[cairo] problem with BitBlt

[Michael Gurnett]

Thank you for confirming what I believed to be the case with BitBlt, and for 
the info to fix it.

Mick
----- Original Message ----- 
From: "Vladimir Vukicevic" <vladimir at pobox.com>
To: <gurnett at macbat.comgurnett@macbat.com>
Cc: <cairo at cairographics.org>
Sent: Monday, September 01, 2008 9:27 PM
Subject: Re: [cairo] problem with BitBlt


>
> 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
>>>>>
>>>>> ________________________________
>>>>>
>>>>> _______________________________________________
>>>>> cairo mailing list
>>>>> cairo at cairographics.org
>>>>> http://lists.cairographics.org/mailman/listinfo/cairo
>>>>>
>>>>> _______________________________________________
>>>>> cairo mailing list
>>>>> cairo at cairographics.org
>>>>> http://lists.cairographics.org/mailman/listinfo/cairo
>>>>
>>>>
>>>>
>>>> --
>>>> 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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