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Area fill algorithm: crayons and coloring book

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Kids know how to use crayons and a coloring book. How do you write such a program?

In my last post (Which pixels do you turn on when you draw a line?) I showed how to draw a line. Now suppose you have some lines or shapes already drawn. How would you write code to fill in an area bounded by the drawn pixels?

IOW, imagine you’ve drawn a circle. You want to fill the circle with a color. Right click inside. What code should run? What if the figure were more complex, like a large block “W” or a curled up snake.

More formally: Given an array of pixels that represents the outline of a shape, and a point’s x,y coordinate within that shape, how would you area fill that shape?

Perhaps you could write some code that will check all pixels to the East, North, West, South until it reached a pixel that was already painted. Then what?

What sort of algorithm and data structure would you use?

Below are C# and VB versions of a simple implementation that isn’t very efficient, but is quite simple.

The code is identical to the last post (the drawing parts) except for the areas delimited by “AREA”. This makes it easier for you to cut/paste the code.

Start Visual Studio 2008

Choose File->New->Project->C# or VB->Windows Forms Application.

Choose View->Code

Paste in the VB or C# version of the code below, hit F5 to run it. Draw a shape, right click to fill.

How would you improve it? Why isn’t it efficient?

Try making the form bigger and paint more pixels. What happens?

 

Clue: look at the source code for MineSweeper that I wrote to take advantage of the (then) new feature Collections. It’s part of the Task Pane for Visual Foxpro 9.0

See also:

Remove double spaces from pasted code samples in blog

 

<C# Sample>

#define AREA

using System;

using System.Collections.Generic;

using System.ComponentModel;

using System.Data;

using System.Drawing;

using System.Linq;

using System.Text;

using System.Windows.Forms;

namespace WindowsFormsApplication1

{

    public partial class Form1 : System.Windows.Forms.Form

    {

        Size m_numCells = new Size(350, 200);// we'll use an array of Cells

        Boolean[,] m_cells; // the array of cells: whether they've been drawn or not

        Size m_cellSize = new Size(8, 8); // cell height & width

        Size m_Offset = new Size(0, 0);

        bool m_MouseDown = false;

        Button btnErase;

        Point? m_PtOld;

        SolidBrush m_brushMouse = new SolidBrush(Color.Red);

        SolidBrush m_brushGenerated = new SolidBrush(Color.Black);

        delegate bool DrawCellDelegate(Point ptcell, Brush br);

        Graphics m_oGraphics;

        public Form1()

        {

         this.Load += new EventHandler(this.Loaded);

        }

        void Loaded(Object o, EventArgs e)

        {

            this.Width = 600;

            this.Height = 400;

            this.btnErase = new Button();

            this.btnErase.Text = "&Erase";

            this.btnErase.Click += new EventHandler(this.btnErase_Click);

            this.Controls.Add(this.btnErase);

            this.BackColor = Color.White;

            btnErase_Click(null, null);

        }

        void btnErase_Click(object sender, System.EventArgs e)

        {

            m_oGraphics = Graphics.FromHwnd(this.Handle);

            m_numCells.Width = this.Width / m_cellSize.Width;

            m_numCells.Height = this.Height / m_cellSize.Height;

            m_cells = new Boolean[m_numCells.Width, m_numCells.Height];

            m_oGraphics.FillRectangle(Brushes.White, new Rectangle(0, 0, this.Width, this.Height));

        }

        Point PointToCell(Point p1)

        {

            Point ptcell = new Point(

            (p1.X - m_Offset.Width) / m_cellSize.Width,

            (p1.Y - m_Offset.Height) / m_cellSize.Height);

            return ptcell;

        }

        protected override void OnMouseDown(MouseEventArgs e)

        {

            if (e.Button == MouseButtons.Left)

            {

                m_MouseDown = true;

                m_PtOld = new Point(e.X, e.Y);

                CheckMouseDown(e);

            }

#if AREA

            else

            {

                AreaFill(PointToCell(new Point(e.X, e.Y)));

            }

#endif

        }

        protected override void OnMouseMove(MouseEventArgs e)

        {

            if (m_MouseDown)

            {

                CheckMouseDown(e);

            }

        }

        protected override void OnMouseUp(MouseEventArgs e)

        {

            m_MouseDown = false;

        }

        void CheckMouseDown(MouseEventArgs e)

        {

            Point ptMouse = new Point(e.X, e.Y);

            Point ptcell = PointToCell(ptMouse);

            if (ptcell.X >= 0 && ptcell.X < m_numCells.Width &&

                ptcell.Y >= 0 && ptcell.Y < m_numCells.Height)

            {

                DrawLineOfCells(PointToCell(m_PtOld.Value), ptcell, new DrawCellDelegate(DrawACell));

                m_PtOld = ptMouse;

            }

        }

        bool DrawACell(Point ptcell, Brush br)

        {

            bool fDidDraw = false;

            if (!m_cells[ptcell.X, ptcell.Y]) // if not drawn already

            {

                m_cells[ptcell.X, ptcell.Y] = true;

