Image Codecs Module

The Image Codecs (imgcodecs) module handles reading and writing images in various formats, supporting both static images and animations with metadata.

Overview

From opencv2/imgcodecs.hpp:48-55:

This module provides image file reading and writing capabilities with support for various formats including JPEG, PNG, TIFF, WebP, AVIF, GIF, and more. It also handles multi-page images, animations, and metadata (EXIF, XMP, ICC profiles).

imread

Load images from files with automatic format detection

imwrite

Save images to files with format-specific parameters

Animations

Read and write animated GIF, AVIF, APNG, and WebP

Metadata

Handle EXIF, XMP, and ICC profile metadata

Reading Images

Basic Image Reading

From imgcodecs.hpp:332-384:

#include <opencv2/imgcodecs.hpp>
using namespace cv;

// Basic usage with default flags
Mat img = imread("photo.jpg");

// Check if image was loaded
if (img.empty()) {
    std::cerr << "Error: Could not load image" << std::endl;
    return -1;
}

// Read as grayscale
Mat gray = imread("photo.jpg", IMREAD_GRAYSCALE);

// Read with alpha channel preserved
Mat rgba = imread("image.png", IMREAD_UNCHANGED);

Image Read Flags

From imgcodecs.hpp:69-85:

enum ImreadModes {
    IMREAD_UNCHANGED = -1,       // Load as-is, including alpha
    IMREAD_GRAYSCALE = 0,        // Convert to grayscale
    IMREAD_COLOR_BGR = 1,        // Convert to 3-channel BGR (default)
    IMREAD_COLOR = 1,            // Same as IMREAD_COLOR_BGR
    IMREAD_ANYDEPTH = 2,         // Load 16-bit/32-bit when available
    IMREAD_ANYCOLOR = 4,         // Read in any color format
    IMREAD_LOAD_GDAL = 8,        // Use GDAL driver
    IMREAD_REDUCED_GRAYSCALE_2 = 16,  // Grayscale at 1/2 size
    IMREAD_REDUCED_COLOR_2 = 17,      // Color at 1/2 size
    IMREAD_REDUCED_GRAYSCALE_4 = 32,  // Grayscale at 1/4 size
    IMREAD_REDUCED_COLOR_4 = 33,      // Color at 1/4 size
    IMREAD_REDUCED_GRAYSCALE_8 = 64,  // Grayscale at 1/8 size
    IMREAD_REDUCED_COLOR_8 = 65,      // Color at 1/8 size
    IMREAD_IGNORE_ORIENTATION = 128,  // Ignore EXIF orientation
    IMREAD_COLOR_RGB = 256           // Convert to RGB instead of BGR
};

// Combine flags with bitwise OR
Mat img = imread("hdr.tiff", IMREAD_ANYDEPTH | IMREAD_ANYCOLOR);

Reading with Metadata

From imgcodecs.hpp:397-412:

// Read image with metadata
std::vector<int> metadataTypes;
std::vector<Mat> metadata;

Mat img = imreadWithMetadata("photo.jpg", metadataTypes,
                             metadata, IMREAD_ANYCOLOR);

// Check what metadata was found
for (size_t i = 0; i < metadataTypes.size(); i++) {
    switch(metadataTypes[i]) {
        case IMAGE_METADATA_EXIF:
            std::cout << "Found EXIF data" << std::endl;
            break;
        case IMAGE_METADATA_XMP:
            std::cout << "Found XMP data" << std::endl;
            break;
        case IMAGE_METADATA_ICCP:
            std::cout << "Found ICC Profile" << std::endl;
            break;
    }
}

Supported Formats

From imgcodecs.hpp:340-355, OpenCV supports:

Always Available

BMP - Windows Bitmap (*.bmp, *.dib)
GIF - Graphics Interchange Format (*.gif)
PBM/PGM/PPM/PXM/PNM - Portable image formats
Sun Rasters - (*.sr, *.ras)
HDR - Radiance HDR (*.hdr, *.pic)

With External Libraries

JPEG - (*.jpeg, *.jpg, *.jpe) - requires libjpeg
JPEG 2000 - (*.jp2) - requires libjasper/OpenJPEG
PNG - (*.png) - requires libpng
WebP - (*.webp) - requires libwebp
AVIF - (*.avif) - requires libavif
TIFF - (*.tiff, *.tif) - requires libtiff
OpenEXR - (*.exr) - requires OpenEXR
PFM - Portable Float Map (*.pfm)
JPEG XL - (*.jxl) - requires libjxl

Format Detection: OpenCV determines the image format by content, not by file extension. The extension is only used when writing images.

