Reading and Writing Images and Video

imdecode

Reads an image from a buffer in memory.

The function reads an image from the specified buffer in the memory. If the buffer is too short or contains invalid data, the empty matrix/image is returned.

See :ocv:func:`imread` for the list of supported formats and flags description.

imencode

Encodes an image into a memory buffer.

The function compresses the image and stores it in the memory buffer that is resized to fit the result. See :ocv:func:`imwrite` for the list of supported formats and flags description.

imread

Loads an image from a file.

The function imread loads an image from the specified file and returns it. If the image cannot be read (because of missing file, improper permissions, unsupported or invalid format), the function returns an empty matrix ( Mat::data==NULL ). Currently, the following file formats are supported:

• Windows bitmaps - *.bmp, *.dib (always supported)
• JPEG files - *.jpeg, *.jpg, *.jpe (see the Notes section)
• JPEG 2000 files - *.jp2 (see the Notes section)
• Portable Network Graphics - *.png (see the Notes section)
• Portable image format - *.pbm, *.pgm, *.ppm (always supported)
• Sun rasters - *.sr, *.ras (always supported)
• TIFF files - *.tiff, *.tif (see the Notes section)

imwrite

Saves an image to a specified file.

The function imwrite saves the image to the specified file. The image format is chosen based on the filename extension (see :ocv:func:`imread` for the list of extensions). Only 8-bit (or 16-bit unsigned (CV_16U) in case of PNG, JPEG 2000, and TIFF) single-channel or 3-channel (with 'BGR' channel order) images can be saved using this function. If the format, depth or channel order is different, use :ocv:func:`Mat::convertTo` , and :ocv:func:`cvtColor` to convert it before saving. Or, use the universal XML I/O functions to save the image to XML or YAML format.

It is possible to store PNG images with an alpha channel using this function. To do this, create 8-bit (or 16-bit) 4-channel image BGRA, where the alpha channel goes last. Fully transparent pixels should have alpha set to 0, fully opaque pixels should have alpha set to 255/65535. The sample below shows how to create such a BGRA image and store to PNG file. It also demonstrates how to set custom compression parameters

#include <vector>
#include <stdio.h>
#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

void createAlphaMat(Mat &mat)
{
    for (int i = 0; i < mat.rows; ++i) {
        for (int j = 0; j < mat.cols; ++j) {
            Vec4b& rgba = mat.at<Vec4b>(i, j);
            rgba[0] = UCHAR_MAX;
            rgba[1] = saturate_cast<uchar>((float (mat.cols - j)) / ((float)mat.cols) * UCHAR_MAX);
            rgba[2] = saturate_cast<uchar>((float (mat.rows - i)) / ((float)mat.rows) * UCHAR_MAX);
            rgba[3] = saturate_cast<uchar>(0.5 * (rgba[1] + rgba[2]));
        }
    }
}

int main(int argv, char **argc)
{
    // Create mat with alpha channel
    Mat mat(480, 640, CV_8UC4);
    createAlphaMat(mat);

    vector<int> compression_params;
    compression_params.push_back(CV_IMWRITE_PNG_COMPRESSION);
    compression_params.push_back(9);

    try {
        imwrite("alpha.png", mat, compression_params);
    }
    catch (runtime_error& ex) {
        fprintf(stderr, "Exception converting image to PNG format: %s\n", ex.what());
        return 1;
    }

    fprintf(stdout, "Saved PNG file with alpha data.\n");
    return 0;
}

VideoCapture

Class for video capturing from video files or cameras. The class provides C++ API for capturing video from cameras or for reading video files. Here is how the class can be used:

#include "opencv2/opencv.hpp"

using namespace cv;

int main(int, char**)
{
    VideoCapture cap(0); // open the default camera
    if(!cap.isOpened())  // check if we succeeded
        return -1;

    Mat edges;
    namedWindow("edges",1);
    for(;;)
    {
        Mat frame;
        cap >> frame; // get a new frame from camera
        cvtColor(frame, edges, CV_BGR2GRAY);
        GaussianBlur(edges, edges, Size(7,7), 1.5, 1.5);
        Canny(edges, edges, 0, 30, 3);
        imshow("edges", edges);
        if(waitKey(30) >= 0) break;
    }
    // the camera will be deinitialized automatically in VideoCapture destructor
    return 0;
}

VideoCapture::VideoCapture

VideoCapture constructors.

VideoCapture::open

Open video file or a capturing device for video capturing

The methods first call :ocv:func:`VideoCapture::release` to close the already opened file or camera.

VideoCapture::isOpened

Returns true if video capturing has been initialized already.

If the previous call to VideoCapture constructor or VideoCapture::open succeeded, the method returns true.

VideoCapture::release

Closes video file or capturing device.

The methods are automatically called by subsequent :ocv:func:`VideoCapture::open` and by VideoCapture destructor.

The C function also deallocates memory and clears *capture pointer.

VideoCapture::grab

Grabs the next frame from video file or capturing device.

The methods/functions grab the next frame from video file or camera and return true (non-zero) in the case of success.

The primary use of the function is in multi-camera environments, especially when the cameras do not have hardware synchronization. That is, you call VideoCapture::grab() for each camera and after that call the slower method VideoCapture::retrieve() to decode and get frame from each camera. This way the overhead on demosaicing or motion jpeg decompression etc. is eliminated and the retrieved frames from different cameras will be closer in time.

Also, when a connected camera is multi-head (for example, a stereo camera or a Kinect device), the correct way of retrieving data from it is to call VideoCapture::grab first and then call :ocv:func:`VideoCapture::retrieve` one or more times with different values of the channel parameter. See http://code.opencv.org/projects/opencv/repository/revisions/master/entry/samples/cpp/kinect_maps.cpp

VideoCapture::retrieve

Decodes and returns the grabbed video frame.

The methods/functions decode and return the just grabbed frame. If no frames has been grabbed (camera has been disconnected, or there are no more frames in video file), the methods return false and the functions return NULL pointer.

VideoCapture::read

Grabs, decodes and returns the next video frame.

The methods/functions combine :ocv:func:`VideoCapture::grab` and :ocv:func:`VideoCapture::retrieve` in one call. This is the most convenient method for reading video files or capturing data from decode and return the just grabbed frame. If no frames has been grabbed (camera has been disconnected, or there are no more frames in video file), the methods return false and the functions return NULL pointer.

VideoCapture::get

Returns the specified VideoCapture property

Note: When querying a property that is not supported by the backend used by the VideoCapture class, value 0 is returned.

VideoCapture::set

Sets a property in the VideoCapture.

VideoWriter

Video writer class.

VideoWriter::VideoWriter

VideoWriter constructors

The constructors/functions initialize video writers. On Linux FFMPEG is used to write videos; on Windows FFMPEG or VFW is used; on MacOSX QTKit is used.

ReleaseVideoWriter

Releases the AVI writer.

The function should be called after you finished using CvVideoWriter opened with :ocv:cfunc:`CreateVideoWriter`.

VideoWriter::open

Initializes or reinitializes video writer.

The method opens video writer. Parameters are the same as in the constructor :ocv:func:`VideoWriter::VideoWriter`.

VideoWriter::isOpened

Returns true if video writer has been successfully initialized.

VideoWriter::write

Writes the next video frame

The functions/methods write the specified image to video file. It must have the same size as has been specified when opening the video writer.