OpenCV 错误：带有 canny 的空指针答案

【问题标题】：OpenCV Error: Null pointer with cannyOpenCV 错误：带有 canny 的空指针
【发布时间】：2015-07-26 10:37:47
【问题描述】：

我正在尝试学习 C 和 OpenCV。我正在使用以下代码，其灵感来自 O'Reilly Learning OpenCV，但出现错误并且图像未按预期显示。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 上得到的输出是：

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

【问题讨论】：

请使用 opencv 的 c++ api，不要使用过时的，不再维护的 c-api

标签： c opencv

【解决方案1】：

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

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

那么你不检查你的doCanny()函数的返回值是否有效。因此，您将一个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);

为了使您的代码能够处理三通道图像，您应该将doCanny() 函数中的in 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，您可以通过将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。

【讨论】：

【解决方案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;
 }

如果问题继续，请访问

Opencv error 1

Opencv error2

【讨论】：