Writing Images

Basic Image Writing

From imgcodecs.hpp:511-569:

// Basic usage
Mat img = imread("input.jpg");
bool success = imwrite("output.png", img);

if (!success) {
    std::cerr << "Error: Could not save image" << std::endl;
}

// Write with compression parameters
vector<int> compression_params;
compression_params.push_back(IMWRITE_JPEG_QUALITY);
compression_params.push_back(95);  // Quality 0-100

imwrite("output.jpg", img, compression_params);

Write Parameters

From imgcodecs.hpp:88-130, format-specific parameters:

// JPEG parameters
vector<int> jpeg_params;
jpeg_params.push_back(IMWRITE_JPEG_QUALITY);
jpeg_params.push_back(95);  // Quality: 0-100 (default 95)
jpeg_params.push_back(IMWRITE_JPEG_PROGRESSIVE);
jpeg_params.push_back(1);   // Enable progressive JPEG
jpeg_params.push_back(IMWRITE_JPEG_OPTIMIZE);
jpeg_params.push_back(1);   // Optimize encoding

imwrite("output.jpg", img, jpeg_params);

// PNG parameters
vector<int> png_params;
png_params.push_back(IMWRITE_PNG_COMPRESSION);
png_params.push_back(9);    // Compression: 0-9 (default 1)
png_params.push_back(IMWRITE_PNG_STRATEGY);
png_params.push_back(IMWRITE_PNG_STRATEGY_DEFAULT);

imwrite("output.png", img, png_params);

// WebP parameters
vector<int> webp_params;
webp_params.push_back(IMWRITE_WEBP_QUALITY);
webp_params.push_back(90);  // Quality: 1-100

imwrite("output.webp", img, webp_params);

// AVIF parameters
vector<int> avif_params;
avif_params.push_back(IMWRITE_AVIF_QUALITY);
avif_params.push_back(95);  // Quality: 0-100
avif_params.push_back(IMWRITE_AVIF_SPEED);
avif_params.push_back(9);   // Speed: 0 (slow) to 10 (fast)

imwrite("output.avif", img, avif_params);

// TIFF parameters
vector<int> tiff_params;
tiff_params.push_back(IMWRITE_TIFF_COMPRESSION);
tiff_params.push_back(IMWRITE_TIFF_COMPRESSION_LZW);

imwrite("output.tiff", img, tiff_params);

Saving Images with Alpha Channel

From imgcodecs.hpp:533-536:

// Create 4-channel BGRA image
Mat bgra(height, width, CV_8UC4);

// Fill with color and alpha
for (int y = 0; y < height; y++) {
    for (int x = 0; x < width; x++) {
        bgra.at<Vec4b>(y, x) = Vec4b(
            b_value,    // Blue
            g_value,    // Green  
            r_value,    // Red
            alpha_value // Alpha (0=transparent, 255=opaque)
        );
    }
}

// Save as PNG with alpha
imwrite("transparent.png", bgra);

Multi-Page Images

From imgcodecs.hpp:414-434:

// Read all pages from multi-page TIFF
vector<Mat> pages;
bool success = imreadmulti("document.tiff", pages);

std::cout << "Loaded " << pages.size() << " pages" << std::endl;

// Read specific range of pages
vector<Mat> selectedPages;
int startPage = 2;
int pageCount = 5;
imreadmulti("document.tiff", selectedPages, startPage, pageCount);

// Write multiple images as TIFF
vector<Mat> images;
// ... fill images vector ...
imwrite("output.tiff", images);

// Count images in file
size_t count = imcount("animation.avif");

Animation Support

From imgcodecs.hpp:290-330, OpenCV supports animated images:

Reading Animations

// Animation structure (imgcodecs.hpp:294-330)
Animation anim;

// Load entire animation
if (imreadanimation("animated.gif", anim)) {
    std::cout << "Frames: " << anim.frames.size() << std::endl;
    std::cout << "Loop count: " << anim.loop_count << std::endl;
    
