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From: News123 on 3 Mar 2010 03:43
Hi Tim,

Tim Roberts wrote:
> News123 <news123(a)free.fr> wrote:
>> I created a grayscale image with PIL.
>>
>> Now I would like to write a C function, which reads a;most all pixels
>> and will modify a few of them.
>>
>> My current approach is:
>> - transform the image to a string()
>> - create a byte array huge enough to contain the resulting image
>> - call my c_function, which copies over the entire image in order
>> to modify a few pixels
>> How can I achieve this with the least amount of copies?
>
> If it were me, I'd be tempted to go peek at the source code for PIL, then
> pass the Image object to my C routine and poke at the innards to find the
> buffer with the pixels.

Yes, this might be an option
Somehow though it didn't feel right for me to depend on internal non
documented data types, which might change between releases of PIL.

N
From: Stefan Behnel on 3 Mar 2010 04:10
News123, 03.03.2010 01:38:
> I created a grayscale image with PIL.
>
> Now I would like to write a C function, which reads a;most all pixels
> and will modify a few of them.
>
> My current approach is:
> - transform the image to a string()
> - create a byte array huge enough to contain the resulting image
> - call my c_function, which copies over the entire image in order
> to modify a few pixels
> How can I achieve this with the least amount of copies?

Take a look at Cython instead, it will allow you to access PIL's image
buffer directly, instead of copying the data. It will also simplify and
speed up your C wrapper code.

Stefan

From: News123 on 3 Mar 2010 04:37
Hi Stefan,

Stefan Behnel wrote:
> News123, 03.03.2010 01:38:
>> I created a grayscale image with PIL.
>>
>> Now I would like to write a C function, which reads a;most all pixels
>> and will modify a few of them.
>>
>> My current approach is:
>> - transform the image to a string()
>> - create a byte array huge enough to contain the resulting image
>> - call my c_function, which copies over the entire image in order
>> to modify a few pixels
>> How can I achieve this with the least amount of copies?
>
> Take a look at Cython instead, it will allow you to access PIL's image
> buffer directly, instead of copying the data. It will also simplify and
> speed up your C wrapper code.
>
> Stefan
>

I don't know Cython. Having looked at the web site I'm not entirely
sure, I understood your suggestion.

Do you mean
- to stay with Python 2.6 and to implement only my extension in Cython.
This might be very attractive. If yes, can you recommend some url's /
tutorials / etc. which might help to implement a cython extension for
python2.6. which reads and modifies a pixel of a PIL image.

or do you mean
- to switch entirely from Python 2.6 to Cython. I would be reluctant to
do so, as I have already a lot of existing code. and I do not have the
time to check the entire code base for portability issues



thanks in advance


N
From: Stefan Behnel on 3 Mar 2010 05:10
News123, 03.03.2010 10:37:
> Stefan Behnel wrote:
>> Take a look at Cython instead, it will allow you to access PIL's image
>> buffer directly, instead of copying the data. It will also simplify and
>> speed up your C wrapper code.
>
> I don't know Cython. Having looked at the web site I'm not entirely
> sure, I understood your suggestion.
>
> Do you mean
> - to stay with Python 2.6 and to implement only my extension in Cython.

Absolutely.


> This might be very attractive. If yes, can you recommend some url's /
> tutorials / etc. which might help to implement a cython extension for
> python2.6. which reads and modifies a pixel of a PIL image.

Check out the tutorial on the web site, and ask on the cython-users mailing
list about integration with PIL. I'm sure you'll get some useful examples.

Here's something closely related, although it doesn't really use PIL (but
the mechanisms are the same):

http://wiki.cython.org/examples/mandelbrot


> or do you mean
> - to switch entirely from Python 2.6 to Cython. I would be reluctant to
> do so, as I have already a lot of existing code. and I do not have the
> time to check the entire code base for portability issues

The nice thing about Cython is that you always have a normal CPython
interpreter running, so you can split your code between the interpreter and
the compiler at any level of granularity. However, the advice was really to
write your image processing algorithm in Cython, not your entire program.

Stefan

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