Question

我正在尝试学习C和OpenCV。我正在使用以下代码，受到 O'Reilly Learning OpenCV 的启发，但是我收到错误并且图像没有按预期显示。这段代码有问题吗？

# include "cv.h"
# include "highgui.h"

IplImage * doCanny(IplImage *in, double lowThresh, double highThresh, int aperture) {

    if(1 != in->nChannels){
        return 0;
    }

    IplImage *out = cvCreateImage(cvGetSize(in), IPL_DEPTH_8U, 1);
    cvCanny(in, out, lowThresh, highThresh, aperture);
    return out;
}

/* This layout works with other functions like cvPyrdown, but for some reason it doesn't work with cvCanny */   
int main() {

    double lowThresh = 50;
    double highThresh = 150;
    int aperture = 3;

    IplImage *img = cvLoadImage("/tmp/lena.jpg", CV_LOAD_IMAGE_UNCHANGED);
    IplImage *out = doCanny(img, lowThresh, highThresh, aperture);

    cvNamedWindow("Example2-6", CV_LOAD_IMAGE_UNCHANGED);
    cvShowImage("Example2-6", out);
    cvWaitKey(0);
    cvReleaseImage(&img);
    cvReleaseImage(&out);
    cvDestroyWindow("Example2-6");

    return 0;
}

我在Mac OS X Yosemite上获得的输出是：

OpenCV错误：cvGetMat中的空指针（传递NULL数组指针），文件/opt/local/var/macports/build/.../opencv-2.4.11/.../array.cpp，第2382行

Answer 1

您的 jpg 图像很可能是三通道图像。您将此图片作为in传递到doCanny()函数中。以下代码将导致doCanny()函数返回0：

if(1 != in->nChannels){
    return 0;
}

然后您不检查doCanny()函数的返回值是否有效。因此，您将空指针传递给以下函数，这可能会导致您的空指针错误：

cvShowImage("Example2-6", out);

为了让您的代码能够处理三通道图像，您应该将in功能中的doCanny()图像转换为单通道图像：

IplImage * doCanny(IplImage *in, double lowThresh, double highThresh, int aperture) {
    IplImage *out = cvCreateImage(cvGetSize(in), IPL_DEPTH_8U, 1);

    /* Convert the three-channel input image into a single-channel image. */
    IplImage* oneChannel = cvCreateImage(cvGetSize(in), IPL_DEPTH_8U, 1); 
    cvCvtColor(in, oneChannel, CV_BGR2GRAY);

    cvCanny(oneChannel, out, lowThresh, highThresh, aperture);
    return out;
}

或者，您可以将CV_LOAD_IMAGE_GRAYSCALE作为第二个参数传递给cvLoadImage()，直接将输入图像加载为单通道图像：

IplImage *img = cvLoadImage("/tmp/lena.jpg", CV_LOAD_IMAGE_GRAYSCALE);

无论如何，您应该始终检查函数返回的值是否有效。

Answer 2

它对我有用：

 #include "opencv/cv.h"
 #include "opencv/highgui.h"
 #include "opencv2/core/core_c.h"

 using namespace cv;
 IplImage * doCanny(IplImage *in, double lowThresh, double highThresh, int aperture) 
  {
    if(1 != in->nChannels)
    { return 0; }

    IplImage *out = cvCreateImage(cvGetSize(in),IPL_DEPTH_8U, 0);
    cvCanny(in, out, lowThresh, highThresh, aperture);
    return (out);
  }


 int main() 
 {
    double lowThresh = 50;
    double highThresh = 150;
    int aperture = 3;

    IplImage *img = cvLoadImage("/tmp/lena.jpg",0);
    IplImage *out = doCanny(img, lowThresh, highThresh, aperture);

    cvNamedWindow("Example2-6", 0);
    cvShowImage("Example2-6", out);
    cvWaitKey(0);
    cvReleaseImage(&img);
    cvReleaseImage(&out);
    cvDestroyWindow("Example2-6");
    return 0;
 }

如果问题仍然存在，请访问

Opencv error 1

Opencv error2

OpenCV错误：带有canny的空指针

2 个答案: