Image file reading and writing

Overview

C API
iOS glue
// enums

enum cv::ImreadModes;
enum cv::ImwriteFlags;
enum cv::ImwritePAMFlags;
enum cv::ImwritePNGFlags;

// global functions

Mat
cv::imdecode(
    InputArray buf,
    int flags
    );

Mat
cv::imdecode(
    InputArray buf,
    int flags,
    Mat* dst
    );

bool
cv::imencode(
    const String& ext,
    InputArray img,
    std::vector<uchar>& buf,
    const std::vector<int>& params = std::vector<int>()
    );

Mat
cv::imread(
    const String& filename,
    int flags = IMREAD_COLOR
    );

bool
cv::imreadmulti(
    const String& filename,
    std::vector<Mat>& mats,
    int flags = IMREAD_ANYCOLOR
    );

bool
cv::imwrite(
    const String& filename,
    InputArray img,
    const std::vector<int>& params = std::vector<int>()
    );

Detailed Documentation

Global Functions

Mat
cv::imdecode(
    InputArray buf,
    int flags
    )

Reads an image from a buffer in memory.

The function imdecode reads an image from the specified buffer in the memory. If the buffer is too short or contains invalid data, the function returns an empty matrix (Mat::data ==NULL ).

See cv::imread for the list of supported formats and flags description.

In the case of color images, the decoded images will have the channels stored in B G R order.

Parameters:

buf Input array or vector of bytes.
flags The same flags as in cv::imread, see cv::ImreadModes. 
Mat
cv::imdecode(
    InputArray buf,
    int flags,
    Mat* dst
    )

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters:

buf  
flags  
dst The optional output placeholder for the decoded matrix. It can save the image reallocations when the function is called repeatedly for images of the same size. 
bool
cv::imencode(
    const String& ext,
    InputArray img,
    std::vector<uchar>& buf,
    const std::vector<int>& params = std::vector<int>()
    )

Encodes an image into a memory buffer.

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

Parameters:

ext File extension that defines the output format.
img Image to be written.
buf Output buffer resized to fit the compressed image.
params Format-specific parameters. See cv::imwrite and cv::ImwriteFlags. 
Mat
cv::imread(
    const String& filename,
    int flags = IMREAD_COLOR
    )

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)
  • WebP - *.webp (see the Notes section)
  • Portable image format - *.pbm, *.pgm, *.ppm *.pxm, *.pnm (always supported)
  • Sun rasters - *.sr, *.ras (always supported)
  • TIFF files - *.tiff, *.tif (see the Notes section)
  • OpenEXR Image files - *.exr (see the Notes section)
  • Radiance HDR - *.hdr, *.pic (always supported)
  • Raster and Vector geospatial data supported by Gdal (see the Notes section)
  • The function determines the type of an image by the content, not by the file extension.
  • In the case of color images, the decoded images will have the channels stored in B G R order.
  • On Microsoft Windows* OS and MacOSX*, the codecs shipped with an OpenCV image (libjpeg, libpng, libtiff, and libjasper) are used by default. So, OpenCV can always read JPEGs, PNGs, and TIFFs. On MacOSX, there is also an option to use native MacOSX image readers. But beware that currently these native image loaders give images with different pixel values because of the color management embedded into MacOSX.
  • On Linux*, BSD flavors and other Unix-like open-source operating systems, OpenCV looks for codecs supplied with an OS image. Install the relevant packages (do not forget the development files, for example, “libjpeg-dev”, in Debian* and Ubuntu*) to get the codec support or turn on the OPENCV_BUILD_3RDPARTY_LIBS flag in CMake.
  • In the case you set WITH_GDAL flag to true in CMake and IMREAD_LOAD_GDAL to load the image, then GDAL driver will be used in order to decode the image by supporting the following formats: Raster, Vector.
  • If EXIF information are embedded in the image file, the EXIF orientation will be taken into account and thus the image will be rotated accordingly except if the flag IMREAD_IGNORE_ORIENTATION is passed.

Parameters:

filename Name of file to be loaded.
flags Flag that can take values of cv::ImreadModes 
bool
cv::imreadmulti(
    const String& filename,
    std::vector<Mat>& mats,
    int flags = IMREAD_ANYCOLOR
    )

Loads a multi-page image from a file.

The function imreadmulti loads a multi-page image from the specified file into a vector of Mat objects.

Parameters:

filename Name of file to be loaded.
flags Flag that can take values of cv::ImreadModes, default with cv::IMREAD_ANYCOLOR.
mats A vector of Mat objects holding each page, if more than one.

See also:

cv::imread

bool
cv::imwrite(
    const String& filename,
    InputArray img,
    const std::vector<int>& params = std::vector<int>()
    )

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 cv::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 Mat::convertTo, and cv::cvtColor to convert it before saving. Or, use the universal FileStorage 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 <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

void createAlphaMat(Mat &mat)
{
    CV_Assert(mat.channels() == 4);
    for (int i = 0; i < mat.rows; ++i) {
        for (int j = 0; j < mat.cols; ++j) {
            Vec4b& bgra = mat.at<Vec4b>(i, j);
            bgra[0] = UCHAR_MAX; // Blue
            bgra[1] = saturate_cast<uchar>((float (mat.cols - j)) / ((float)mat.cols) * UCHAR_MAX); // Green
            bgra[2] = saturate_cast<uchar>((float (mat.rows - i)) / ((float)mat.rows) * UCHAR_MAX); // Red
            bgra[3] = saturate_cast<uchar>(0.5 * (bgra[1] + bgra[2])); // Alpha
        }
    }
}

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(IMWRITE_PNG_COMPRESSION);
    compression_params.push_back(9);

    try {
        imwrite("alpha.png", mat, compression_params);
    }
    catch (cv::Exception& 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;
}

Parameters:

filename Name of the file.
img Image to be saved.
params Format-specific parameters encoded as pairs (paramId_1, paramValue_1, paramId_2, paramValue_2, … .) see cv::ImwriteFlags