在可写位图图像中仅显示ARGB中的绿色分量颜色

Question

I have a WriteableBitmap image and I want to show only green component of it on a buttonclick event.

Earlier I was trying to first convert it to Grayscale and then i was hoping that i would be able to show only Green component after this,But anyhow i am not able to crack that.

Below is the code what i am currently trying for converting the image to a Grayscale image:

public static WriteableBitmap ToGrayScale(WriteableBitmap bitmapImage) {

    for (var y = 0; y < bitmapImage.PixelHeight; y++) {
        for (var x = 0; x < bitmapImage.PixelWidth; x++) {
            var pixelLocation = bitmapImage.PixelWidth * y + x;
            var pixel = bitmapImage.Pixels[pixelLocation];
            var pixelbytes = BitConverter.GetBytes(pixel);
            var bwPixel = (byte)(.299 * pixelbytes[2] + .587 * pixelbytes[1] + .114 * pixelbytes[0]);
            pixelbytes[0] = bwPixel;
            pixelbytes[1] = bwPixel;
            pixelbytes[2] = bwPixel;
            bitmapImage.Pixels[pixelLocation] = BitConverter.ToInt32(pixelbytes, 0);
        }
    }

    return bitmapImage;
}

I am getting this error: 'Windows.UI.Xaml.Media.Imaging.WriteableBitmap' does not contain a definition for 'Pixels' and no extension method 'Pixels' accepting a first argument of type 'Windows.UI.Xaml.Media.Imaging.WriteableBitmap' could be found (are you missing a using directive or an assembly reference?)

Some help would be very grateful to me.Thank you.

Answer 1

You have to copy the 2nd channel of the image:

public static WriteableBitmap ToGrayScale(WriteableBitmap bitmapImage) {

    for (var y = 0; y < bitmapImage.PixelHeight; y++) {
        for (var x = 0; x < bitmapImage.PixelWidth; x++) {
            var pixelLocation = bitmapImage.PixelWidth * y + x;
            var pixel = bitmapImage.Pixels[pixelLocation];
            var pixelbytes = BitConverter.GetBytes(pixel);
            var greenChannel = (byte)(pixelbytes[1]);
            pixelbytes[0] = greenChannel;
            pixelbytes[1] = greenChannel;
            pixelbytes[2] = greenChannel;
            bitmapImage.Pixels[pixelLocation] = BitConverter.ToInt32(pixelbytes, 0);
        }
    }

    return bitmapImage;
}

(In this code I am assuming that your gray conversion is correct. Windows has the tendency to store RGB images as BGR in memory. I am not sure where the alpha channel goes. You should check that with a test image, which has regions of pure colours.)

Answer 2

As noted, Windows.UI.Xaml.Media.Imaging.WriteableBitmap does not have a Pixels property: it has a PixelBuffer which returns the pixels as an IBuffer.

You can access the IBuffer with a DataReader & DataWriter or by converting to a stream with the AsStream extension method or to to a byte[] with the ToArray extension method. Once you have a stream or array it's straightforward to loop through, convert the bytes to green or grayscale, and set the modified pixels back.

My colleague Jeff demonstrates the grayscale conversion via AsStream in his blog entry How to: Convert an Image to Grayscale

The open source WriteableBitmapEx project also adds its own extensions to WriteableBitmap to access the PixelBuffer.