Image file reading and writing

Modules
	C API
	iOS glue

Enumerations
enum	cv::ImreadModes {   cv::IMREAD_UNCHANGED = -1,   cv::IMREAD_GRAYSCALE = 0,   cv::IMREAD_COLOR = 1,   cv::IMREAD_ANYDEPTH = 2,   cv::IMREAD_ANYCOLOR = 4,   cv::IMREAD_LOAD_GDAL = 8,   cv::IMREAD_REDUCED_GRAYSCALE_2 = 16,   cv::IMREAD_REDUCED_COLOR_2 = 17,   cv::IMREAD_REDUCED_GRAYSCALE_4 = 32,   cv::IMREAD_REDUCED_COLOR_4 = 33,   cv::IMREAD_REDUCED_GRAYSCALE_8 = 64,   cv::IMREAD_REDUCED_COLOR_8 = 65 }
	Imread flags. More...
enum	cv::ImwriteFlags {   cv::IMWRITE_JPEG_QUALITY = 1,   cv::IMWRITE_JPEG_PROGRESSIVE = 2,   cv::IMWRITE_JPEG_OPTIMIZE = 3,   cv::IMWRITE_JPEG_RST_INTERVAL = 4,   cv::IMWRITE_JPEG_LUMA_QUALITY = 5,   cv::IMWRITE_JPEG_CHROMA_QUALITY = 6,   cv::IMWRITE_PNG_COMPRESSION = 16,   cv::IMWRITE_PNG_STRATEGY = 17,   cv::IMWRITE_PNG_BILEVEL = 18,   cv::IMWRITE_PXM_BINARY = 32,   cv::IMWRITE_WEBP_QUALITY = 64 }
	Imwrite flags. More...
enum	cv::ImwritePNGFlags {   cv::IMWRITE_PNG_STRATEGY_DEFAULT = 0,   cv::IMWRITE_PNG_STRATEGY_FILTERED = 1,   cv::IMWRITE_PNG_STRATEGY_HUFFMAN_ONLY = 2,   cv::IMWRITE_PNG_STRATEGY_RLE = 3,   cv::IMWRITE_PNG_STRATEGY_FIXED = 4 }
	Imwrite PNG specific flags used to tune the compression algorithm. More...

Functions
Mat	cv::imdecode (InputArray buf, int flags)
	Reads an image from a buffer in memory. More...
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 >())
	Encodes an image into a memory buffer. More...
Mat	cv::imread (const String &filename, int flags=IMREAD_COLOR)
	Loads an image from a file. More...
bool	cv::imreadmulti (const String &filename, std::vector< Mat > &mats, int flags=IMREAD_ANYCOLOR)
	Loads a multi-page image from a file. More...
bool	cv::imwrite (const String &filename, InputArray img, const std::vector< int > &params=std::vector< int >())
	Saves an image to a specified file. More...

Detailed Description

Enumeration Type Documentation

enum cv::ImreadModes

Imread flags.

Enumerator
IMREAD_UNCHANGED	If set, return the loaded image as is (with alpha channel, otherwise it gets cropped).
IMREAD_GRAYSCALE	If set, always convert image to the single channel grayscale image.
IMREAD_COLOR	If set, always convert image to the 3 channel BGR color image.
IMREAD_ANYDEPTH	If set, return 16-bit/32-bit image when the input has the corresponding depth, otherwise convert it to 8-bit.
IMREAD_ANYCOLOR	If set, the image is read in any possible color format.
IMREAD_LOAD_GDAL	If set, use the gdal driver for loading the image.
IMREAD_REDUCED_GRAYSCALE_2	If set, always convert image to the single channel grayscale image and the image size reduced 1/2.
IMREAD_REDUCED_COLOR_2	If set, always convert image to the 3 channel BGR color image and the image size reduced 1/2.
IMREAD_REDUCED_GRAYSCALE_4	If set, always convert image to the single channel grayscale image and the image size reduced 1/4.
IMREAD_REDUCED_COLOR_4	If set, always convert image to the 3 channel BGR color image and the image size reduced 1/4.
IMREAD_REDUCED_GRAYSCALE_8	If set, always convert image to the single channel grayscale image and the image size reduced 1/8.
IMREAD_REDUCED_COLOR_8	If set, always convert image to the 3 channel BGR color image and the image size reduced 1/8.

enum cv::ImwriteFlags

Imwrite flags.

