
      TileY := StrToInt(Items[I].SubItems[2]);
      DrawTileOnMap(imgTileset, TileX, TileY, MapX, MapY, MapEditor.Canvas);
    end;
  end;

  PaintGrid(MapEditor.Canvas, FMapWidth, FMapHeight, 32, 1, $00543B1B);
end;

{ ---------------------------------------------------------------------------- }

procedure TMainForm.MouseTimerTimer(Sender: TObject);
var
  Ctrl: TControl;
  Pt: TPoint;
begin
  FMapTileColumn := -1;
  FMapTileRow := -1;
  StatusBar.Panels[2].Text := '';

  // check if the cursor is above the map editor...
  Ctrl := FindControlAtPosition(Mouse.CursorPos, True);
  if Ctrl <> nil then
  begin
    if (Ctrl = MapEditor) then
    begin
      Pt := Mouse.CursorPos;
      Pt := MapEditor.ScreenToClient(Pt);
      ConvertToSnapPosition(Pt.X, Pt.Y, FSnapX, FSnapY, FMapTilePos);

      // assign the tile column and row, then update in statusbar
      FMapTileColumn := MapTilePositionToColumn(FMapTilePos.X);
      FMapTileRow := MapTilePositionToRow(FMapTilePos.Y);

      // check if the mouse is inside the map editor...
      if (FMapTileColumn > -1) and (FMapTileRow > -1) then
      begin
        // check if drawing and draw tile
        if FIsDrawing then
        begin
          DoDrawTile(FMapTilePos.X, FMapTilePos.Y);
        end;

        // check if deleting and delete tile
        if FIsDeleting then
        begin
          DeleteTileAtPosition(FMapTilePos.X, FMapTilePos.Y, lvwRecords);
        end;
      end;
    end;
  end;
end;

{ ---------------------------------------------------------------------------- }

end.

maputils.pas

unit maputils;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, Controls, Graphics, ExtCtrls, ComCtrls;

procedure PaintGrid(MapCanvas: TCanvas; MapWidth, MapHeight: Integer;
  CellSize: Integer; LineWidth: Integer; GridColor: TColor);    
procedure ConvertToSnapPosition(X, Y: Integer; SnapX, SnapY: Integer;
  var APoint: TPoint);    
procedure PositionTilesetCursor(const Tileset, TilesetCursor: TImage;
  X, Y: Integer);
procedure PositionMapCursor(const Map, MapCursor: TControl; X, Y: Integer);
procedure DrawTileOnMap(const Tileset: TImage; TileX, TileY: Integer;
  MapX, MapY: Integer; OutCanvas: TCanvas);
function GetMapTilesColumnCount(MapWidth: Integer): Integer;
function GetMapTilesRowCount(MapHeight: Integer): Integer;
function MapTilePositionToColumn(MapX: Integer): Integer;
function MapTilePositionToRow(MapY: Integer): Integer;
function MapTileColumnIndexToPosition(ColumnIndex: Integer): Integer;
function MapTileRowIndexToPosition(RowIndex: Integer): Integer;
function IsTileAtPosition(MapX, MapY: Integer;
  const TileRecords: TListView): Boolean;
procedure DeleteTileAtPosition(MapX, MapY: Integer;
  const TileRecords: TListView);
procedure CleanObsoleteMapTiles(const TileRecords: TListView);

const
  FTileHeight = 32;         // height of each tile
  FTileWidth  = 32;         // width of each tile
  FSnapX      = 32;         // size of the X Snap
  FSnapY      = 32;         // size of the Y Snap

  FMapHeight  = 1280;       // height of the map
  FMapWidth   = 1280;       // width of the map

var
  FTilesetPos: TPoint;      // tile position in tileset
  FMapTilePos: TPoint;      // tile position in map
  FMapTileColumn: Integer;
  FMapTileRow: Integer;
  FIsDrawing: Boolean;      // flag to determine if drawing tile on map.
  FIsDeleting: Boolean;     // flag to determine if deleting tile from map.

