[cairo] problem with BitBlt

[Gerdus van Zyl]

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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