Python-list Digest, Vol 52, Issue 19

Jair Trejo jairtrejo at yahoo.com.mx
Wed Jan 2 20:19:16 CET 2008


> > De: Fredrik Lundh <fredrik at pythonware.com>
> A: python-list at python.org
> Fecha: Wed, 02 Jan 2008 15:39:11 +0100
> Asunto: Re: PyCairo, PIL and StringIO
> 
> Jair Trejo wrote:
> 
> > I'm doing some image processing in PIL, and I want
> to
> > display the results in a GTK window using PyCairo,
> so
> > I create a Cairo image surface from the PIL Image
> like
> > this:
> > data
> >         mfile = StringIO.StringIO()
> >         final.save(mfile, format="PNG")
> >         ima =
> > cairo.ImageSurface.create_from_png(mfile)
> >         mfile.close()
> >         return ima
> > 
> > Where final is a PIL image. The problem is, I get
> a
> > IOError: error while reading from Input Stream.
> > 
> > ¿Any idea of why is this happening?
> 
> "save" leaves the file pointer at an undefined
> position (usually at the 
> end), so my guess is that you have to rewind the
> file before you pass it
> to the next function:
> 
>      final.save(mfile, format="PNG")
>      mfile.seek(0) # rewind
> 
> also note that compressing and decompressing will
> introduce unnecessary 
> overhead; chances are that you might get a lot
> better performance if you 
> just shuffle the raw pixels.  I haven't used PyCairo
> myself, but from a 
> quick look at the ImageSurface documentation,
> something like this should 
> work (untested):
> 
>     if final.mode != "RGB":
>         final = final.convert("RGB")
>     w, h = final.size
>     data = final.tostring() # get packed RGB buffer
>     ima = cairo.ImageSurface.create(data,
> FORMAT_RGB24, w, h, w*3)
> 
> (tweak as necessary)
> 
> </F>

Thank, you, it worked!
I tried rewinding the file, and it worked OK. But you
were right about performance issues, so I tweaked your
code and left it as:

from array import array

...

        w,h = final.size
        data=array('c')
        data.fromstring(final.tostring())
        ima=cairo.ImageSurface.create_for_data(data,
cairo.FORMAT_ARGB32,w,h,w*4)
        return ima

Which is around 10 times faster. The problem is, it
swaps the R and B bands. I could probably write my own
Image.tostring(), but i found it more convenient to
swap the channels before processing, and it worked
just fine.


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