improving speed of GetPixel function in c#

Question

i need to read getpixel of bmp with speed but is very low i used of LockBits

     private void LockUnlockBitsExample(Bitmap bmp)
    {

        Rectangle rect = new Rectangle(0, 0, bmp.Width, bmp.Height);
        System.Drawing.Imaging.BitmapData bmpData =
            bmp.LockBits(rect, System.Drawing.Imaging.ImageLockMode.ReadWrite,
            bmp.PixelFormat);
        IntPtr ptr = bmpData.Scan0;

        int bytes  = Math.Abs(bmpData.Stride) * bmp.Height;
        rgbValues = new byte[bytes];

        System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes);
        bmp.UnlockBits(bmpData);
    }

and this function

        private Color GetMyPixel(byte[] rgbValues,Bitmap bmp, int x,int y )
    {

        int index= (bmp.Width*y+x)*3;
        Color MyColor = Color.FromArgb(rgbValues[index], rgbValues[index + 1], rgbValues[index + 2]);
        return MyColor;
    }

but output of my function is different from original getpixel

What is the color format of the original image? Looks like Your GetMyPixel function assume the images are 24 bits per pixel. — sam1589914, Sep 20 '12 at 10:36

score 2 · Answer 1 · answered Sep 20 '12 at 10:37

2

In this line:

int index= (bmp.Width*y+x)*3;

I believe that bmp.Stride must be used instead of bmp.Width. Also check assumption that PixelFormat is 24 bits per pixel.

Another thing are color indexes: blue is first (index), then green (index+1), then red (index + 2).

answered Sep 20 '12 at 10:37

Grzegorz Pływacz

166
1
4

Since this is the accepted answer, I want to say that simply replacing bmp.Width with bmp.Stride would be incorrect as you would not want to multiply the stride by y and by 3 (bytes per pixel). Width is the length is pixels, so it is correct to multiply that by 3, but it is not correct to multiply the stride by 3. – Jason Sep 20 '12 at 11:04
@JasonHermann You are absolutely right. I had to missed that. – Grzegorz Pływacz Sep 20 '12 at 11:17

score 1 · Accepted Answer · answered Sep 20 '12 at 10:38

I have code in VB for some reason that does almost the exact same thing as you, so I hope this helps. You can try the following modification to GetMyPixel:

Use Stride instead of Width and invert the byte order in your call to FromArgb.

private Color GetMyPixel(byte[] rgbValues,Bitmap bmp, int x,int y )
{
   int index= (bmp.Stride*y+x*3);        
   if (index > rgbValues.Length - 3)
   index = rgbValues.Length - 3;
   Color MyColor = Color.FromArgb(rgbValues[index+2], rgbValues[index + 1], rgbValues[index]);         
    return MyColor;
}

score 1 · Answer 3 · answered Sep 20 '12 at 10:40

1

You should check this post out :working with lockbits

it helped me a lot when i did something similar

answered Sep 20 '12 at 10:40

meirrav

761
1
9
27

improving speed of GetPixel function in c#

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