Hi,

I am currently using the following routine to export an image which has transparent background from an Metafile. I am using the following routine. But the exported images has black background. Also please note that these images are transparent bitmap and pngs.


Is there any way to avoid blackbackground, my intention is to have transparent background.

private Bitmap GetBitmap( int imageOffset, uint imgSize, IntPtr ptr, IntPtr bitmapInfoPtr,
int iUsageSrc )
{
imageOffset -= ( 2 * IntSize );
IntPtr imgPtr = new IntPtr( ptr.ToInt64() + imageOffset );
byte[] imgData = new byte[ imgSize ];
Marshal.Copy( imgPtr, imgData, 0, imgData.Length );

IntPtr hBitmap = GdiApi.CreateDIBitmap(Objects.Handle, bitmapInfoPtr,
WinGdiConst.CBM_INIT, imgData, bitmapInfoPtr, (uint)iUsageSrc);

Bitmap bmp = null;

if( hBitmap != IntPtr.Zero )
{
// Clone image for preventing disposing it before document save.
bmp = Image.FromHbitmap( hBitmap );
bmp = bmp.Clone() as Bitmap;
}
else
{
Debug.WriteLine( "Can't create bitmap" );
}

GdiApi.DeleteObject( hBitmap );

return bmp;
}

Thanks in advance,
Karthik

Well, that's pretty unclear. We have no idea what pixel format you chose. And GDI doesn't know beans about transparency. Use Bitmap.LockBits() instead. And Bitmap.MakeTransparent() to select a background color as the transparency if you don't have a format that supports alpha.

---

Hans Passant.

Hi Hans,

Regarding the PixelFormat, the created bitmap is in 24bppRGB and not {ARGB}. I have already tried with
Bitmap.MakeTransparent() but it wasn't helped me.

Also regarding my application, my application basically processes metafiles and export its contents to PDF.

I am getting this background color problem, only with the text and images. Also these are
of record type. EMR_ALPHABLEND.

Thanks in advance,
Karthik

Hi,

I found the following formula from the emf reference manual. This
formula deals with tow bitmaps for alpha-blending. Could anyone let
me know how do I apply the following equation in a single bitmap which
has the black color in place of the transparent areas.

src.Red = src.Red * (SrcConstantAlpha/255.0)
src.Green = src.Green * (SrcConstantAlpha/255.0)
src.Blue = src.Blue * (SrcConstantAlpha/255.0)

dst.Red = src.Red + (1.0 - (src.Alpha/255.0)) * dst.Red
dst.Green = src.Green + (1.0 - (src.Alpha/255.0)) * dst.Green
dst.Blue = src.Blue + (1.0 - (src.Alpha/255.0)) * dst.Blue

If the destination bitmap has an alpha channel, then it is blended as
follows:
src.Alpha = src.Alpha * (SrcConstantAlpha)/255.0)
dst.Alpha = src.Alpha + (1.0 - (src.Alpha/255.0)) * dst.Alpha
Note that if SrcConstantAlpha is 0xFF, there is in effect no
premultiplication of the source values.

Thanks in advance,
Karthik