CODEWITHSUNDEEP
c#imagepointersimage-processing
badbod99
badbod99

Reputation: 7526

Greyscale darkening problem with Bitmap image in C#

I'm loading a 8bppIndexed greyscale image into memory and reading the pixel values. The problem is that the values I am getting from the pixels do not seem to match the actual image, they are always darker. My image is a simple grey gradient like this:

alt text

The bottom right pixel is returning 191 and the top left 0. The top left is actually 64 and bottom right is 255.

Here is how I am loading my image:

Bitmap threshImg = new Bitmap(@"C:\grey.bmp");

Checking the PixelFormat confirms it is in Format8bppIndexed. So I read the bottom right pixel and top left like so:

BitmapData data = threshImg.LockBits(new Rectangle(0, 0, rectWidth, rectHeight), ImageLockMode.ReadOnly, PixelFormat.Format8bppIndexed);
unsafe
{
    byte* pixel = (byte*)data.Scan0.ToPointer();
    int topVal = (int)(byte)pixel[0];
    int bottomVal = (int)(byte)pixel[((threshImg.Height * threshImg.Width)) - 1];
}
threshImg.UnlockBits(data);

If I convert the image to 24bppRbg (and adjust the code accordingly) I see the correct colour values in the respective corners.

Anyone know why I'm getting darker values when using an 8bppIndexed image?

Upvotes: 1

Views: 2039

Answers (2)

Robert Paulson
Robert Paulson

Reputation: 18061

The value in the 8bpp indexed image isn't the color itself (or gray value) but the index. Try to look up the color value in the palette.

Upvotes: 3

ChrisF
ChrisF

Reputation: 137148

With an indexed image there are only a certain number of colours (or in this case shades of grey) available - usually 256. It's probable that there aren't enough to get the full range of shades in the original image.

As having the exact shades is important I'd shift to a 24bpp image.

Upvotes: 1

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