problem with Getpixel()

Question

1.00/5 (1 vote)

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hi,,,this is my code,,,when i call Gfill,,,after getpixel method a value of c is 0,,,what is my problem?

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 GFill
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        int MouseX, MouseY;
       // Color ColorOfPixel(Point P);
        private void button1_Click(object sender, EventArgs e)
        {
           

        }

        private void GFill(int x, int y, int bcolor, int Fcolor)
        {
            Graphics g = this.CreateGraphics();
            Bitmap bmp = new Bitmap(this.Width,this.Height);
            bmp.SetPixel(0, 0, Color.Blue);
            int c = bmp.GetPixel(x, y).ToArgb();
                if ((x >= 0) && (x <= 640) && (y >= 0) && (y <= 480))
                {
                    if ((c != bcolor) && (c != Fcolor))
                    {
                        g.DrawImageUnscaled(bmp, x, y);
                        GFill(x + 1, y, bcolor, Fcolor);
                        GFill(x, y + 1, bcolor, Fcolor);
                        GFill(x - 1, y, bcolor, Fcolor);
                        GFill(x, y - 1, bcolor, Fcolor);
                    }
                }
            }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            Rectangle rectangle1 = new Rectangle(100, 200, 200, 100);
            e.Graphics.DrawRectangle(Pens.Black, rectangle1);
        }

        private void Form1_MouseClick(object sender, MouseEventArgs e)
        {
            MouseX = MousePosition.X;
            MouseY = MousePosition.Y;
            GFill(MouseX, MouseY, this.BackColor.ToArgb(), this.ForeColor.ToArgb());
            Application.DoEvents();
        }


    }
}

Posted 4-Dec-13 9:53am

ali_vampire

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BillWoodruff 5-Dec-13 0:13am

Do keep in mind that where a mouse is clicked in a displayed graphic at run-time may correspond to any number of bitmap pixels ... depending on bitmap resolution and color-depth, and monitor resolution and color-depth.

3 solutions

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Matt T Heffron · Accepted Answer · 2013-12-04T10:01:00

You're creating a new Bitmap in each invocation of GFill, so you're always looking at the newly created bitmap, not the actual image of the form!

Create one Bitmap in Form1_MouseClick, copy the initial image of the form into the bitmap, and pass it to GFill. Do the filling operation and then call the g.DrawImageUnscaled(bmp, 0, 0); after GFill returns.

Aside: GetPixel and SetPixel are notorously slow. Search here on CodeProject (or with Google) to find better ways of doing this.

[Edit: getting the initial client area bitmap left as an exercise... :-) Google is your friend here!]

C#

private void Form1_MouseClick(object sender, MouseEventArgs e)
{
  Graphics g = this.CreateGraphics();
  Rectangle rect = this.ClientRectangle;
  Bitmap bmp = new Bitmap(rect.Width, rect.Height, g);
  //Get form's client area bitmap into bmp
  BitmapData bmpData = bmp.LockBits(rect, ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);
  int[] pixels = new int[bmpData.Height * bmpData.Width];
  Marshal.Copy(bmpData.Scan0, pixels, 0, pixels.Length);
  GFill(Color.Blue.ToArgb(), e.X, e.Y, this.BackColor.ToArgb(), this.ForeColor.ToArgb(), bmp.Width, bmp.Height, pixels);
  Marshal.Copy(pixels, 0, bmpData.Scan0, pixels.Length);
  bmp.UnlockBits(bmpData);
  g.DrawImageUnscaled(bmp, 0, 0);
}

private void GFill(int newColor, int x, int y, int bcolor, int Fcolor, int width, int height, int[] pixels)
{
  if ((x >= 0) && (x < width) && (y >= 0) && (y < height))
  {
    int offset = x + y * width;
    int c = pixels[offset];
    if ((c != newColor) && (c != bcolor) && (c != Fcolor))
    {
      pixels[offset] = newColor;
      GFill(newColor, x + 1, y, bcolor, Fcolor, width, height, pixels);
      GFill(newColor, x, y + 1, bcolor, Fcolor, width, height, pixels);
      GFill(newColor, x - 1, y, bcolor, Fcolor, width, height, pixels);
      GFill(newColor, x, y - 1, bcolor, Fcolor, width, height, pixels);
    }
  }
}

Notice: I added a check for the pixel at x,y already newColor to prevent re-filling areas already done.
This fill routine will overflow the stack fairly quickly!!!

Sergey Alexandrovich Kryukov · Accepted Answer · 2013-12-04T10:39:00

In addition to the correct Solution 1:

For such a massive bit operations, you should never use GetPixel/SetPixel, which is prohibitively slow and would be good only for working with very few pixels. Instead, use System.Drawing.Bitmap.LockBits:
http://msdn.microsoft.com/en-us/library/system.drawing.bitmap.lockbits%28v=vs.110%29.aspx[^].

The code samples and description of these two methods in MSDN will help you to understand what they do and how to use them.

Another pretty bad thing is Application.DoEvents. If you need to do something in parallel, use threads; but this is a different story.

[EDIT]

Please see my comments below. If you need to pick up a pixel from screen, you are doing it wrong.
See, for example:
http://stackoverflow.com/questions/1483928/how-to-read-the-color-of-a-screen-pixel[^],
http://bobpowell.net/eyedropper.aspx[^].

The clean way to do it without using P/Invoke is also this: get a screen coordinate of your click, take a copy of a little fragment of the screen (ideally, of a size of one pixel), and use System.Drawing.Graphics.CopyFromScreen:
http://msdn.microsoft.com/en-us/library/system.drawing.graphics.copyfromscreen%28v=vs.110%29.aspx[^].

It will give you an instance of System.Drawing.Graphics to read from screen, possibly even with GetPixel.

To get screen coordinates of the click, use http://msdn.microsoft.com/en-us/library/system.windows.forms.control.pointtoscreen%28v=vs.110%29.aspx[^].

—SA

Adam Zgagacz · Accepted Answer · 2013-12-04T10:02:00

Solution 2

In your code you set pixel 0,0 to blue and in the next line you are examining value of pixel x, y.
Almost certainly x!=0 and y!=0.

C#

Bitmap bmp = new Bitmap(this.Width,this.Height);
bmp.SetPixel(0, 0, Color.Blue);
int c = bmp.GetPixel(x, y).ToArgb();

Posted 4-Dec-13 10:02am

Adam Zgagacz

Comments

ali_vampire 4-Dec-13 16:06pm

so how can i use getpixel?
i use this line but i have an error again

bmp.SetPixel(x,y, Color.Blue);

Adam Zgagacz 4-Dec-13 16:24pm

I'm not sure I understand your problem. In the piece of code you posted you assign value to the single pixel, then you are checking different pixel and wondering why you don't see value you put somewhere else.

if you do:

bmp.SetPixel(0, 0, Color.Blue);
int c = bmp.GetPixel(0, 0).ToArgb();

you will se value.

ali_vampire 4-Dec-13 16:37pm

for example when i clicked on a(204,305) on the form,,,i need getpixel of these points,,,

Adam Zgagacz 4-Dec-13 16:55pm

See solution #3 by SA. There is no point to talk more about SetPixel GetPixel.