Black border around circle after copying bitmap to another bitmap

Question

I have bitmap extracted from BitmapSource (RenderTargetBitmap) with blue circle in it. RenderTargetBitmap is created with PixelFormats.Pbgra32.

PixelFormats Pbgra32 pre-multiplies each color channel with alpha value. So, when I try to convert bitmap to cursor I was getting less opaque image than is should have.

I found solution to the problem here which clone the bitmap to Format24bppRgb and manually set R,B,G and alpha values. However, solutions works perfectly fine but for cloned bitmap I see black border around visual.

Can I get rid of that black border in cloned bitmap? (I suspect it's something inside SafeCopy method)

Methods used from the link are:

private static void SafeCopy(BitmapData srcData, BitmapData dstData, byte alphaLevel)
{
  for (int y = 0; y < srcData.Height; y++)
    for (int x = 0; x < srcData.Width; x++)
    {
       byte b = Marshal.ReadByte(srcData.Scan0, y * srcData.Stride + x * 3);
       byte g = Marshal.ReadByte(srcData.Scan0, y * srcData.Stride + x * 3 + 1);
       byte r = Marshal.ReadByte(srcData.Scan0, y * srcData.Stride + x * 3 + 2);

       Marshal.WriteByte(dstData.Scan0, y * dstData.Stride + x * 4, b);
       Marshal.WriteByte(dstData.Scan0, y * dstData.Stride + x * 4 + 1, g);
       Marshal.WriteByte(dstData.Scan0, y * dstData.Stride + x * 4 + 2, r);
       Marshal.WriteByte(dstData.Scan0, y * dstData.Stride + x * 4 + 3, alphaLevel);
    }
}

private static Cursor CreateCustomCursorInternal(Bitmap bitmap, double opacity)
{
    Bitmap cursorBitmap = null;
    IconInfo iconInfo = new IconInfo();
    Rectangle rectangle = new Rectangle(0, 0, bitmap.Width, bitmap.Height);

    try
    {
        byte alphaLevel = System.Convert.ToByte(byte.MaxValue * opacity);

        // Here, the pre-multiplied alpha channel is specified
        cursorBitmap = new Bitmap(bitmap.Width, bitmap.Height, 
                                   PixelFormat.Format32bppPArgb);

        // Assuming the source bitmap can be locked in a 24 bits per pixel format
        BitmapData bitmapData = bitmap.LockBits(rectangle, ImageLockMode.ReadOnly, 
                                                PixelFormat.Format24bppRgb);
        BitmapData cursorBitmapData = cursorBitmap.LockBits(rectangle, 
                               ImageLockMode.WriteOnly, cursorBitmap.PixelFormat);

        // Use SafeCopy() to set the bitmap contents
        SafeCopy(bitmapData, cursorBitmapData, alphaLevel);

        cursorBitmap.UnlockBits(cursorBitmapData);
        bitmap.UnlockBits(bitmapData);

        .......
}

Original bitmap:

Cloned bitmap:

Answer 1

The simplest way to convert a WPF 32bit PBGRA bitmap to a WinForms PARGB bitmap and at the same time apply a global opacity seems to be just multiplying all A, R, G and B values with the opacity factor (a float value between 0 and 1) like in the method shown below. However, I would have expected that it would also be necessary to swap the bytes, but apparently it isn't.

private static void CopyBufferWithOpacity(byte[] sourceBuffer,
    System.Drawing.Imaging.BitmapData targetBuffer, double opacity)
{
    for (int i = 0; i < sourceBuffer.Length; i++)
    {
        sourceBuffer[i] = (byte)Math.Round(opacity * sourceBuffer[i]);
    }

    Marshal.Copy(sourceBuffer, 0, targetBuffer.Scan0, sourceBuffer.Length);
}

Given a 32bit PBGRA bitmap pbgraBitmap (e.g. a RenderTargetBitmap), you would use the method like this:

var width = pbgraBitmap.PixelWidth;
var height = pbgraBitmap.PixelHeight;
var stride = width * 4;
var buffer = new byte[stride * height];
pbgraBitmap.CopyPixels(buffer, stride, 0);

var targetFormat = System.Drawing.Imaging.PixelFormat.Format32bppPArgb;
var bitmap = new System.Drawing.Bitmap(width, height, targetFormat);
var bitmapData = bitmap.LockBits(
    new System.Drawing.Rectangle(0, 0, width, height),
    System.Drawing.Imaging.ImageLockMode.WriteOnly,
    targetFormat);

CopyBufferWithOpacity(buffer, bitmapData, 0.6);

bitmap.UnlockBits(bitmapData);