waves from selected pixel (openGL C++)

Question

I've created a program that draws a waving flag and I want to add a functionality that will create new wave on selected pixel, but I can't make it start where I want it to start an that even make the flag stop waving (prob. because of synced sin).

Here's my display func.

const int W = 800;
const int H = 600;
// simulates Frame Buffer
unsigned char pixels[H][W][3] = { 0 }; // 3 is for RGB
    void display()
    {
        glClear(GL_COLOR_BUFFER_BIT); // clean frame buffer
    
        createFlag();
        int i, j;
        double dist;
        offset += 0.25;
        for (i = 0; i < H; i++)
            for (j = 0; j < W; j++)
            {
                dist = sqrt(pow(i + H / 2.0, 2) + pow(j + W / 2.0, 2));
                pixels[i][j][0] += 135 + 55 * (1 + 1 * sin(dist / 25 - offset)) / 2; // red
                pixels[i][j][1] += 135 + 85 * (1 + 1 * sin(dist / 25 - offset)) / 2; // green
                pixels[i][j][2] += 135 + 105 * (1 + 1 * sin(dist / 25 - offset)) / 2; // blue
    
            }
        // draws the matrix pixels
        glDrawPixels(W, H, GL_RGB, GL_UNSIGNED_BYTE, pixels);
    
        glutSwapBuffers(); // show all
    }

And here is my mouse func.

void mouse(int button, int state, int x, int y)
{
    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN)
    {
        double dist;
        offset += 0.1;
        for (y = 0; y < H; y++)
            for (x = 0; x < W; x++)
            {
                dist = sqrt(pow(H/2.0 -(H - y), 2) + pow(W/2.0 -x, 2)); //problem is prob. here
                pixels[y][x][0] += 135+ 55 * (1 + 1 * sin(dist / 50.0 - offset)) / 2; // red
                pixels[y][x][1] += 135+ 85 * (1 + 1 * sin(dist / 50.0 - offset)) / 2; // green
                pixels[y][x][2] += 135+105 * (1 + 1 * sin(dist / 50.0 - offset)) / 2; // blue
                if (offset < 0.3)
                    offset += 0.05;
            }
    }
}

Answer 1

Here are some points that I see:

• in your mouse function you don't use your arguments x and y which are the position of your mouse click, instead you create local variables x and y. If you want to start a wave from the pixel you clicked on, you have to center your dist to this point, so an idea of code could be (inspired by GLUT mouse button down):

  void mouse(int button, int state, int x, int y)
  {
      // save the left button state
      if (button == GLUT_LEFT_BUTTON)
      {
          leftMouseButtonDown = (state == GLUT_DOWN);
      }
  
      // save the mouse position
      mouseXPos = x;
      mouseYPos = y;
  }
  
  void display()
  {
      glClear(GL_COLOR_BUFFER_BIT); // clean frame buffer
  
      createFlag();
      double dist;
      offset += 0.25;
      for (i = 0; i < H; i++)
          for (j = 0; j < W; j++)
          {
              dist = sqrt(pow(i + H / 2.0, 2) + pow(j + W / 2.0, 2));
              pixels[i][j][0] += 135 + 55 * (1 + 1 * sin(dist / 25 - offset)) / 2; // red
              pixels[i][j][1] += 135 + 85 * (1 + 1 * sin(dist / 25 - offset)) / 2; // green
              pixels[i][j][2] += 135 + 105 * (1 + 1 * sin(dist / 25 - offset)) / 2; // blue
          }
  
      if (leftMouseButtonDown)   // the way you can get leftMouseButtonDown depends on your 
                                 // configuration of your different files 
                                 // (main.cpp, scene.cpp, ...)
          new_wave() ; //see below
  
      // draws the matrix pixels
      glDrawPixels(W, H, GL_RGB, GL_UNSIGNED_BYTE, pixels);
  
      glutSwapBuffers(); // show all
  }
  
  void new_wave()
  {
      x = mouseXPos ;   // same remark as leftMouseButtonDown 
      y = mouseYPos ;
      offset = 0.1;
      for (i = 0; i < H; i++)     // i and not y
          for (j = 0; j < W; j++)
          {
              dist = sqrt(pow(i - y, 2) + pow(j - x, 2));    // change here
              pixels[i][j][0] += 135+ 55 * (1 + 1 * sin(dist / 50.0 - offset)) / 2; // red
              pixels[i][j][1] += 135+ 85 * (1 + 1 * sin(dist / 50.0 - offset)) / 2; // green
              pixels[i][j][2] += 135+105 * (1 + 1 * sin(dist / 50.0 - offset)) / 2; // blue
              if (offset < 0.3)
                  offset += 0.05;   
  // I don't really get what you do here with the offset, but another parameter to
  // play with could be the amplitude of this wave that starts from the point 
  // you clicked, like when a drop falls on water
          }
  }
  

• if you want a real-time waving flag you have to take time as a parameter of your pixels color value

• I haven't tested it but I'm not sure on how openGL reacts to RGB coloring with values over 255, which happens in your case, maybe keep that in mind if you still have bugs

• dist = sqrt(pow(i + H / 2.0, 2) + pow(j + W / 2.0, 2)); this is centered in (-W/2.0, -H/2.0), is that what you want? (maybe yes just want to make sure, if you want to simulate some wind you could set wind's origin where you want, which is what you do here)

• int i, j; in your display isn't useful (just to clear some code)

So these are some remarks I would have made myself, this piece of code probably won't be your final one. Let me know if I misunderstood what you are aiming to do, or if something I wrote is unclear.