passing multi dimensional array from cffi to c

[Prashant Saxena]

Hi,

I have a numpy array like this:

buffer = numpy.ones((height,width,3), dtype=numpy.uint8)

a structure that represent a pixel like this:

typedef struct{
    unsigned char r, g, b;
} pixel;

Now how do pass this array from python-cffi to a c function where I can easily set values. Something like this:

buffer[23][34].r = 255;
buffer[23][34].g = 0;
buffer[23][34].b = 255;

Regards
Prashant

[Armin Rigo]

Hi,

On 7 May 2016 at 13:19, Prashant Saxena <anima...@gmail.com> wrote:
> Now how do pass this array from python-cffi to a c function where I can
> easily set values. Something like this:
> buffer[23][34].r = 255;
> buffer[23][34].g = 0;
> buffer[23][34].b = 255;

NumPy arrays support the buffer interface, so you can use ffi.from_buffer():
http://cffi.readthedocs.io/en/latest/ref.html#ffi-buffer-ffi-from-buffer

Example: x=lib.somefunction(ffi.from_buffer(my_numpy_array))


A bientôt,

Armin

[Prashant Saxena]

Thanks Armin,

Could you please let me know the definition of 'somefunction' in C in this case?
I tried this way:
void somefunction(unsigned char *buffer){
pixel *p;
p = (pixel*)buffer[0][0];
p->r, p->g, p->b
...
}

Prashant

[Armin Rigo]

Hi Prashant,

On 7 May 2016 at 13:46, Prashant Saxena <anima...@gmail.com> wrote:
> Could you please let me know the definition of 'somefunction' in C in this
> case?

That's a C question. It is a bit outside the scope of this mailing
list, but let me try.

> void somefunction(unsigned char *buffer){
> pixel *p;
> p = (pixel*)buffer[0][0];
> p->r, p->g, p->b
> ...
> }

An "unsigned char *" argument is a pointer to the start of the data,
which is a contiguous array, "rgbrgbrgbrgb...". There is no
information about the fact that this was originally organized as a 2D
matrix. If you need this, you need to pass extra arguments to the C
function, like "int width, int height". Then you access the pixel at
(x, y) with this kind of code:

pixel *corner, *line_start, *p;
corner = (pixel *)buffer;
line_start = corner + y * width;
p = line_start + x;


A bientôt,

Armin.