C++ 在 opencv 中使用 Mat::at(i,j) 作为二维 Mat 对象
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Using Mat::at(i,j) in opencv for a 2-D Mat object
提问by Chani
I am using Ubuntu 12.04 and OpenCV 2
我正在使用 Ubuntu 12.04 和 OpenCV 2
I have written the following code :
我写了以下代码：
IplImage* img =0;
img = cvLoadImage("nature.jpg");
if(img != 0)
{
Mat Img_mat(img);
std::vector<Mat> RGB;
split(Img_mat, RGB);
int data = (RGB[0]).at<int>(i,j)); /*Where i, j are inside the bounds of the matrix size .. i have checked this*/
}
The problem is I am getting negative values and very large values in the data variable. I think I have made some mistake somewhere. Can you please point it out.
I have been reading the documentation (I have not finished it fully.. it is quite large. ) But from what I have read, this should work. But it isnt. What is going wrong here?
问题是我在数据变量中得到负值和非常大的值。我想我在某个地方犯了一些错误。你能不能指出来。
我一直在阅读文档（我还没有完全读完……它很大。）但是从我读过的内容来看，这应该有效。但它不是。这里出了什么问题？
回答by rotating_image
Img_matis a 3 channeled image. Each channel consists of pixel values ucharin data type.
So with split(Img_mat, BGR)the Img_matis split into 3 planes of blue, green and red which are collectively stored in a vector BGR. So BGR[0]is the first (blue) plane with uchardata type pixels...hence it will be
Img_mat是一个 3 通道图像。每个通道由uchar数据类型的像素值组成。因此，split(Img_mat, BGR)它Img_mat被分成蓝色、绿色和红色的 3 个平面，它们共同存储在一个向量中BGR。所以，BGR[0]是第一个（蓝色）平面与uchar数据类型像素...因此，这将是
int dataB = (int)BGR[0].at<uchar>(i,j);
int dataG = (int)BGR[1].at<uchar>(i,j);
so on...
很快...
回答by flowfree
You have to specify the correct type for cv::Mat::at(i,j). You are accessing the pixel as int, while it should be a vector of uchar. Your code should look something like this:
您必须为 指定正确的类型cv::Mat::at(i,j)。您正在访问像素为int，而它应该是 的向量uchar。您的代码应如下所示：
IplImage* img = 0;
img = cvLoadImage("nature.jpg");
if(img != 0)
{
Mat Img_mat(img);
std::vector<Mat> BGR;
split(Img_mat, BGR);
Vec3b data = BGR[0].at<Vec3b>(i,j);
// data[0] -> blue
// data[1] -> green
// data[2] -> red
}
回答by aLu
Why are you loading an IplImage first? You are mixing the C and C++ interfaces. Loading a cv::Mat with imread directly would be more straight-forward.
为什么要先加载 IplImage？您正在混合 C 和 C++ 接口。直接用 imread 加载 cv::Mat 会更直接。
This way you can also specify the type and use the according type in your at call.
通过这种方式，您还可以指定类型并在通话中使用相应的类型。