    // Process each frame
    for (size_t i = 0; i < anim.frames.size(); i++) {
        Mat frame = anim.frames[i];
        int duration = anim.durations[i];  // milliseconds
        
        // Process frame...
    }
}

// Load specific range of frames
Animation partialAnim;
int startFrame = 10;
int frameCount = 20;
imreadanimation("video.avif", partialAnim, startFrame, frameCount);

Writing Animations

// Create animation
Animation anim;
anim.loop_count = 0;  // 0 = infinite loop
anim.bgcolor = Scalar(255, 255, 255, 255);  // BGRA background

// Add frames
for (int i = 0; i < numFrames; i++) {
    Mat frame = generateFrame(i);
    anim.frames.push_back(frame);
    anim.durations.push_back(100);  // 100ms per frame
}

// Save as animated GIF
vector<int> gif_params;
gif_params.push_back(IMWRITE_GIF_QUALITY);
gif_params.push_back(5);  // Quality 1-8
imwriteanimation("output.gif", anim, gif_params);

// Save as animated AVIF
vector<int> avif_params;
avif_params.push_back(IMWRITE_AVIF_QUALITY);
avif_params.push_back(90);
imwriteanimation("output.avif", anim, avif_params);

GIF Frame Duration: GIF durations must be multiples of 10ms due to format limitations. Values are automatically rounded down.

Memory Buffer Operations

From imgcodecs.hpp:595-694:

Decoding from Memory

// Decode from buffer
vector<uchar> buffer;
// ... fill buffer with image data ...

Mat img = imdecode(buffer, IMREAD_COLOR);

// Decode with existing Mat (reuses memory)
Mat output;
imdecode(buffer, IMREAD_COLOR, &output);

// Decode with metadata
vector<int> metadataTypes;
vector<Mat> metadata;
Mat img = imdecodeWithMetadata(buffer, metadataTypes,
                                metadata, IMREAD_ANYCOLOR);

Encoding to Memory

// Encode to buffer
vector<uchar> buffer;
vector<int> params;
params.push_back(IMWRITE_JPEG_QUALITY);
params.push_back(90);

bool success = imencode(".jpg", img, buffer, params);

// Buffer now contains JPEG-encoded image
std::cout << "Encoded size: " << buffer.size() << " bytes" << std::endl;

// Can send over network, save to database, etc.

Format Capabilities

Checking Format Support

From imgcodecs.hpp:696-726:

// Check if file can be read
if (haveImageReader("photo.jpg")) {
    Mat img = imread("photo.jpg");
}

// Check if format can be written
if (haveImageWriter(".png")) {
    imwrite("output.png", img);
}

// Check by extension
if (haveImageWriter(".avif")) {
    std::cout << "AVIF encoding is available" << std::endl;
}

Image Collections

From imgcodecs.hpp:728-758, iterate through multi-page images:

// Create collection (lazy loading)
ImageCollection collection("document.tiff");

// Iterate through pages
for (auto it = collection.begin(); it != collection.end(); ++it) {
    Mat page = *it;
    // Process page...
}

// Random access (less efficient)
Mat page5 = *collection.at(5);

// Release cached pages to save memory
collection.releaseCache();

Practical Examples

Convert Image Format

#include <opencv2/imgcodecs.hpp>

int main() {
    // Read any format
    Mat img = imread("input.bmp", IMREAD_UNCHANGED);
    
    if (img.empty()) {
        std::cerr << "Failed to load image" << std::endl;
        return -1;
    }
    
    // Save as different format with quality settings
    vector<int> params;
    params.push_back(IMWRITE_PNG_COMPRESSION);
    params.push_back(9);  // Maximum compression
    
    if (imwrite("output.png", img, params)) {
        std::cout << "Conversion successful" << std::endl;
    }
    
    return 0;
}

Batch Process Images

#include <opencv2/imgcodecs.hpp>
#include <opencv2/imgproc.hpp>
#include <filesystem>

namespace fs = std::filesystem;

void processDirectory(const string& inputDir, const string& outputDir) {
    for (const auto& entry : fs::directory_iterator(inputDir)) {
        string path = entry.path().string();
        
        // Check if file can be read
        if (!haveImageReader(path)) continue;
        
        // Load and process
        Mat img = imread(path, IMREAD_COLOR);
        if (img.empty()) continue;
        
