C# print pixel value

Question

I have a 8 bit bitmap color image. when i do a

Color pixelcolor = b.GetPixel(j,i);    
Console.Write(pixelcolor.ToString() + " " );

I get

 Color [A=255, R=255, G=255, B=255]

I need to get only the 8 bit value. not 24 bit seperate values for R,G,B ,A.

Answer 1

There is no way to do this using the Bitmap class directly. However, you can use the LockBits method to access the pixels directly.

Using unsafe code: (remember to enable unsafe code in your project first)

public static unsafe Byte GetIndexedPixel(Bitmap b, Int32 x, Int32 y)
{
    if (b.PixelFormat != PixelFormat.Format8bppIndexed) throw new ArgumentException("Image is not in 8 bit per pixel indexed format!");
    if (x < 0 || x >= b.Width) throw new ArgumentOutOfRangeException("x", string.Format("x should be in 0-{0}", b.Width));
    if (y < 0 || y >= b.Height) throw new ArgumentOutOfRangeException("y", string.Format("y should be in 0-{0}", b.Height));
    BitmapData data = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadOnly, b.PixelFormat);
    try
    {
        Byte* scan0 = (Byte*)data.Scan0;
        return scan0[x + y * data.Stride];
    }
    finally
    {
        if (data != null) b.UnlockBits(data);
    }
}

The safe alternative, using Marshal.Copy:

public static Byte GetIndexedPixel(Bitmap b, Int32 x, Int32 y)
{
    if (b.PixelFormat != PixelFormat.Format8bppIndexed) throw new ArgumentException("Image is not in 8 bit per pixel indexed format!");
    if (x < 0 || x >= b.Width) throw new ArgumentOutOfRangeException("x", string.Format("x should be in 0-{0}", b.Width));
    if (y < 0 || y >= b.Height) throw new ArgumentOutOfRangeException("y", string.Format("y should be in 0-{0}", b.Height));
    BitmapData data = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadOnly, b.PixelFormat);
    try
    {
        Byte[] pixel = new Byte[1];
        Marshal.Copy(new IntPtr(data.Scan0.ToInt64() + x + y * data.Stride), pixel, 0, 1);
        return pixel[0];
    }
    finally
    {
        if (data != null) b.UnlockBits(data);
    }
}

Answer 2

The methods in the Bitmap class doesn't let you get the palette index directly.

You can get the palette for the image using the Palette property, and look for the color there, but that's a bit of a workaround.

To get the palette index directly, you would use the LockBits method to get access to the image data directly. You would either have to use marshalling to copy the data into an array, or use pointers in unsafe mode to access it.

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The A property in a Color value is the Alpha component. It can have the value 0 to 255, where 0 is fully transparent and 255 is fully solid.