How to Create a Gdiplus::Bitmap from an HBITMAP, retaining the alpha channel information?

Question

When I create a new Gdiplus::Bitmap using the Bitmap::FromHBITMAP function, the resulting Bitmap is opaque - none of the partial transparency from the original HBITMAP is preserved.

Is there a way to create a Gdiplus::Bitmap from an HBITMAP which brings across the alpha channel data?

Almanac

Do you know for sure that the HBITMAP has an alpha channel to start with? For instance, did you create it as a DIB with BI_RGB and 32bpp or some other way? – Cushat

Yes, it does have transparency. It draws nicely using the ::AlphaBlend() function. – Almanac

(I created it in C# using new Bitmap(string filename) and the calling GetHBitmap() on the .NET bitmap object) – Almanac

Answer 1

It turns out that GDI+ never brings across the alpha channel when creating a Bitmap from an HBITMAP.

The answer is to:

• Use GetObject passing in a BITMAP and the HBITMAP, to get the width and height (and if the input bitmap is a DIB, the pixel data) of the input HBITMAP.
• Create a Bitmap of the correct size with 32 bit PARGB pixel format.
• Use LockBits to get hold of the pixelData memory of your new Bitmap.
• If you got the pixels from GetObject, copy the ARGB values across using memcpy.
• Call UnlockBits on the new Bitmap.

In my case, the format of the input HBITMAP is correct for doing a straight memcpy from input bitmap pixel data to the new Bitmap pixel data.

If you didnt get the input pixel data from GetObject, use GetDIBits to get a copy in the correct format.

Almanac

Answer 2

I think working code is more useful than instructions, so:

#include <GdiPlus.h>
#include <memory>

Gdiplus::Status HBitmapToBitmap( HBITMAP source, Gdiplus::PixelFormat pixel_format, Gdiplus::Bitmap** result_out )
{
  BITMAP source_info = { 0 };
  if( !::GetObject( source, sizeof( source_info ), &source_info ) )
    return Gdiplus::GenericError;

  Gdiplus::Status s;

  std::auto_ptr< Gdiplus::Bitmap > target( new Gdiplus::Bitmap( source_info.bmWidth, source_info.bmHeight, pixel_format ) );
  if( !target.get() )
    return Gdiplus::OutOfMemory;
  if( ( s = target->GetLastStatus() ) != Gdiplus::Ok )
    return s;

  Gdiplus::BitmapData target_info;
  Gdiplus::Rect rect( 0, 0, source_info.bmWidth, source_info.bmHeight );

  s = target->LockBits( &rect, Gdiplus::ImageLockModeWrite, pixel_format, &target_info );
  if( s != Gdiplus::Ok )
    return s;

  if( target_info.Stride != source_info.bmWidthBytes )
    return Gdiplus::InvalidParameter; // pixel_format is wrong!

  CopyMemory( target_info.Scan0, source_info.bmBits, source_info.bmWidthBytes * source_info.bmHeight );

  s = target->UnlockBits( &target_info );
  if( s != Gdiplus::Ok )
    return s;

  *result_out = target.release();

  return Gdiplus::Ok;
}

Yemane

I had similar problem with a CBitmap, from which I retrieved the HBITMAP. In that case ::GetObject() left source_info.bmBits set to NULL. I could solve this by calling myCBitmapPtr->GetBitmapBits(source_info.bmWidthBytes * source_info.bmHeight, target_info.Scan0) – Mcintosh

BTW, the return values are nicely checked in your example but if source_info.bmBits is NULL, there's an access violation in the call to CopyMemory(). – Mcintosh