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?

Solution 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.

Solution 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;
}

Comments

[Johann Gerell]

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?

[mackenir]

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

[mackenir]

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

[foraidt]

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)

[foraidt]

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().