How to programmatically detect if a bitmap has alpha channel?

Question

As subject. Preferibly using C code.

score 3 · Answer 1 · edited Feb 15 '12 at 13:58

=========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;
    }

score 1 · Answer 2 · edited May 23 '17 at 12:23

1

With ::GetDIBits as described on #333559

edited May 23 '17 at 12:23

Community

1
1

answered Mar 26 '09 at 13:14

Aidan Fitzpatrick

1,950
1
21
26

score 1 · Answer 3 · answered Mar 26 '09 at 13:22

1

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,

answered Mar 26 '09 at 13:22

if you use bits[3], it probably 24-bit bitmap with BGR[B]GR, so you will get Blue value, not alpha. – dns Jan 03 '16 at 03:46

score 1 · Answer 4 · answered Mar 28 '09 at 10:21

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!

score 0 · Answer 5 · answered Jan 03 '16 at 03:44

0

Use GetDIBits(hdc, hbmp, 0, 1, NULL, &bmi, DIB_RGB_COLORS).

And then see if bmi.bmiHeader.biBitCount has value of 32 then it has alpha channel, otherwise it doesn't have alpha.

answered Jan 03 '16 at 03:44

dns

2,753
1
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How to programmatically detect if a bitmap has alpha channel?

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