answer_26.cpp

#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <iostream>
#include <math.h>


// bilinear
cv::Mat bilinear(cv::Mat img, double rx, double ry){
  // get height and width
  int width = img.cols;
  int height = img.rows;
  int channel = img.channels();


  // get resized shape
  int resized_width = (int)(width * rx);
  int resized_height = (int)(height * ry);
  int x_before, y_before;
  double dx, dy;
  double val;

  // output image
  cv::Mat out = cv::Mat::zeros(resized_height, resized_width, CV_8UC3);

  // bi-linear interpolation
  for (int y = 0; y < resized_height; y++){
    y_before = (int)floor(y / ry);
    y_before = fmin(y_before, height - 1);
    dy = y / ry - y_before;
    
    for (int x = 0; x < resized_width; x++){
      x_before = (int)floor(x / rx);
      x_before = fmin(x_before, width - 1);
      dx = x / rx - x_before;
      
      // compute bi-linear
      for (int c = 0; c < channel; c++){
        val = (1. - dx) * (1. - dy) * img.at<cv::Vec3b>(y_before, x_before)[c] +
          dx * (1. - dy) * img.at<cv::Vec3b>(y_before, x_before + 1)[c] +
          (1. - dx) * dy * img.at<cv::Vec3b>(y_before + 1, x_before)[c] +
          dx * dy * img.at<cv::Vec3b>(y_before + 1, x_before)[c];

        // assign pixel to new position
        out.at<cv::Vec3b>(y, x)[c] = (uchar)val;
      }
    }
  }

  return out;
}


int main(int argc, const char* argv[]){
  // read image
  cv::Mat img = cv::imread("imori.jpg", cv::IMREAD_COLOR);

  // bilinear
  cv::Mat out = bilinear(img, 1.5, 1.5);
  
  //cv::imwrite("out.jpg", out);
  cv::imshow("answer", out);
  cv::waitKey(0);
  cv::destroyAllWindows();
 
  return 0;
}