如何把OpenCV Python获取的图像传递到C层处理

用OpenCV Python来开发，如果想要用到一些C/C++的图像处理库，就需要创建Python的C/C++扩展，然后把数据从Python传递到底层处理。这里分享下如何在C/C++层获取数据。

参考原文：How to Convert OpenCV Image Data from Python to C

作者：Xiao Ling

翻译：yushulx

开发环境

Dynamsoft Barcode Reader 4.2
Python version: 2.7.0
OpenCV version: 2.4.10. Download cv2.pyd
Windows 10
USB webcam

Python C/C++扩展

把DynamsoftBarcodeReaderx86.dll和cv2.pyd拷贝到目录Python27\Lib\site-packages。

OpenCV Python获取的图像数据类型是numpy.ndarray:

> rval, frame = vc.read();
> print type(frame)
> <type 'numpy.ndarray'>

在C层我们希望能获取到数据的指针。查看OpenCV源码文件opencv\modules\python\src2\cv2.cv.hpp可以找到方法：

PyObject *o;
if (!PyArg_ParseTuple(args, "O", &o))
return NULL;

PyObject *ao = PyObject_GetAttrString(o, "__array_struct__");
PyObject *retval;

if ((ao == NULL) || !PyCObject_Check(ao)) {
PyErr_SetString(PyExc_TypeError, "object does not have array interface");
return NULL;
}

PyArrayInterface *pai = (PyArrayInterface*)PyCObject_AsVoidPtr(ao);
if (pai->two != 2) {
PyErr_SetString(PyExc_TypeError, "object does not have array interface");
Py_DECREF(ao);
return NULL;
}

// Construct data with header info and image data
char *buffer = (char*)pai->data; // The address of image data
int width = pai->shape[1];       // image width
int height = pai->shape[0];      // image height
int size = pai->strides[0] * pai->shape[0]; // image size = stride * height

这样就可以了。现在可以用这个数据做点事情，比如调用barcode接口来做检测。我依然用Dynamsoft Barcode Reader SDK做示例。首先需要构建一下数据：

char *total = (char *)malloc(size + 40); // buffer size = image size + header size
memset(total, 0, size + 40);
BITMAPINFOHEADER bitmap_info = {40, width, height, 0, 24, 0, size, 0, 0, 0, 0};
memcpy(total, &bitmap_info, 40);

// Copy image data to buffer from bottom to top
char *data = total + 40;
int stride = pai->strides[0];
for (int i = 1; i <= height; i++) {
memcpy(data, buffer + stride * (height - i), stride);
data += stride;
}

接下来就可以检测barcode了：

// Dynamsoft Barcode Reader initialization
__int64 llFormat = (OneD | QR_CODE | PDF417 | DATAMATRIX);
int iMaxCount = 0x7FFFFFFF;
ReaderOptions ro = {0};
pBarcodeResultArray pResults = NULL;
ro.llBarcodeFormat = llFormat;
ro.iMaxBarcodesNumPerPage = iMaxCount;
printf("width: %d, height: %d, size:%d\n", width, height, size);
int iRet = DBR_DecodeBuffer((unsigned char *)total, size + 40, &ro, &pResults);
printf("DBR_DecodeBuffer ret: %d\n", iRet);
free(total); // Do not forget to release the constructed buffer

// Get results
int count = pResults->iBarcodeCount;
pBarcodeResult* ppBarcodes = pResults->ppBarcodes;
pBarcodeResult tmp = NULL;
retval = PyList_New(count); // The returned Python object
PyObject* result = NULL;
for (int i = 0; i < count; i++)
{
tmp = ppBarcodes[i];
result = PyString_FromString(tmp->pBarcodeData);
printf("result: %s\n", tmp->pBarcodeData);
PyList_SetItem(retval, i, Py_BuildValue("iN", (int)tmp->llFormat, result)); // Add results to list
}
// release memory
DBR_FreeBarcodeResults(&pResults);

在Windows上构建Python扩展需要先设置一下，不然会出错。我使用Visual Studio 2015。命令行如下：

SET VS90COMNTOOLS=%VS140COMNTOOLS%
python setup.py build install

好了。现在可以用Python脚本来调用了。首先打开摄像头：

import cv2
from dbr import *
import time

vc = cv2.VideoCapture(0)

接下来读取一帧的数据：

cv2.imshow(windowName, frame)
rval, frame = vc.read();

现在可以实时检测barcode了：

initLicense("<license>") # Invalid license is fine.
results = decodeBuffer(frame)
if (len(results) > 0):
print "Total count: " + str(len(results))
for result in results:
print "Type: " + types[result[0]]
print "Value: " + result[1] + "\n"

 

源码

https://github.com/yushulx/opencv-python-webcam-barcode-reader