[cairo] problem with BitBlt

[Vladimir Vukicevic]

A win32 surface created with a DC is always RGB24 -- that is, no  
alpha.  Whatever the original contents of the HDC is retained. If you  
need a RGBA surface, you should use  
cairo_win32_surface_create_with_dib, or cairo_surface_create_similar.   
BitBlt doesn't know about the alpha channel at all; so if you BitBlt a  
24-bit surface to an ARGB surface, you'll get garbage for the alpha  
channel... similarly if you go the other way, the alpha channel will  
just be discarded.

I don't know MFC, so I don't know what format a TBitmap is created as  
-- but given GDI's overall lack of alpha support (AlphaBlend and  
StretchDIBits w/HALFTONE being the exceptions), I would be surprised  
if it included an alpha channel.

     - Vlad

On Sep 1, 2008, at 5:30 AM, <gurnett at macbat.com> <gurnett at macbat.com>  
wrote:

> I tried
>
> (cr,CAIRO_OPERATOR_CLEAR);
>> cairo_paint(cr);
>
> but with no luck. Everything was working properly until I introduced  
> the
> TImage and Bitblt bits, so I ought to be with one of these that the  
> problem
> occurs
>
>
> Thanks
>
> Mick
> ----- Original Message -----
> From: "Gerdus van Zyl" <gerdusvanzyl at gmail.com>
> To: <cairo at cairographics.org>
> Sent: Monday, September 01, 2008 11:50 AM
> Subject: Re: [cairo] problem with BitBlt
>
>
>> Have you tried:
>>
>> cairo_set_operator(cr,CAIRO_OPERATOR_CLEAR);
>> cairo_paint(cr);
>>
>> You should also check if the Graphics::TBitmap is not initialized  
>> with
>> a white background and try using direct GDI to clear it if the above
>> does not work.
>>
>> ~Gerdus
>>
>> On Sun, Aug 31, 2008 at 12:11 PM, Michael Gurnett <gurnett at glocalnet.net 
>> >
>> wrote:
>>> Why would you want to paint the background red with and 100%  
>>> opaque (no
>>> transparancy). What you explain later on is the problem I have. The
>>> background colour would not be unexpected if the correct procedure  
>>> as
>>> used.
>>> It is just a question of the correct approach.
>>>
>>> ----- Original Message -----
>>> From: Mukul Sharma
>>> To: Michael Gurnett
>>> Cc: cairo at cairographics.org
>>> Sent: Sunday, August 31, 2008 11:23 AM
>>> Subject: Re: [cairo] problem with BitBlt
>>> Ohk.
>>>
>>> Use cairo_set_source_rgba(1,0,0,1);
>>> and then use cairo_paint();
>>>
>>> I think, this would work. On a sidenote, having a transparent  
>>> background
>>> can
>>> give you unexpected background color. I had tried doing the same  
>>> thing,
>>> but
>>> eventually settled on having a known background color as compared to
>>> having
>>> a transparent background. For example, when I had a transparent
>>> background,
>>> I obtained a black background. Thus, I painted it all in white and  
>>> then
>>> performed my drawings.
>>>
>>> I cannot support your guess that BitBlt does not support Alpha  
>>> values.
>>> Because, BitBlt is simply a method to transfer a value of colour.  
>>> The
>>> value
>>> of color is calculated at Cairo's End and rendered at the Display  
>>> end.
>>> Thus,
>>> if Alpha values are not supported somewhere, it would be your  
>>> display and
>>> AFAK it cannot be BitBlt part or Cairo part.
>>>
>>> Regards
>>> Mukul
>>>
>>> On Sun, Aug 31, 2008 at 2:10 AM, Michael Gurnett <gurnett at glocalnet.net 
>>> >
>>> wrote:
>>>>
>>>> Tried with cairo_paint_with_alpha, but that didn't seem to work.
>>>>
>>>>
>>>>
>>>> ----- Original Message -----
>>>> From: Michael Gurnett
>>>> To: cairo at cairographics.org
>>>> Sent: Sunday, August 31, 2008 2:27 AM
>>>> Subject: [cairo] problem with BitBlt
>>>> I have the following bit of code
>>>>
>>>> bckgrnd = new Graphics::TBitmap;
>>>>   bckgrnd->Width = ClientWidth;
>>>>   bckgrnd->Height = ClientHeight;
>>>> HDC hdc;
>>>> hdc=bckgrnd->Canvas->Handle;
>>>>  cairo_pattern_t *pat;
>>>> cairo_surface_t *surface;
>>>>
>>>> surface = cairo_win32_surface_create (hdc);
>>>> background = cairo_create (surface);
>>>>
>>>> ....
>>>>
>>>>
>>>> The idea is the to draw on the background surface and BitBlt it  
>>>> on my
>>>> paint routine. However, with the code above it appears that the  
>>>> initial
>>>> drawing space is filled white (guessing rgba are all 1). Is there  
>>>> a way
>>>> to
>>>> get the initial drawing region to be transparant.
>>>>
>>>> Thanks
>>>>
>>>> Michael
>>>>
>>>> ________________________________
>>>>
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>>>
>>>
>>>
>>> --
>>> Thanks and Regards,
>>>
>>> Mukul Sharma
>>> Centre for High Performance Embedded Systems
>>> Nanyang Technological University, Singapore
>>>
>>> "A person who runs from responsibilities, runs from life."
>>>
>>> "It is easy to hate and it is difficult to love. This is how the  
>>> whole
>>> scheme of things works. All good things are difficult to achieve;  
>>> and bad
>>> things are very easy to get."
>>>
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>>> cairo mailing list
>>> cairo at cairographics.org
>>> http://lists.cairographics.org/mailman/listinfo/cairo
>>>
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