How to programmatically detect if a bitmap has alpha channel ?

Question

As subject. Preferibly using C code. Thanks,

Answer 1

Check out the answer to this StackOverflow question: Question 333559

Answer 2

With ::GetDIBits as described on #333559

Answer 3

On that answer it is not clear what to do with GetDiBits():

GetDIBits( hDC, hBmp, 0, 1, (void**) &bits, &bmi, DIB_RGB_COLORS );

bits[3] == alpha of topleft pixel;

Should bits[3] be tested against zero ? what to do with that value ? thanks,

Answer 4

I have implemented a prototype but it is not working fine, so there may be something wrong in the code. I share it here with the hope that we can fix it together:

BOOL HasAlphaChannel( HBITMAP hBmp )
{
   HDC hDC = CreateCompatibleDC( NULL );
   BITMAPINFO bmi; 
   void * bits;
   unsigned long ul; 
   BOOL bAlphaChannel = FALSE;

   memset( &bmi, 0, sizeof( BITMAPINFO ) ); 
   bmi.bmiHeader.biSize = sizeof( BITMAPINFOHEADER );  

   GetDIBits( hDC, hBmp, 0, 1, NULL, &bmi, DIB_RGB_COLORS ); 

   bits = malloc( bmi.bmiHeader.biSizeImage );

   // this is returning zero wich means error (why???) 
   GetDIBits( hDC, hBmp, 0, bmi.bmiHeader.biHeight, &bits, &bmi, DIB_RGB_COLORS ); 

   for( ul = 0; ul < bmi.bmiHeader.biSizeImage; ul += 4 )
      if( ( ( char * ) bits )[ ul + 3 ] != 0 )
         bAlphaChannel = TRUE;    

   free( bits );

   DeleteDC( hDC );

   return bAlphaChannel;
}

Thanks!

Answer 5

=========MFC++ version

private: static Boolean __gc* BitmapHasAlpha(BitmapData __gc* bmpData)

{ if ((bmpData->PixelFormat != PixelFormat::Format32bppArgb) && (bmpData->PixelFormat != PixelFormat::Format32bppRgb)) { return false; } for (Int32 __gc* i = 0; (i < bmpData->Height); i++) { Int32 __gc* num2 = (i * bmpData->Stride); for (Int32 __gc* j = 3; (j < (bmpData->Width * 4)); j += 4) { Byte __gc** numPtr = *static_cast<__box Byte __gc***>(((bmpData->Scan0->ToPointer() + num2) + j)); if (numPtr[0] != 0) { return true; } } } return false; }

=========C# version

private static unsafe bool BitmapHasAlpha(BitmapData bmpData)
    {
        if ((bmpData.PixelFormat != PixelFormat.Format32bppArgb) && (bmpData.PixelFormat != PixelFormat.Format32bppRgb))
        {
            return false;
        }
        for (int i = 0; i < bmpData.Height; i++)
        {
            int num2 = i * bmpData.Stride;
            for (int j = 3; j < (bmpData.Width * 4); j += 4)
            {
                byte* numPtr = ((byte*)bmpData.Scan0.ToPointer()) + num2 + j;
                if (numPtr[0] != 0)
                {
                    return true;
                }
            }
        }
        return false;
    }