opencv
Pixel Access
Access individual pixel values with cv::Mat::at()
Example
To access pixel values in an OpenCV cv::Mat object, you first have to know the type of your matrix.
The most common types are:
CV_8UC1for 8-bit 1-channel grayscale images;CV_32FC1for 32-bit floating point 1-channel grayscale images;CV_8UC3for 8-bit 3-channel color images; andCV_32FC3for 32-bit floating point 3-channel color images.
The default setting with cv::imread will create a CV_8UC3 matrix.
To access individual pixels, the safest way, though not the most efficient, is to use cv::Mat::at<T>(r,c) method where r is the row of the matrix and c is the column. The template argument depends on the type of the matrix.
Let us say you have a cv::Mat image. According to its type, the access method and the pixel color type will be different.
- For
CV_8UC1:uchar pixelGrayValue = image.at<uchar>(r,c). - For
CV_8UC3:cv::Vec3b pixelColor = image.at<cv::Vec3b>(r,c). Thecv::Vec3bobject represents a triplet ofucharvalues (integers between 0 and 255). - For
CV_32FC1:float pixelGrayValue = image.at<float>(r,c). - For
CV_32FC3:cv::Vec3f pixelColor = image.at<cv::Vec3f>(r,c). Thecv::Vec3fobject represents a triplet offloatvalues.
Note that OpenCV represents images in row-major order, like, e.g. Matlab or as the convention in Algebra. Thus, if your pixel coordinates are (x,y), then you will access the pixel using image.at<..>(y,x).
Alternatively, at<> also support access via a single cv::Point argument.
In this case, the access is done in column-major:
image.at<..>(cv::Point(x,y));
Take a look at OpenCV documentation for more details on this method.