                //*

                m_oGraphics.FillRectangle(br,

                    m_Offset.Width + ptcell.X * m_cellSize.Width,

                    m_Offset.Height + ptcell.Y * m_cellSize.Height,

                    m_cellSize.Width,

                    m_cellSize.Height);

                /*/

                 g.DrawRectangle(new Pen(Color.Blue,1),

                    m_Offset.Width + ptcell.X * m_cellSize.Width,

                    m_Offset.Height + ptcell.Y * m_cellSize.Height,

      m_cellSize.Width,

                    m_cellSize.Height);

                

                  //*/

                fDidDraw = true;

            }

            return fDidDraw;

        }

        void DrawLineOfCells(Point p1, Point p2, DrawCellDelegate drawit)

        {

            // https://en.wikipedia.org/wiki/Bresenham%27s\_line\_algorithm

            Brush br = m_brushMouse;

            int x0 = p1.X;

            int y0 = p1.Y;

            int x1 = p2.X;

            int y1 = p2.Y;

          int x, cx, deltax, xstep,

                y, cy, deltay, ystep,

                 error;

            bool st;

            // find largest delta for pixel steps

            st = (Math.Abs(y1 - y0) > Math.Abs(x1 - x0));

            // if deltay > deltax then swap x,y

            if (st)

            {

                x0 ^= y0; y0 ^= x0; x0 ^= y0; // swap(x0, y0);

                x1 ^= y1; y1 ^= x1; x1 ^= y1; // swap(x1, y1);

            }

            deltax = Math.Abs(x1 - x0);

            deltay = Math.Abs(y1 - y0);

            error = (deltax / 2);

            y = y0;

            if (x0 > x1) { xstep = -1; }

            else { xstep = 1; }

            if (y0 > y1) { ystep = -1; }

            else { ystep = 1; }

            for (x = x0; (x != (x1 + xstep)); x += xstep)

            {

                cx = x; cy = y; // copy of x, copy of y

                // if x,y swapped above, swap them back now

                if (st)

                {

                    cx ^= cy; cy ^= cx; cx ^= cy;

                }

                if (drawit(new Point(cx, cy), br))

                {

                    br = m_brushGenerated;

                }

                error -= deltay; // converge toward end of line

                if (error < 0)

                { // not done yet

                    y += ystep;

                    error += deltax;

                }

            }

        }

#if AREA

        SolidBrush m_brushFill = new SolidBrush(Color.Blue);

        Color m_oColor = Color.Black;

  /*

        /*/

                void AreaFill(Point ptcell)

                {

                    if (ptcell.X >= 0 && ptcell.X < m_numCells.Width)

                    {

                        if (ptcell.Y >= 0 && ptcell.Y < m_numCells.Height)

                        {

        // System.Threading.Thread.Sleep(100);

                            if (DrawACell(ptcell, m_brushFill))

                            {

                                m_oColor = Color.FromArgb((int)(((((uint)m_oColor.ToArgb() & 0xffffff) + 140) & 0xffffff) | 0xff000000));

                                m_brushFill = new SolidBrush(m_oColor);

                                AreaFill(new Point(ptcell.X - 1, ptcell.Y));

                                AreaFill(new Point(ptcell.X + 1, ptcell.Y));

                                AreaFill(new Point(ptcell.X, ptcell.Y + 1));

                                AreaFill(new Point(ptcell.X, ptcell.Y - 1));

                         }

                        }

                    }

                }

        //*/

#endif

    }

}

</C# Sample>

 

<VB Sample>

#Const AREA = True

Public Class Form1

    Dim m_numCells = New Size(350, 300) ' we'll use an array of cells

    Dim m_cells(,) As Boolean ' the array of cells: whether they've been drawn or not

    Dim m_cellSize = New Size(8, 8) ' cell size & width

    Dim m_Offset = New Size(0, 0)

    Dim m_MouseDown = False

    Dim WithEvents btnErase As Button

    Dim m_PtOld As Point?

    Dim m_brushGenerated = New SolidBrush(Color.Black)

    Dim m_brushMouse = New SolidBrush(Color.Red)

    Dim m_oGraphics As Graphics

    Delegate Function DrawCellDelegate(ByVal ptCell As Point, ByVal br As Brush) As Boolean

    Sub Form_Load() Handles Me.Load

        Me.Width = 600

        Me.Height = 400

        Me.btnErase = New Button()

        Me.btnErase.Text = "&Erase"

        Me.Controls.Add(Me.btnErase)

        Me.BackColor = Color.White

        btnErase_Click()

    End Sub

    Sub btnErase_Click() Handles btnErase.Click

        m_oGraphics = Graphics.FromHwnd(Me.Handle)

        m_numCells.Width = Me.Width / m_cellSize.Width

        m_numCells.Height = Me.Height / m_cellSize.Height

        ReDim m_cells(m_numCells.Width, m_numCells.Height)

        m_oGraphics.FillRectangle(Brushes.White, New Rectangle(0, 0, Me.Width, Me.Height))

