Transparency support on wxDC

[Chris Spencer]

Hi all,

I need support for an alpha value on a wxBrush when drawing a
rectangle on a wxDC object (so you can still what's behind the
rectangle, just obscured). The manual for wxDC says it works for Mac
OS X when using Core Graphics, but doesn't say for other wx ports.
Does anyone know about the others? I'd rather find out beforehand
before I discover I've wasted my effort..

Chris.

[Bryan Petty]

It _only_ works on OS X when using Core Graphics. You can however
accomplish this task easily on all platforms by using the newer
wxGraphicsContext API instead of wxDC.

Regards,
Bryan Petty

[Chris Spencer]

2009/9/8 Bryan Petty <br...@ibaku.net>:

> It _only_ works on OS X when using Core Graphics. You can however
> accomplish this task easily on all platforms by using the newer
> wxGraphicsContext API instead of wxDC.
>
> Regards,
> Bryan Petty

Perfect. Thanks Bryan.

Chris.

[Robin Dunn]

Or for an easier transition you can use the wxGCDC, which implements the
wxDC API using the wxGraphicsContext. (Not always a perfect match, but
it's close.)

--
Robin Dunn
Software Craftsman
http://wxPython.org

[Uli Hertlein]

Robin Dunn wrote:
> Or for an easier transition you can use the wxGCDC, which implements the
> wxDC API using the wxGraphicsContext. (Not always a perfect match, but
> it's close.)

Is there a particular reason why wxGCDC isn't in the online docs yet? Is it
deprecated?

Also, is it possible to use wxGraphicsContext to draw into a bitmap?

I thought I've seen something related to this but couldn't find it anymore in
2.8.10. The wxMemoryDC constructor is MSW-only.

Cheers,
/uli

--
Uli Hertlein
Research and Development email: u...@xdt.com.au
XDT Pty Ltd Web: www.xdt.com.au

[Chris Spencer]

Ok I've started using wxGraphicsContext and it does exactly what I
want, but on Windows it flickers a bit. Here is the paint event of my
custom control:

wxPaintDC dc(this);
wxGraphicsContext *gc = wxGraphicsContext::Create(dc);
gc->SetBrush(wxBrush(wxColour(255, 204, 229, 63)));
gc->DrawBitmap(label_, 0, 0, label_.GetWidth(), label_.GetHeight());
gc->DrawRectangle(0, lines_, label_.GetWidth(), label_.GetHeight() - lines_);

This is called every half second or so using Refresh(). Quite
frequently there will be a very noticable flicker where the bitmap is
shown before the rectangle is drawn, or even a white background is
shown before even the bitmap is drawn. I tried using Freeze() and
Thaw() but it didn't make a difference. I works as intended in wxGTK.

First, am I using wxGraphicsContext correctly, and if so, any ideas
how to fix this?

On a related note, is it necessary to delete the graphics context at
the end of the function?

Chris.

[Robin Dunn]

Uli Hertlein wrote:
> Robin Dunn wrote:
>> Or for an easier transition you can use the wxGCDC, which implements the
>> wxDC API using the wxGraphicsContext. (Not always a perfect match, but
>> it's close.)
>
> Is there a particular reason why wxGCDC isn't in the online docs yet? Is it
> deprecated?
>
> Also, is it possible to use wxGraphicsContext to draw into a bitmap?

Yes.

>
> I thought I've seen something related to this but couldn't find it anymore in
> 2.8.10. The wxMemoryDC constructor is MSW-only.

I think it still works on the other platforms though because the wxDC
inheritance hierarchy is slightly different.

[Robin Dunn]

Chris Spencer wrote:
> Ok I've started using wxGraphicsContext and it does exactly what I
> want, but on Windows it flickers a bit. Here is the paint event of my
> custom control:
>
> wxPaintDC dc(this);
> wxGraphicsContext *gc = wxGraphicsContext::Create(dc);
> gc->SetBrush(wxBrush(wxColour(255, 204, 229, 63)));
> gc->DrawBitmap(label_, 0, 0, label_.GetWidth(), label_.GetHeight());
> gc->DrawRectangle(0, lines_, label_.GetWidth(), label_.GetHeight() - lines_);
>
> This is called every half second or so using Refresh(). Quite
> frequently there will be a very noticable flicker where the bitmap is
> shown before the rectangle is drawn, or even a white background is
> shown before even the bitmap is drawn. I tried using Freeze() and
> Thaw() but it didn't make a difference.

Freeze/Thaw simply suspend the sending of Paint events, but since you
are already in the handler for a paint event it doesn't really do
anything. In a nutshell the issue is that on Windows the intermediate
steps in a set of drawing operations can be flushed to the screen at any
time, and so it's possible to see steps along the way.

> First, am I using wxGraphicsContext correctly, and if so, any ideas
> how to fix this?

Use a wxBufferedPaintDC on Windows instead of the wxPaintDC.

>
> On a related note, is it necessary to delete the graphics context at
> the end of the function?

Yes.

[Chris Spencer]

2009/9/9 Robin Dunn <ro...@alldunn.com>:

> Use a wxBufferedPaintDC on Windows instead of the wxPaintDC.

That's sorted it, thanks for that Robin.