WICBitmapSource.CopyPixels to GDI Bitmap Scan0 - interop

Question

I've modified the C# IWICBitmapSource.CopyPixels interface to allow both array marshaling and passing a pointer:
void CopyPixels(
WICRect prc,
uint cbStride,
uint cbBufferSize,
[Out]
[MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 2)]
byte[] pbBuffer
);
new void CopyPixels(
WICRect prc,
uint cbStride,
uint cbBufferSize,
IntPtr pbBuffer
);
I'm calling it like this
public static Bitmap FromWic(IWICBitmapSource source) {
Guid format;
source.GetPixelFormat(out format);
PixelFormat gdiFormat = ConversionUtils.GetPixelFormat(format);
uint w, h;
source.GetSize(out w, out h);
Bitmap b = new Bitmap((int)w, (int)h, gdiFormat);
BitmapData bd = b.LockBits(new Rectangle(0, 0, (int)w, (int)h),
ImageLockMode.ReadWrite, b.PixelFormat);
try {
//Copy unmanaged-to-unmanaged
source.CopyPixels(
new WICRect { X = 0, Y = 0, Width = (int)w, Height = (int)h },
(uint)bd.Stride, (uint)(bd.Stride * bd.Height), bd.Scan0);
} finally {
b.UnlockBits(bd);
}
return b;
}
The code executes without errors, and all the width, height, stride, and buffer size values are correct, but the bitmap remains black, as if WIC never touched it.
Any ideas why this could be happening? Is there something wrong with the .NET marshalling?

Answer 1

It seems that WPF uses a proxy method to copy data to unmanaged memory. Based on that, I found a solution. The following code works and should be extremely efficient.
[DllImport("WindowsCodecs.dll", EntryPoint = "IWICBitmapSource_CopyPixels_Proxy")]
internal static extern int CopyPixels(IWICBitmapSource bitmap, IntPtr rect, uint cbStride, uint cbBufferSize, IntPtr pvPixels);
public static Bitmap FromWic(IWICBitmapSource source) {
Guid format; //Get the WIC pixel format
source.GetPixelFormat(out format);
//Get the matching GDI format
PixelFormat gdiFormat = ConversionUtils.GetPixelFormat(format);
//If it's not GDI-supported format, convert it to one.
IWICComponentFactory factory = null;
IWICFormatConverter converter = null;
try {
if (gdiFormat == PixelFormat.Undefined) {
factory = (IWICComponentFactory)new WICImagingFactory();
converter = factory.CreateFormatConverter();
converter.Initialize(source, Consts.GUID_WICPixelFormat32bppBGRA, WICBitmapDitherType.WICBitmapDitherTypeNone, null, 0.9f, WICBitmapPaletteType.WICBitmapPaletteTypeCustom);
gdiFormat = PixelFormat.Format32bppArgb;
}
IWICBitmapSource data = converter != null ? converter : source;
//Get the dimensions of the WIC bitmap
uint w, h;
data.GetSize(out w, out h);
Bitmap b = new Bitmap((int)w, (int)h, gdiFormat);
BitmapData bd = b.LockBits(new Rectangle(0, 0, (int)w, (int)h), ImageLockMode.WriteOnly, b.PixelFormat);
try {
long result = CopyPixels(data, IntPtr.Zero, (uint)bd.Stride, (uint)(bd.Stride * bd.Height), bd.Scan0);
if (result == 0x80070057) throw new ArgumentException();
if (result < 0) throw new Exception("HRESULT " + result);
return b;
} finally {
b.UnlockBits(bd);
}
} finally {
if (converter != null) Marshal.ReleaseComObject(converter);
if (source != null) Marshal.ReleaseComObject(factory);
}
}
Note that I modified the IWICBitmapSource_CopyPixels_Proxy dllimport to use IWICBitmapSource instead of SafeMILHandle, and I modified ref Int32Rect to IntPtr, so that I could legally pass IntPtr.Zero (I get an HRESULT error if I pass in a rectangle to copy - and I know my stucture is value, I use it in other CopyPixels calls.