TIL Day 25

2023.10.24

Today's agenda

1. Color image process
2. Color space
3. Color extract

1. Color image process

In opencv using CV_8UC3 type can make 256 256 256 color using red, blue, green space
Actually we called RGB space in real life, but in opencv called BGR space

RGB space to Greyscale space

#include <iostream>
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

#define RGB2GRAY(r, g, b) ((4899*(r) + 9617*(g) + 1868*(b)) >> 14)

int main()
{
	Mat src = imread("mandrill.bmp");

	if (src.empty()) {
		cerr << "Image laod failed!" << endl;
		return;
	}

#if 0
	Mat dst;
	cvtColor(src, dst, COLOR_BGR2GRAY);
#else
	Mat dst(src.rows, src.cols, CV_8UC1);

	for (int y = 0; y < src.rows; y++) {
		for (int x = 0; x < src.cols; x++) {
			uchar b = src.at<Vec3b>(y, x)[0];
			uchar g = src.at<Vec3b>(y, x)[1];
			uchar r = src.at<Vec3b>(y, x)[2];

			//uchar gray = (uchar)(0.299 * r + 0.587 * g + 0.114 * b + 0.5);
			//uchar gray = (uchar)((299 * r + 587 * g + 114 * b) / 1000);
			//dst.at<uchar>(y, x) = gray;
			
			dst.at<uchar>(y, x) = (uchar)RGB2GRAY(r, g, b);
		}
	}
#endif

	imshow("src", src);
	imshow("dst", dst);
	waitKey();
	destroyAllWindows();
}

void cvtColor(InputArray src, OutputArray dst, int code, int dstCn = 0);

parameters

• src : source image
• dst : destination image
• code : color convert code
  - COLOR_BGR2GRAY / COLOR_GRAY2BGR : BGR <-> GRAY
  • COLOR_BGR2RGB / COLOR_RGB2BGR : BGR <-> RGB
  • COLOR_BSG2HSV / COLOR_HSV2BGR : BGR <-> HSV
  • COLOR_BGR2YCrCb / COLOR_YCrCb2BGR : BGR <-> YCrCb
• dstCn : channel of destination image. if 0 decided automatically

2. Color space

#include <iostream>
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

void split_bgr();
void split_hsv();
void split_ycrcb();

int main()
{
	split_bgr();
	split_hsv();
	split_ycrcb();
}

void split_bgr()
{
	Mat src = imread("candies.png", IMREAD_COLOR);

	if (src.empty()) {
		cerr << "Image load failed!" << endl;
		return;
	}

	vector<Mat> bgr_planes;
	split(src, bgr_planes);

	imshow("src", src);
	imshow("B plane", bgr_planes[0]);
	imshow("G plane", bgr_planes[1]);
	imshow("R plane", bgr_planes[2]);

	waitKey();
	destroyAllWindows();
}

void split_hsv()
{
	Mat src = imread("candies.png", IMREAD_COLOR);

	if (src.empty()) {
		cerr << "Image load failed!" << endl;
		return;
	}

	Mat src_hsv;
	cvtColor(src, src_hsv, COLOR_BGR2HSV);

	vector<Mat> hsv_planes;
	split(src_hsv, hsv_planes);

	imshow("src", src);
	imshow("H plane", hsv_planes[0]);
	imshow("S plane", hsv_planes[1]);
	imshow("V plane", hsv_planes[2]);

	waitKey();
	destroyAllWindows();
}

void split_ycrcb()
{
	Mat src = imread("candies.png", IMREAD_COLOR);

	if (src.empty()) {
		cerr << "Image load failed!" << endl;
		return;
	}

	Mat src_ycrcb;
	cvtColor(src, src_ycrcb, COLOR_BGR2YCrCb);

	vector<Mat> ycrcb_planes;
	split(src_ycrcb, ycrcb_planes);

	imshow("src", src);
	imshow("Y plane", ycrcb_planes[0]);
	imshow("Cr plane", ycrcb_planes[1]);
	imshow("Cb plane", ycrcb_planes[2]);

	waitKey();
	destroyAllWindows();
}

Color image histogram equzlization

Previous i leaned only do histogram equalization on greyscale image
but if i want to do histogram equalization on color image, i have to split all color space and do histogram equalization and merge
below image is doing histogram equalization on rgb image but other color type(YCrCb, HSV) also do same order.

void equalizeHist(InputArray src, OutputArray dst);

parameters

• src : input image(8bit 1channel)
• dst : outptu image(8bit 1channel)

3. Color extract

void inRange(InputArray src, InputArray lowerb, InputArray upperb, OutputArray dst);

parameters

• src : source image
• lowerb : lower value(Mat or Scalar)
• upperb : upper value(mat or Scalar)
• dst : same size of src.(CV_8UC1)

#include <iostream>
#include <algorithm>
#include "opencv2/opencv.hpp"

using namespace std;
using namespace cv;

int pos_hue1 = 50, pos_hue2 = 80, pos_sat1 = 150, pos_sat2 = 255;
Mat src, src_hsv, dst, mask;

void on_hsv_changed(int, void*)
{
	Scalar lowerb(pos_hue1, pos_sat1, 0);
	Scalar upperb(pos_hue2, pos_sat2, 255);
	inRange(src_hsv, lowerb, upperb, mask);

	cvtColor(src, dst, COLOR_BGR2GRAY);
	cvtColor(dst, dst, COLOR_GRAY2BGR);
	src.copyTo(dst, mask);

	imshow("mask", mask);
	imshow("dst", dst);
}

int main()
{
	src = imread("candies.png", IMREAD_COLOR);

	if (src.empty()) {
		cerr << "Image load failed!" << endl;
		return -1;
	}

	cvtColor(src, src_hsv, COLOR_BGR2HSV);

	namedWindow("src");
	namedWindow("mask");
	namedWindow("dst");

	imshow("src", src);

	createTrackbar("Lower Hue", "dst", &pos_hue1, 179, on_hsv_changed);
	createTrackbar("Upper Hue", "dst", &pos_hue2, 179, on_hsv_changed);
	createTrackbar("Lower Sat", "dst", &pos_sat1, 255, on_hsv_changed);
	createTrackbar("Upper Sat", "dst", &pos_sat2, 255, on_hsv_changed);
	on_hsv_changed(0, 0);

	waitKey();
}