OpenCV  4.10.0-dev
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An example using PCA for dimensionality reduction while maintaining an amount of variance

* pca.cpp
* Author:
* Kevin Hughes <kevinhughes27[at]gmail[dot]com>
* Special Thanks to:
* Philipp Wagner <bytefish[at]gmx[dot]de>
* This program demonstrates how to use OpenCV PCA with a
* specified amount of variance to retain. The effect
* is illustrated further by using a trackbar to
* change the value for retained variance.
* The program takes as input a text file with each line
* begin the full path to an image. PCA will be performed
* on this list of images. The author recommends using
* the first 15 faces of the AT&T face data set:
* so for example your input text file would look like this:
* <path_to_at&t_faces>/orl_faces/s1/1.pgm
* <path_to_at&t_faces>/orl_faces/s2/1.pgm
* <path_to_at&t_faces>/orl_faces/s3/1.pgm
* <path_to_at&t_faces>/orl_faces/s4/1.pgm
* <path_to_at&t_faces>/orl_faces/s5/1.pgm
* <path_to_at&t_faces>/orl_faces/s6/1.pgm
* <path_to_at&t_faces>/orl_faces/s7/1.pgm
* <path_to_at&t_faces>/orl_faces/s8/1.pgm
* <path_to_at&t_faces>/orl_faces/s9/1.pgm
* <path_to_at&t_faces>/orl_faces/s10/1.pgm
* <path_to_at&t_faces>/orl_faces/s11/1.pgm
* <path_to_at&t_faces>/orl_faces/s12/1.pgm
* <path_to_at&t_faces>/orl_faces/s13/1.pgm
* <path_to_at&t_faces>/orl_faces/s14/1.pgm
* <path_to_at&t_faces>/orl_faces/s15/1.pgm
#include <iostream>
#include <fstream>
#include <sstream>
#include <opencv2/core.hpp>
using namespace cv;
using namespace std;
// Functions
static void read_imgList(const string& filename, vector<Mat>& images) {
std::ifstream file(filename.c_str(), ifstream::in);
if (!file) {
string error_message = "No valid input file was given, please check the given filename.";
CV_Error(Error::StsBadArg, error_message);
string line;
while (getline(file, line)) {
images.push_back(imread(line, IMREAD_GRAYSCALE));
static Mat formatImagesForPCA(const vector<Mat> &data)
Mat dst(static_cast<int>(data.size()), data[0].rows*data[0].cols, CV_32F);
for(unsigned int i = 0; i < data.size(); i++)
Mat image_row = data[i].clone().reshape(1,1);
Mat row_i = dst.row(i);
return dst;
static Mat toGrayscale(InputArray _src) {
Mat src = _src.getMat();
// only allow one channel
if(src.channels() != 1) {
CV_Error(Error::StsBadArg, "Only Matrices with one channel are supported");
// create and return normalized image
Mat dst;
cv::normalize(_src, dst, 0, 255, NORM_MINMAX, CV_8UC1);
return dst;
struct params
Mat data;
int ch;
int rows;
PCA pca;
string winName;
static void onTrackbar(int pos, void* ptr)
cout << "Retained Variance = " << pos << "% ";
cout << "re-calculating PCA..." << std::flush;
double var = pos / 100.0;
struct params *p = (struct params *)ptr;
p->pca = PCA(p->data, cv::Mat(), PCA::DATA_AS_ROW, var);
Mat point = p->pca.project(p->data.row(0));
Mat reconstruction = p->pca.backProject(point);
reconstruction = reconstruction.reshape(p->ch, p->rows);
reconstruction = toGrayscale(reconstruction);
imshow(p->winName, reconstruction);
cout << "done! # of principal components: " << p->pca.eigenvectors.rows << endl;
// Main
int main(int argc, char** argv)
cv::CommandLineParser parser(argc, argv, "{@input||image list}{help h||show help message}");
if (parser.has("help"))
// Get the path to your CSV.
string imgList = parser.get<string>("@input");
if (imgList.empty())
// vector to hold the images
vector<Mat> images;
// Read in the data. This can fail if not valid
try {
read_imgList(imgList, images);
} catch (const cv::Exception& e) {
cerr << "Error opening file \"" << imgList << "\". Reason: " << e.msg << endl;
// Quit if there are not enough images for this demo.
if(images.size() <= 1) {
string error_message = "This demo needs at least 2 images to work. Please add more images to your data set!";
CV_Error(Error::StsError, error_message);
// Reshape and stack images into a rowMatrix
Mat data = formatImagesForPCA(images);
// perform PCA
PCA pca(data, cv::Mat(), PCA::DATA_AS_ROW, 0.95); // trackbar is initially set here, also this is a common value for retainedVariance
// Demonstration of the effect of retainedVariance on the first image
Mat point = pca.project(data.row(0)); // project into the eigenspace, thus the image becomes a "point"
Mat reconstruction = pca.backProject(point); // re-create the image from the "point"
reconstruction = reconstruction.reshape(images[0].channels(), images[0].rows); // reshape from a row vector into image shape
reconstruction = toGrayscale(reconstruction); // re-scale for displaying purposes
// init highgui window
string winName = "Reconstruction | press 'q' to quit";
namedWindow(winName, WINDOW_NORMAL);
// params struct to pass to the trackbar handler
params p; = data; = images[0].channels();
p.rows = images[0].rows;
p.pca = pca;
p.winName = winName;
// create the tracbar
int pos = 95;
createTrackbar("Retained Variance (%)", winName, &pos, 100, onTrackbar, (void*)&p);
// display until user presses q
imshow(winName, reconstruction);
char key = 0;
while(key != 'q')
key = (char)waitKey();
return 0;
Designed for command line parsing.
Definition utility.hpp:820
Class passed to an error.
Definition core.hpp:115
String msg
the formatted error message
Definition core.hpp:134
n-dimensional dense array class
Definition mat.hpp:812
CV_NODISCARD_STD Mat clone() const
Creates a full copy of the array and the underlying data.
Mat row(int y) const
Creates a matrix header for the specified matrix row.
Mat reshape(int cn, int rows=0) const
Changes the shape and/or the number of channels of a 2D matrix without copying the data.
int channels() const
Returns the number of matrix channels.
void convertTo(OutputArray m, int rtype, double alpha=1, double beta=0) const
Converts an array to another data type with optional scaling.
Principal Component Analysis.
Definition core.hpp:2513
This is the proxy class for passing read-only input arrays into OpenCV functions.
Definition mat.hpp:160
Mat getMat(int idx=-1) const
void normalize(InputArray src, InputOutputArray dst, double alpha=1, double beta=0, int norm_type=NORM_L2, int dtype=-1, InputArray mask=noArray())
Normalizes the norm or value range of an array.
#define CV_32F
Definition interface.h:78
#define CV_8UC1
Definition interface.h:88
#define CV_Error(code, msg)
Call the error handler.
Definition base.hpp:320
void imshow(const String &winname, InputArray mat)
Displays an image in the specified window.
int waitKey(int delay=0)
Waits for a pressed key.
void namedWindow(const String &winname, int flags=WINDOW_AUTOSIZE)
Creates a window.
int createTrackbar(const String &trackbarname, const String &winname, int *value, int count, TrackbarCallback onChange=0, void *userdata=0)
Creates a trackbar and attaches it to the specified window.
int main(int argc, char *argv[])
Definition highgui_qt.cpp:3
PyParams params(const std::string &tag, const std::string &model, const std::string &weights, const std::string &device)
"black box" representation of the file storage associated with a file on disk.
Definition core.hpp:102
STL namespace.