gravity/object.js

import {
  ACCELERATION_VECTOR_ARROWHEAD,
  ACCELERATION_VECTOR_COLOR,
  ACCELERATION_VECTOR_SCALE,
  ACCELERATION_VECTOR_WIDTH,
  OFFSCREEN_OBJECT_ARROWHEAD_LENGTH,
  OFFSCREEN_OBJECT_LINE_SCALE,
  OFFSCREEN_OBJECT_LINE_WIDTH,
  PATH_TRACES_COLOR,
  PATH_TRACES_DASHED_OPACITY,
  PATH_TRACES_OPACITY,
  PATH_TRACES_WIDTH,
  VELOCITY_VECTOR_ARROWHEAD,
  VELOCITY_VECTOR_COLOR,
  VELOCITY_VECTOR_SCALE,
  VELOCITY_VECTOR_WIDTH,
} from './config.js';

export class MassObject {
  mass = 0;
  density = 1;
  position = {x: undefined, y: undefined};
  velocity = {x: 0, y: 0};
  acceleration = {x: 0, y: 0};
  color = {r: undefined, g: undefined, b: undefined};
  created = undefined;
  forces = []; // [{x, y}]
  history = [];
  alive = true;

  constructor(x, y) {
    this.position.x = x;
    this.position.y = y;
    this.color.r = Math.random() * 256;
    this.color.g = Math.random() * 256;
    this.color.b = Math.random() * 256;
    this.created = document.timeline.currentTime;
  }

  get age() {
    return document.timeline.currentTime - this.created;
  }

  get radius() {
    // radius should be proportional to cube root of mass
    return Math.pow(this.mass / this.density, 1 / 3);
  }

  getAcceleration() {
    let ax = 0;
    let ay = 0;
    for (let {x, y} of this.forces) {
      ax += x;
      ay += y;
    }
    return {
      x: ax / this.mass,
      y: ay / this.mass,
    };
  }

  drawObject(sim) {
    const {
      color: {r, g, b},
      position: {x, y},
      velocity: {x: vx, y: vy},
      acceleration,
      radius,
    } = this;

    const {
      display: {
      ctx,
        height: H,
        width: W,
        viewOrigin: {x: ox, y: oy},
      }
    } = sim;

    const cx = ox + W / 2;
    const cy = oy + H / 2;

    // Draw path traces
    if (sim.getOption('display.traces') && this.history?.length) {
      const dashedTraces = sim.getOption('display.dashedTraces');
      const opacity = dashedTraces ? PATH_TRACES_DASHED_OPACITY : PATH_TRACES_OPACITY;
      ctx.strokeStyle = PATH_TRACES_COLOR === 'object color' ?
        `rgba(${r}, ${g}, ${b}, ${opacity})` : PATH_TRACES_COLOR;
      ctx.lineWidth = PATH_TRACES_WIDTH / this.scale;
      ctx.beginPath();
      let dash = false;
      for (let i = 0; i < this.history.length; i++) {
        // if (i % 2 > 0) continue;
        const {position: {x, y}} = this.history[i];
        if (dashedTraces) {
          if (dash) {
            ctx.lineTo(x, y);
          } else {
            ctx.moveTo(x, y);
          }
          dash = !dash;
        } else {
          ctx.lineTo(x, y);
        }
      }
      ctx.stroke();
    }

    if (!this.alive) return;

    // If the object is outside the display area, draw an arrow at the edge of the display
    if (Math.abs(x - cx) - radius >= W / 2 ||
      Math.abs(y - cy) - radius >= H / 2) {
      // Find where a line from center of display to object intersects display edge
      let px, py;
      if (y <= cy) {
        // Line intersects y = 0:
        const y0px = cx + (H / 2) / (cy - y) * (x - cx);
        if (Math.abs(y0px - cx) <= W / 2) {
          px = y0px;
          py = oy;
        }
      } else {
        // Line intersects y = H
        const yHpx = cx + (H / 2) / (y - cy) * (x - cx);
        if (Math.abs(yHpx - cx) <= W / 2) {
          px = yHpx;
          py = oy + H;
        }
      }
      if (px === undefined) {
        if (x <= cx) {
          // Line intersects x = 0:
          px = ox;
          py = cy + (W / 2) / (cx - x) * (y - cy);
        } else {
          // Line intersects x = W:
          px = ox + W;
          py = cy + (W / 2) / (x - cx) * (y - cy);
        }
      }

      const arrowDirection = Math.atan2(py - cy, px - cx);
      // Length of arrow based on distance (logarithmic scale)
      const distance = Math.sqrt((x - px) ** 2, (y - py) ** 2) * this.scale;
      const arrowLength = Math.log(distance) * OFFSCREEN_OBJECT_LINE_SCALE / this.scale;
      const startAx = px - arrowLength * Math.cos(arrowDirection);
      const startAy = py - arrowLength * Math.sin(arrowDirection);
      sim.display.drawArrow(startAx, startAy, px, py, {
        style: `rgb(${r}, ${g}, ${b})`,
        width: OFFSCREEN_OBJECT_LINE_WIDTH,
        arrowheadLength: OFFSCREEN_OBJECT_ARROWHEAD_LENGTH,
        fill: false,
        ifShort: 'head',
      });

      return;
    }

    // Draw filled circle for the object
    ctx.fillStyle = `rgb(${r}, ${g}, ${b})`;
    ctx.beginPath();
    ctx.arc(x, y, radius, 0, 2 * Math.PI);
    ctx.fill();

    // Draw arrow for the velocity
    if (sim.getOption('display.velocity')) {
      const speed = Math.sqrt(vx ** 2 + vy ** 2);
      const endVx = x + VELOCITY_VECTOR_SCALE * vx / speed * Math.log(speed);
      const endVy = y + VELOCITY_VECTOR_SCALE * vy / speed * Math.log(speed);
      const style = VELOCITY_VECTOR_COLOR === 'object color' ?
        `rgb(${r}, ${g}, ${b})` : VELOCITY_VECTOR_COLOR;
      sim.display.drawArrow(x, y, endVx, endVy, {
        style,
        width: VELOCITY_VECTOR_WIDTH,
        arrowhead: VELOCITY_VECTOR_ARROWHEAD,
        fill: false,
        ifShort: 'head'
      });
    }

    // Draw arrow for acceleration
    if (sim.getOption('display.acceleration')) {
      const accelerationMagnitude = Math.sqrt(acceleration.x ** 2 + acceleration.y ** 2);
      const endAx = x + ACCELERATION_VECTOR_SCALE * acceleration.x /
        accelerationMagnitude * Math.log(accelerationMagnitude);
      const endAy = y + ACCELERATION_VECTOR_SCALE * acceleration.y /
        accelerationMagnitude * Math.log(accelerationMagnitude);
      const style = ACCELERATION_VECTOR_COLOR === 'object color' ?
        `rgb(${r}, ${g}, ${b})` : ACCELERATION_VECTOR_COLOR;
      sim.display.drawArrow(x, y, endAx, endAy, {
        style,
        width: ACCELERATION_VECTOR_WIDTH,
        arrowhead: ACCELERATION_VECTOR_ARROWHEAD,
        fill: false,
        ifShort: 'tail'
      });
    }

  }

}