Re: [PATCH 2/2] comctl32: Paint 32-bpp bitmaps with an alpha channel using GdiAlphaBlend.

[Alexandre Julliard]

Dmitry Timoshkov  writes:
> Alexandre Julliard  wrote:
> > Dmitry Timoshkov  writes:
> > > +static HBITMAP create_alpha_bitmap( HBITMAP hbitmap )
> > > +{
> > > - HBITMAP alpha = 0;
> > > - BITMAPINFO *info = NULL;
> > > - BITMAP bm;
> > > - HDC src, dst;
> > > - void *bits;
> > > - DWORD i;
> > > - const unsigned char *ptr;
> > > - BOOL has_alpha = FALSE;
> > > -
> > > - if (!GetObjectW( hbitmap, sizeof(bm), &bm )) return 0;
> > > - if (bm.bmBitsPixel != 32) return 0;
> > > -
> > > - if (!(src = CreateCompatibleDC( 0 ))) return 0;
> > > - if (!(dst = CreateCompatibleDC( src ))) goto done;
> > > - if (!(info = HeapAlloc( GetProcessHeap(), 0, FIELD_OFFSET( BITMAPINFO, bmiColors[256] )))) goto done;
> > > - info->bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
> > > - info->bmiHeader.biWidth = bm.bmWidth;
> > > - info->bmiHeader.biHeight = -bm.bmHeight;
> > > - info->bmiHeader.biPlanes = 1;
> > > - info->bmiHeader.biBitCount = 32;
> > > - info->bmiHeader.biCompression = BI_RGB;
> > > - info->bmiHeader.biSizeImage = bm.bmWidth * bm.bmHeight * 4;
> > > - info->bmiHeader.biXPelsPerMeter = 0;
> > > - info->bmiHeader.biYPelsPerMeter = 0;
> > > - info->bmiHeader.biClrUsed = 0;
> > > - info->bmiHeader.biClrImportant = 0;
> > > - if (!(alpha = CreateDIBSection( dst, info, DIB_RGB_COLORS, &bits, NULL, 0 ))) goto done;
> > > -
> > > - SelectObject( src, hbitmap );
> > > - SelectObject( dst, alpha );
> > > - BitBlt(dst, 0, 0, bm.bmWidth, bm.bmHeight, src, 0, 0, SRCCOPY);
> >
> > BitBlt() is not guaranteed to preserve alpha, it's not the right
> > function to use here.
>
> I've heard that before but didn't see the reasoning, in which cases it's
> supposed to fail? Are you probably suggesting to use GdiAlphaBlend() or
> something else?

GetDIBits() is a better choice. Though in general you'd have a DIB
section already, in which case you don't need to copy the bits at all.
It's not clear that an alpha channel is even meaningful with a DDB, did
you test that case?