winex11: Fix alpha blending in X11DRV_UpdateLayeredWindow.

[Alexandre Julliard]

Sebastian Lackner <sebastian at fds-team.de> writes:

> On 07.02.2017 17:08, Alexandre Julliard wrote:
>> Sebastian Lackner <sebastian at fds-team.de> writes:
>> 
>>> @@ -2627,14 +2627,12 @@ BOOL CDECL X11DRV_UpdateLayeredWindow( HWND hwnd, const UPDATELAYEREDWINDOWINFO
>>>      {
>>>          IntersectRect( &rect, &rect, info->prcDirty );
>>>          memcpy( src_bits, dst_bits, bmi->bmiHeader.biSizeImage );
>>> -        PatBlt( hdc, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top, BLACKNESS );
>>> -    }
>>> -    ret = GdiAlphaBlend( hdc, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top,
>>> -                         info->hdcSrc,
>>> -                         rect.left + (info->pptSrc ? info->pptSrc->x : 0),
>>> -                         rect.top + (info->pptSrc ? info->pptSrc->y : 0),
>>> -                         rect.right - rect.left, rect.bottom - rect.top,
>>> -                         (info->dwFlags & ULW_ALPHA) ? *info->pblend : blend );
>>> +    }
>>> +    ret = BitBlt( hdc, rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top,
>>> +                  info->hdcSrc,
>>> +                  rect.left + (info->pptSrc ? info->pptSrc->x : 0),
>>> +                  rect.top + (info->pptSrc ? info->pptSrc->y : 0),
>>> +                  SRCCOPY );
>> 
>> BitBlt is not guaranteed to set a valid alpha channel.
>
> Could you please clarify in which situation this code will not work as expected?
> I have checked the relevant code, but so far I have not found any reason why it wouldn't work here.
> Also, it works perfectly fine with the test app written to demonstrate this bug
> (which also uses alpha channels). Am I missing something?

If the source doesn't have alpha, it's assumed to be all 0xff.  In that
case, GdiAlphaBlend will set the alpha correctly in the destination, but
BitBlt won't.

-- 
Alexandre Julliard
julliard at winehq.org