Asp Forum - Re: Python-list Digest, Vol 52, Issue 19

Jair Trejo

1/2/2008 7:19:00 PM

> > De: Fredrik Lundh <fredrik@pythonware.com>
> A: python-list@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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Re: Python-list Digest, Vol 52, Issue 19

Jair Trejo

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