How to read a image sequence with OpenCV

This rough post contains information about the stored image sequence management for still images processing in OpenCV. This post is a reminder, not a tutorial.

Simple assumptions

• Your image sequence contains images with file name like this: image000001.png. Normally you store images with this sequential file name convention (consider also the case of auto file naming)
• Your images are in png format (It is only for this example)
• Your folder absolute path is /folder1/folder2/folder3/ (please consider that Unix and Win paths are different, so use carefully the \ or / chars)

ImageList file

you can use the image list file creator inside the opencv samples, to build a xml file containing the list of the images inside the target folder.
Then you can import the list of images as a vector inside your code

VideoCapture

0. some variables
cv::Mat image_frame;
std::string str_sequenceFileName = "/folder1/folder2/folder3/image%06d.png ";

1. create a videocapture object and then open it, check if is opened and eventually send an error on the shell.

cv::VideoCapture videoCapture;
videoCapture.open(str_sequenceFileName); // open the default camera
if (!videoCapture.isOpened()) {  // check if we succeeded
std::cerr << "[EE] opening video capture OFFLINE device" << std::endl;
return -1;
}
}

2. use the videocapture object instance to grab the current frame. the videocapture grabs the frame from the image sequence folder. You can use it directly inside your main image processing loop.

videoCapture >> image_frame;

Glob

0. some variables
cv::String str_sequenceFolder = "/folder1/folder2/folder3"
cv::String glob_folder = str_sequenceFolder + "/*.png";
std::vector<cv::String> imageFileNamesList;
imageFileNamesList.clear();
cv::Mat image_frame;

1. fill the vector with the image names inside the sequence folder, to obtain the image names list vector

cv::glob(glob_folder, imageFileNamesList);

2. sort the vector (maybe it is not useful)

std::sort(imageFileNamesList.begin(), imageFileNamesList.end());

3. read the current image from the sequence folder

for (int n_frameNumber=0; n_frameNumber < imageFileNamesList.size(); ++n_frameNumber) 
{
    image_frame = cv::imread(imageFileNamesList[n_frameNumber]);
    //some image processing
}