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

[Alexandre Julliard]

Dmitry Timoshkov <dmitry at baikal.ru> writes:

> Alexandre Julliard <julliard at winehq.org> wrote:
>
>> Dmitry Timoshkov <dmitry at baikal.ru> 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?

-- 
Alexandre Julliard
julliard at winehq.org