        // Resize
        Mat resized;
        resize(img, resized, Size(800, 600));
        
        // Save with JPEG compression
        string filename = entry.path().filename().string();
        string outPath = outputDir + "/" + filename + ".jpg";
        
        vector<int> params = {IMWRITE_JPEG_QUALITY, 85};
        imwrite(outPath, resized, params);
    }
}

Create Thumbnail Grid

#include <opencv2/imgcodecs.hpp>
#include <opencv2/imgproc.hpp>

Mat createThumbnailGrid(const vector<string>& imagePaths,
                        int thumbSize, int cols) {
    vector<Mat> thumbs;
    
    // Load and resize images
    for (const auto& path : imagePaths) {
        Mat img = imread(path, IMREAD_COLOR);
        if (img.empty()) continue;
        
        Mat thumb;
        resize(img, thumb, Size(thumbSize, thumbSize));
        thumbs.push_back(thumb);
    }
    
    // Calculate grid dimensions
    int rows = (thumbs.size() + cols - 1) / cols;
    Mat grid(rows * thumbSize, cols * thumbSize, CV_8UC3,
             Scalar(255, 255, 255));
    
    // Place thumbnails
    for (size_t i = 0; i < thumbs.size(); i++) {
        int row = i / cols;
        int col = i % cols;
        Rect roi(col * thumbSize, row * thumbSize,
                thumbSize, thumbSize);
        thumbs[i].copyTo(grid(roi));
    }
    
    return grid;
}

Best Practices

Always Check Return Values:

Mat img = imread("photo.jpg");
if (img.empty()) {
    // Handle error - file might not exist or be corrupted
}

bool success = imwrite("output.png", img);
if (!success) {
    // Handle error - disk full, permissions, invalid path
}

Choose the Right Format:

JPEG - Best for photographs, lossy compression
PNG - Lossless, supports transparency, larger files
WebP/AVIF - Modern formats with better compression
TIFF - Multi-page, lossless, supports 16/32-bit
EXR - High dynamic range (HDR) images

Memory Considerations: For large images or batch processing:

// Read at reduced size
Mat thumb = imread("large.jpg", IMREAD_REDUCED_COLOR_4);

// Or use ImageCollection for multi-page
ImageCollection col("huge.tiff");
for (auto it = col.begin(); it != col.end(); ++it) {
    processPage(*it);
    col.releaseCache();  // Free memory
}

Color Channel Order

Important: BGR vs RGBOpenCV uses BGR channel order by default (not RGB). When reading images:

// Image is loaded as BGR
Mat img = imread("photo.jpg");
// img[y][x] = [Blue, Green, Red]

// To get RGB order
Mat rgb = imread("photo.jpg", IMREAD_COLOR_RGB);
// Or convert after loading
cvtColor(img, rgb, COLOR_BGR2RGB);

Core Module - Mat data structure
Image Processing - Process loaded images
High-Level GUI - Display images

Source Reference

Main header: ~/workspace/source/modules/imgcodecs/include/opencv2/imgcodecs.hpp

​Overview

imread

imwrite

Animations

Metadata

​Reading Images

​Basic Image Reading

​Image Read Flags

​Reading with Metadata

​Supported Formats

​Always Available

​With External Libraries

​Writing Images

​Basic Image Writing

​Write Parameters

​Saving Images with Alpha Channel

​Multi-Page Images

​Animation Support

​Reading Animations

​Writing Animations

​Memory Buffer Operations

​Decoding from Memory

​Encoding to Memory

​Format Capabilities

​Checking Format Support

​Image Collections

​Practical Examples

​Convert Image Format

​Batch Process Images

​Create Thumbnail Grid

​Best Practices

​Color Channel Order

​Related Modules

​Source Reference

Overview

Reading Images

Basic Image Reading

Image Read Flags

Reading with Metadata

Supported Formats

Always Available

With External Libraries

Writing Images

Basic Image Writing

Write Parameters

Saving Images with Alpha Channel

Multi-Page Images

Animation Support

Reading Animations

Writing Animations

Memory Buffer Operations

Decoding from Memory

Encoding to Memory

Format Capabilities

Checking Format Support

Image Collections

Practical Examples

Convert Image Format

Batch Process Images

Create Thumbnail Grid

Best Practices

Color Channel Order

Related Modules

Source Reference