In wx, bitmap masks are monochrome bit masks.  If the value of the bit 
is 1, then the bitmap pixel is painted.  If it is 0, then the 
destination pixel is left alone.  What you have in the image's alpha 
channel is a blending factor of source and destination:

    color = src.color*(1-src.alpha) + dst.color*src.alpha
    (where X.color is a vector of RGB color information, and
     source.alpha = A / 255.)

Note that alpha channels can implement bitmasks, but can also do a lot 
more in the way of blending two images.

Now, as for alpha blending support in wx, I have not seen it; but the 
support *is* in PIL.  My first guess at the process would be to capture 
the draw event, copy the existing bitmap into a PIL canvas, do the 
blending in PIL, copy the PIL data into an image and then a bitmap, then 
blit the resultant bitmap on in the appropriate place.  Sounds painful!  
But it wouldn't be as bad if you could do all of the drawing in PIL though.


So, in conclusion, I don't think there's a way to do it in wx, but there 
is a path in PIL, albeit a long one.

Good luck!
-Shane


Alberto Griggio wrote:

> Hello everybody,
> first I wanted to report what it seems a bug to me: according to the
> doc, wx.Timer.Start returns True or False depending on the success of
> the operation. But here (wxPython 2.4.0.6  GTK - but I think also MSW)
> it always returns None. I can't test it on 2.4.0.7 at the moment, so
> sorry if it has been already fixed.
> 
> Now, the question. I'm using wxPython together with the PIL to handle
> images. Browsing the wiki, I found that the suggested way to get a
> wx.Bitmap from a PIL image is:
> 
>     img = wx.EmptyImage(*pil_image.size)
>     img.SetData(pil_image.convert('RGB').tostring()) bmp =
>     wx.BitmapFromImage(img)
> 
> This works like a charm on most cases, but if the image is originally
> in RGBA format, it causes a loss of information. This is annoying when
> the image has transparency, since it is completely lost.
> 
> As a workaround, I'm doing something like:
> 
>     if pil_image.mode == 'RGBA':
>         alpha = pil_image.split()[3]
>         mask = wx.EmptyImage(*alpha.size)
>         mask.SetData(alpha.convert('1').convert('RGB').tostring())
>         bmp.SetMask(wx.Mask(wx.BitmapFromImage(mask, 1)))
> 
> But the results are not perfect (see
> http://web.tiscali.it/agriggio/wx_pil.html)
> 
> So I'd like to know if there's a better way to do this.
> 
> Thanks in advance.
> 
> <spam>
> You can safely stop reading here ;-) I'm asking this because I'm
> writing an image viewer. If you want to try it out, you can get it at
> http://web.tiscali.it/agriggio/cornice.html </spam>
> 
> Alberto
> 
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