Enumerator
IMWRITE_JPEG_QUALITY	For JPEG, it can be a quality from 0 to 100 (the higher is the better). Default value is 95.
IMWRITE_JPEG_PROGRESSIVE	Enable JPEG features, 0 or 1, default is False.
IMWRITE_JPEG_OPTIMIZE	Enable JPEG features, 0 or 1, default is False.
IMWRITE_JPEG_RST_INTERVAL	JPEG restart interval, 0 - 65535, default is 0 - no restart.
IMWRITE_JPEG_LUMA_QUALITY	Separate luma quality level, 0 - 100, default is 0 - don't use.
IMWRITE_JPEG_CHROMA_QUALITY	Separate chroma quality level, 0 - 100, default is 0 - don't use.
IMWRITE_PNG_COMPRESSION	For PNG, it can be the compression level from 0 to 9. A higher value means a smaller size and longer compression time. Default value is 3. Also strategy is changed to IMWRITE_PNG_STRATEGY_DEFAULT (Z_DEFAULT_STRATEGY).
IMWRITE_PNG_STRATEGY	One of cv::ImwritePNGFlags, default is IMWRITE_PNG_STRATEGY_DEFAULT.
IMWRITE_PNG_BILEVEL	Binary level PNG, 0 or 1, default is 0.
IMWRITE_PXM_BINARY	For PPM, PGM, or PBM, it can be a binary format flag, 0 or 1. Default value is 1.
IMWRITE_WEBP_QUALITY	For WEBP, it can be a quality from 1 to 100 (the higher is the better). By default (without any parameter) and for quality above 100 the lossless compression is used.

enum cv::ImwritePNGFlags

Imwrite PNG specific flags used to tune the compression algorithm.

These flags will be modify the way of PNG image compression and will be passed to the underlying zlib processing stage.

• The effect of IMWRITE_PNG_STRATEGY_FILTERED is to force more Huffman coding and less string matching; it is somewhat intermediate between IMWRITE_PNG_STRATEGY_DEFAULT and IMWRITE_PNG_STRATEGY_HUFFMAN_ONLY.
• IMWRITE_PNG_STRATEGY_RLE is designed to be almost as fast as IMWRITE_PNG_STRATEGY_HUFFMAN_ONLY, but give better compression for PNG image data.
• The strategy parameter only affects the compression ratio but not the correctness of the compressed output even if it is not set appropriately.
• IMWRITE_PNG_STRATEGY_FIXED prevents the use of dynamic Huffman codes, allowing for a simpler decoder for special applications.

Enumerator
IMWRITE_PNG_STRATEGY_DEFAULT	Use this value for normal data.
IMWRITE_PNG_STRATEGY_FILTERED	Use this value for data produced by a filter (or predictor).Filtered data consists mostly of small values with a somewhat random distribution. In this case, the compression algorithm is tuned to compress them better.
IMWRITE_PNG_STRATEGY_HUFFMAN_ONLY	Use this value to force Huffman encoding only (no string match).
IMWRITE_PNG_STRATEGY_RLE	Use this value to limit match distances to one (run-length encoding).
IMWRITE_PNG_STRATEGY_FIXED	Using this value prevents the use of dynamic Huffman codes, allowing for a simpler decoder for special applications.

Function Documentation

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.

Note

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)

Note

• 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.

  Parameters

  filename	Name of file to be loaded.
  flags	Flag that can take values of cv::ImreadModes

Examples:

demhist.cpp, distrans.cpp, edge.cpp, ffilldemo.cpp, fitellipse.cpp, grabcut.cpp, houghcircles.cpp, houghlines.cpp, lsd_lines.cpp, morphology2.cpp, pca.cpp, and watershed.cpp.

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