implementation

{ ---------------------------------------------------------------------------- }

procedure PaintGrid(MapCanvas: TCanvas; MapWidth, MapHeight: Integer;
  CellSize: Integer; LineWidth: Integer; GridColor: TColor);
var
  ARect: TRect;
  X, Y: Integer;
begin
  ARect := Rect(0, 0, MapWidth, MapHeight);

  with MapCanvas do
  begin
    Pen.Mode  := pmCopy;
    Pen.Style := psSolid;
    Pen.Width := LineWidth;

    // horizontal lines
    Y := ARect.Top + CellSize;
    Pen.Color := GridColor;
    while Y <= ARect.Bottom do
    begin
      MoveTo(ARect.Left, Y -1);
      LineTo(ARect.Right, Y -1);
      Inc(Y, CellSize);
    end;

    // vertical lines
    X := ARect.Left + CellSize;
    Pen.Color := GridColor;
    while X <= ARect.Right do
    begin
      MoveTo(X -1, ARect.Top);
      LineTo(X -1, ARect.Bottom);
      Inc(X, CellSize);
    end;

    // draw left border
    MoveTo(LineWidth-1, LineWidth-1);
    LineTo(LineWidth-1, MapHeight);

    // draw top border
    MoveTo(LineWidth-1, LineWidth-1);
    LineTo(MapWidth, LineWidth-1);
  end;
end;

{ ---------------------------------------------------------------------------- }

procedure ConvertToSnapPosition(X, Y: Integer; SnapX, SnapY: Integer;
  var APoint: TPoint);
begin
  if (X > 0) then APoint.X := X div SnapX * SnapY;
  if (Y > 0) then APoint.Y := Y div SnapY * SnapX;
end;

{ ---------------------------------------------------------------------------- }

procedure PositionTilesetCursor(const Tileset, TilesetCursor: TImage;
  X, Y: Integer);
var
  Pt: TPoint;
begin
  ConvertToSnapPosition(X, Y, FSnapX, FSnapY, Pt);
  if (X > 0) and (X < Tileset.Width) then TilesetCursor.Left := Pt.X;
  if (Y > 0) and (Y < Tileset.Height) then TilesetCursor.Top := Pt.Y;
end;

{ ---------------------------------------------------------------------------- }

procedure PositionMapCursor(const Map, MapCursor: TControl; X, Y: Integer);
var
  Pt: TPoint;
begin
  ConvertToSnapPosition(X, Y, FSnapX, FSnapY, Pt);
  if (X > 0) and (X < Map.Width) then MapCursor.Left := Pt.X;
  if (Y > 0) and (Y < Map.Height) then MapCursor.Top := Pt.Y;
end;

{ ---------------------------------------------------------------------------- }

procedure DrawTileOnMap(const Tileset: TImage; TileX, TileY: Integer;
  MapX, MapY: Integer; OutCanvas: TCanvas);
var
  Bitmap: TBitmap;
begin
  Bitmap := TBitmap.Create;
  try
    Bitmap.PixelFormat := pf24Bit;
    Bitmap.SetSize(FTileWidth, FTileHeight);
    Bitmap.Canvas.CopyRect(
      Rect(0, 0, FTileWidth, FTileHeight),
      Tileset.Canvas,
      Rect(TileX, TileY, TileX + FTileWidth, TileY + FTileHeight));
    OutCanvas.Draw(MapX, MapY, Bitmap);
  finally
    Bitmap.Free;
  end;
end;

{ ---------------------------------------------------------------------------- }

function GetMapTilesColumnCount(MapWidth: Integer): Integer;
var
  LCount: Integer;
begin
  LCount := 0;
  Result := 0;

  repeat
    Inc(LCount, FTileWidth);
  until
    LCount = MapWidth;

  Result := LCount div FTileWidth;
end;

{ ---------------------------------------------------------------------------- }

function GetMapTilesRowCount(MapHeight: Integer): Integer;
var
  LCount: Integer;
begin
  LCount := 0;
  Result := 0;

  repeat
    Inc(LCount, FTileHeight);
  until
    LCount = MapHeight;

  Result := LCount div FTileHeight;
end;

