问题
I have a numpy array which came from a cv2.imread and so has dtype = np.uint8 and ndim = 3.
I want to convert it to a Cython unsigned int* for use with an external cpp library.
I am trying cdef unsigned int* buff = <unsigned int*>im.data however I get the error Python objects cannot be cast to pointers of primitive types
What am I doing wrong?
Thanks
回答1:
thanks for your comments. solved by:
cdef np.ndarray[np.uint32_t, ndim=3, mode = 'c'] np_buff = np.ascontiguousarray(im, dtype = np.uint32)
cdef unsigned int* im_buff = <unsigned int*> np_buff.data
回答2:
The more modern way would be to use a memoryview rather than a pointer:
cdef np.uint32_t[:,:,::1] mv_buff = np.ascontiguousarray(im, dtype = np.uint32)
The [:,;,::1] syntax tells Cython the memoryview is 3D and C contiguous in memory. The advantage of defining the type to be a memoryview rather than a numpy array is
- it can accept any type that defines the buffer interface, for example the built in array module, or objects from the PIL imaging library.
- Memoryviews can be passed without holding the GIL, which is useful for parallel code
To get the pointer from the memoryview get the address of the first element:
cdef np.uint32_t* im_buff = &mv_buff[0,0,0]
This is better than doing <np.uint32_t*>mv_buff.data because it avoids a cast, and casts can often hide errors.
来源：https://stackoverflow.com/questions/10718699/convert-numpy-array-to-cython-pointer