    End Sub

    Function PointToCell(ByVal p1 As Point) As Point

        Dim ptcell = New Point( _

            (p1.X - m_Offset.Width) / m_cellSize.Width, _

       (p1.Y - m_Offset.Height) / m_cellSize.Height)

        Return ptcell

    End Function

    Protected Overrides Sub OnMouseDown(ByVal e As System.Windows.Forms.MouseEventArgs)

        If e.Button = Windows.Forms.MouseButtons.Left Then

            m_MouseDown = True

            m_PtOld = New Point(e.X, e.Y)

            CheckMouseDown(e)

#If AREA Then

        Else

            AreaFill(PointToCell(New Point(e.X, e.Y)))

#End If

        End If

    End Sub

    Protected Overrides Sub OnMouseMove(ByVal e As System.Windows.Forms.MouseEventArgs)

        If m_MouseDown Then

            CheckMouseDown(e)

        End If

    End Sub

    Protected Overrides Sub OnMouseUp(ByVal e As System.Windows.Forms.MouseEventArgs)

        m_MouseDown = False

    End Sub

    Sub CheckMouseDown(ByVal e As MouseEventArgs)

        Dim ptMouse = New Point(e.X, e.Y)

        Dim ptcell = PointToCell(ptMouse)

        If (ptcell.X >= 0 And ptcell.X < m_numCells.Width And _

            ptcell.Y >= 0 And ptcell.Y < m_numCells.Height) Then

            DrawLineOfCells(PointToCell(m_PtOld.Value), ptcell, New DrawCellDelegate(AddressOf DrawACell))

            m_PtOld = ptMouse

        End If

    End Sub

    Function DrawACell(ByVal ptCell As Point, ByVal br As Brush) As Boolean

        Dim fDidDraw = False

        If Not m_cells(ptCell.X, ptCell.Y) Then

            m_cells(ptCell.X, ptCell.Y) = True

            m_oGraphics.FillRectangle(br, _

                    m_Offset.Width + ptCell.X * m_cellSize.Width, _

                    m_Offset.Height + ptCell.Y * m_cellSize.Height, _

                    m_cellSize.Width, _

                    m_cellSize.Height)

            fDidDraw = True

        End If

        Return fDidDraw

    End Function

    Sub DrawLineOfCells(ByVal p0 As Point, ByVal p1 As Point, ByVal drawit As DrawCellDelegate)

        Dim br = m_brushMouse

        Dim x0 = p0.X

        Dim y0 = p0.Y

        Dim x1 = p1.X

        Dim y1 = p1.Y

        Dim fSwapped = False

        Dim dx = Math.Abs(x1 - x0)

        Dim dy = Math.Abs(y1 - y0)

        If dy > dx Then

            fSwapped = True ' swap x0<=>y0, x1<->y1

            x0 = p0.Y

            y0 = p0.X

            x1 = p1.Y

            y1 = p1.X

            dx = Math.Abs(x1 - x0)

       dy = Math.Abs(y1 - y0)

        End If

        Dim err = CInt(dx / 2)

        Dim y = y0

        Dim xstep = 1

        If x0 > x1 Then

            xstep = -1

        End If

        Dim ystep = 1

        If y0 > y1 Then

            ystep = -1

        End If

        Dim x = x0

        While x <> x1 + xstep

            Dim cx = x, cy = y ' copy of x,y

            If fSwapped Then

                cx = y

                cy = x

            End If

            If drawit(New Point(cx, cy), br) Then ' if it wasn't already drawn

                br = m_brushGenerated

            End If

            err -= dy

            If err < 0 Then

                y += ystep

                err += dx

            End If

            x += xstep

        End While

    End Sub

#If AREA Then

    Dim m_brushFill = New SolidBrush(Color.Blue)

    Dim m_oColor = Color.Black

    Sub AreaFill(ByVal ptcell As Point)

        If (ptcell.X >= 0 And ptcell.X < m_numCells.Width) Then

            If (ptcell.Y >= 0 And ptcell.Y < m_numCells.Height) Then

                ' System.Threading.Thread.Sleep(100);

                If (DrawACell(ptcell, m_brushFill)) Then

                    Me.m_oColor = Color.FromArgb((((Me.m_oColor.ToArgb And &HFFFFFF) + 140) And &HFFFFFF) Or &HFF000000)

                    ' m_oColor = Color.FromArgb((int)(((((uint)m_oColor.ToArgb() & 0xffffff) + 140) & 0xffffff) | 0xff000000));

                    m_brushFill = New SolidBrush(m_oColor)

                  AreaFill(New Point(ptcell.X - 1, ptcell.Y)) ' West

                    AreaFill(New Point(ptcell.X + 1, ptcell.Y)) ' East

                    AreaFill(New Point(ptcell.X, ptcell.Y + 1)) ' South

                    AreaFill(New Point(ptcell.X, ptcell.Y - 1)) ' North

                End If

            End If

        End If

    End Sub

#End If

End Class

 

</VB Sample>

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