{ ---------------------------------------------------------------------------- }

function MapTilePositionToColumn(MapX: Integer): Integer;
begin
  Result := MapX div FTileWidth;
end;

{ ---------------------------------------------------------------------------- }

function MapTilePositionToRow(MapY: Integer): Integer;
begin
  Result := MapY div FTileHeight;
end;

{ ---------------------------------------------------------------------------- }

function MapTileColumnIndexToPosition(ColumnIndex: Integer): Integer;
begin
  Result := ColumnIndex * FTileWidth;
end;

{ ---------------------------------------------------------------------------- }

function MapTileRowIndexToPosition(RowIndex: Integer): Integer;
begin
  Result := RowIndex * FTileHeight;
end;

{ ---------------------------------------------------------------------------- }

function IsTileAtPosition(MapX, MapY: Integer;
  const TileRecords: TListView): Boolean;
var
  I: Integer;
  LMapX, LMapY: Integer;
begin
  Result := False;

  with TileRecords do
  begin
    for I := 0 to Items.Count -1 do
    begin
      LMapX := StrToInt(Items[I].Caption);
      LMapY := StrToInt(Items[I].SubItems[0]);
      if (MapX = LMapX) and (MapY = LMapY) then
      begin
        Result := True;
        Break;
      end;
    end;
  end;
end;

{ ---------------------------------------------------------------------------- }

procedure DeleteTileAtPosition(MapX, MapY: Integer;
  const TileRecords: TListView);
var
  I: Integer;
  LMapX, LMapY: Integer;
begin
  if IsTileAtPosition(MapX, MapY, TileRecords) then
  begin
    with TileRecords do
    begin
      for I := Items.Count -1 downto 0 do
      begin
        LMapX := StrToInt(Items[I].Caption);
        LMapY := StrToInt(Items[I].SubItems[0]);

        if (MapX = LMapX) and (MapY = LMapY) then
        begin
          Items.Delete(I);
        end;
      end;
    end;
  end;
end;

{ ---------------------------------------------------------------------------- }

procedure CleanObsoleteMapTiles(const TileRecords: TListView);
var
  I, J: Integer;
begin
  with TileRecords do
  begin
    Items.BeginUpdate;
    try
      SortType := stText;

      for I := Items.Count -1 downto 0 do
      begin
        for J := Items.Count -1 downto I + 1 do
        begin
          if  SameText(Items[I].Caption, Items[J].Caption) and
              SameText(Items[I].SubItems[0], Items[J].SubItems[0]) and
              SameText(Items[I].SubItems[1], Items[J].SubItems[1]) and
              SameText(Items[I].SubItems[2], Items[J].SubItems[2]) then
          begin
            Items.Delete(J);
          end;
        end;
      end;
      TileRecords.SortType := stNone;
    finally
      TileRecords.Items.EndUpdate;
    end;
  end;
end;

{ ---------------------------------------------------------------------------- }

end.

A few notes:

• when dealing with the X,Y coordinates assume that we snap to a grid of 32x32, for example: if X=3 then the cell is 96 etc.
• MapEditor is the name of the paintbox.
• lvwRecords is just a quick and dirty way of storing the tile positions in a TListView, later I will use proper classes to store the data.

Using the listview to store the tile positions looks like this (as I say this was just for quick testing until I use proper classes or array records):

Thank you.

Answer 1

Don't use a TTimer to control your drawing. When the mouse moves around the PaintBox, set your flags as needed, and also keep track of the current mouse coordinates, and then call the PaintBox's Invalidate() method to trigger a repaint when flow control returns back to the message queue. Whenever the PaintBox's OnPaint event is triggered for any reason, draw your map and tiles as needed, and if a tile is being dragged around then draw it at the saved mouse coordinates.

Also, in your DrawTileOnMap() method, you don't need to copy the image to a temp TBitmap, you can copy from your source TImage directly to your target TCanvas.

Try something more like this:

const
  FTileHeight = 32;         // height of each tile
  FTileWidth  = 32;         // width of each tile
  FSnapX      = 32;         // size of the X Snap
  FSnapY      = 32;         // size of the Y Snap

  FMapHeight  = 1280;       // height of the map 
  FMapWidth   = 1280;       // width of the map 

var
  FTilesetPos: TPoint;      // tile position in tileset
  FMapTilePos: TPoint;      // tile position in map
  FMapTileColumn: Integer;
  FMapTileRow: Integer;
  FIsDrawing: Boolean;      // flag to determine if drawing tile on map.

procedure DrawTileOnMap(const Tileset: TImage; TileX, TileY: Integer;
  MapX, MapY: Integer; OutCanvas: TCanvas);
begin
  OutCanvas.CopyRect(
    Rect(MapX, MapY, MapX + FTileWidth, MapY + FTileHeight),
    Tileset.Canvas,
    Rect(TileX, TileY, TileX + FTileWidth, TileY + FTileHeight));
end; 

procedure TMainForm.FormCreate(Sender: TObject);
begin
  FTilesetPos := Point(-1, -1);
  FMapTilePos := Point(-1, -1);
  FMapTileColumn = -1;
  FMapTileRow := -1;
  FIsDrawing := False;
end;

procedure TMainForm.MapEditorMouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  if Button = mbMiddle then Exit;

  if Button = mbLeft then
    FIsDrawing := True
  end else
    DeleteTileAtPosition(FMapTilePos.X, FMapTilePos.Y, lvwRecords);

  MapEditor.Invalidate;
end;

procedure TMainForm.MapEditorMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
begin
  ConvertToSnapPosition(X, Y, FSnapX, FSnapY, FMapTilePos);

  FMapTileColumn := MapTilePositionToColumn(FMapTilePos.X);
  FMapTileRow := MapTilePositionToRow(FMapTilePos.Y);

  if (Button = mbLeft) and FDrawing then
    MapEditor.Invalidate;
end;    

procedure TMainForm.MapEditorMouseUp(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
  if Button = mbLeft then
  begin
    FIsDrawing := False
    MapEditor.Invalidate;
  end;
end;

procedure TMainForm.MapEditorPaint(Sender: TObject);
var
  I, J: Integer;
  TileX, TileY: Integer;
  MapX, MapY: Integer;
begin
  // draw empty/water tiles << NEEDS OPTIMIZATION AS VERY SLOW >>
  {for I := 0 to GetMapTilesColumnCount(FMapWidth) do
  begin
    for J := 0 to GetMapTilesRowCount(FMapHeight) do
    begin
      DrawTileOnMap(Image1, 0, 0, I * FTileWidth, J * FTileHeight, MapEditor.Canvas);
    end;
  end;}

  // draw tiles
  with lvwRecords do
  begin
    for I := 0 to Items.Count -1 do
    begin
      MapX := StrToInt(Items[I].Caption);
      MapY := StrToInt(Items[I].SubItems[0]);
      TileX := StrToInt(Items[I].SubItems[1]);
      TileY := StrToInt(Items[I].SubItems[2]);
      DrawTileOnMap(imgTileset, TileX, TileY, MapX, MapY, MapEditor.Canvas);
    end;
  end;

  PaintGrid(MapEditor.Canvas, FMapWidth, FMapHeight, 32, 1, $00543B1B); 

  if (FMapTileColumn > -1) and (FMapTileRow > -1) and FDrawing then
    DoDrawTile(FMapTilePos.X, FMapTilePos.Y);
end; 

Answer 2

Your approach is wrong. The delay you suffer is mainly because the timer.

Here are some guidances:

1. Draw on a single (offscreen) bitmap.
2. On the PaintBox OnPaint event just copy the offscreen bitmap.
3. Draw on MouseMove, MouseUp directly (if needed)
4. Preload your tiles as different bitmaps or combine them into a bigger one.
5. Never create bitmaps in a time when you should draw.

Answer 3

You should collect the mouse coordinates inside of the OnMouseMove event, otherwise you'll only get a new mouse position when the timer fires.

In addition to that, use GetMouseMovePointsEx in order to get up to 64 mouse positions you missed.