gravity/display.js

import { 
  VELOCITY_VECTOR_SCALE, 
  VELOCITY_VECTOR_COLOR,
  VELOCITY_VECTOR_WIDTH,
  VELOCITY_VECTOR_ARROWHEAD,
  ARROWHEAD_LENGTH,
  ARROWHEAD_WIDTH,
  OFFSCREEN_OBJECT_LINE_SCALE,
  OFFSCREEN_OBJECT_LINE_WIDTH,
  OFFSCREEN_OBJECT_ARROWHEAD_LENGTH,
} from './config.js';

export class Display {
  sim = undefined;
  scale = 1;
  viewOrigin = {x: 0, y: 0};

  constructor(sim) {
    this.sim = sim;
    // Create canvas that fills the window
    // If the window resizes, also resize the canvas
    const canvas = document.createElement('canvas')
    this.canvas = canvas;
    this.sim.div.appendChild(canvas);
    canvas.style.position = 'absolute';
    canvas.style.top = 0;
    canvas.style.left = 0;
    this.fullscreen();
    window.addEventListener('resize', () => this.fullscreen());
  }

  get ctx() {
    const ctx = this.canvas.getContext("2d");
    ctx.resetTransform();
    ctx.scale(this.scale, this.scale);
    ctx.translate(-this.viewOrigin.x, -this.viewOrigin.y);
    return ctx;
  }

  get width() {
    return this.canvas.width / this.scale;
  }

  get height() {
    return this.canvas.height / this.scale;
  }

  screenToSim(x, y) {
    return {
      x: x / this.scale + this.viewOrigin.x,
      y: y / this.scale + this.viewOrigin.y,
    };
  }

  fullscreen() {
    this.canvas.width = document.documentElement.clientWidth;
    this.canvas.height = document.documentElement.clientHeight;
    // this.info['Canvas'] = `${this.canvas.width} x ${this.canvas.height}`;
  }

  fillCanvas() {
    const ctx = this.ctx;
    ctx.fillStyle = '#000';
    ctx.fillRect(this.viewOrigin.x, this.viewOrigin.y, this.width, this.height);
  }

  drawObject(idx) {
    const obj = this.sim.objects.object(idx);
    const ctx = this.ctx;
    const {r, g, b} = obj.color;
    const {x, y} = obj.position;
    const {x: vx, y: vy} = obj.velocity;
    const radius = obj.radius;
    const {height: H, width: W} = this;
    const ox = this.viewOrigin.x;
    const oy = this.viewOrigin.y;
    const cx = ox + W / 2;
    const cy = oy + H / 2;
    // 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 = this.viewOrigin.y;
        }
      } else {
        // Line intersects y = H
        const yHpx = cx + (H / 2) / (y - cy) * (x - cx);
        if (Math.abs(yHpx - cx) <= W / 2) {
          px = yHpx;
          py = this.viewOrigin.y + H;
        }
      }
      if (px === undefined) {
        if (x <= cx) {
          // Line intersects x = 0:
          px = this.viewOrigin.x;
          py = cy + (W / 2) / (cx - x) * (y - cy);
        } else {
          // Line intersects x = W:
          px = this.viewOrigin.x + 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);
      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);
      this.drawArrow(startAx, startAy, px, py, {
        style: `rgb(${r}, ${g}, ${b})`,
        width: OFFSCREEN_OBJECT_LINE_WIDTH,
        arrowheadLength: OFFSCREEN_OBJECT_ARROWHEAD_LENGTH
      });
      
      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
    const endVx = x + VELOCITY_VECTOR_SCALE * vx;
    const endVy = y + VELOCITY_VECTOR_SCALE * vy;
    const style = VELOCITY_VECTOR_COLOR === 'object color' ? ctx.fillStyle : VELOCITY_VECTOR_COLOR;
    this.drawArrow(x, y, endVx, endVy, {
      style, 
      width: VELOCITY_VECTOR_WIDTH,
      arrowhead: VELOCITY_VECTOR_ARROWHEAD
    });

    // TODO: Draw arrow for acceleration
  }

  drawObjects() {
    for (let i = 0; i < this.sim.objects.length; i++) {
      this.drawObject(i);
    }
  }

  drawArrow(startX, startY, endX, endY, {style, width, arrowhead, arrowheadLength}) {
    arrowhead = arrowhead ?? true;
    arrowheadLength = arrowheadLength ?? ARROWHEAD_LENGTH;
    const ctx = this.ctx;
    ctx.strokeStyle = style;
    // Keep arrows at normal scale
    ctx.lineWidth = width / this.scale;
    const scaledArrowheadLength = arrowheadLength / this.scale;
    const arrowDirection = Math.atan2(endY - startY, endX - startX);
    const endAx = arrowhead ? endX - (scaledArrowheadLength - 1) * Math.cos(arrowDirection) : endX;
    const endAy = arrowhead ? endY - (scaledArrowheadLength - 1) * Math.sin(arrowDirection) : endY;
    ctx.beginPath();
    ctx.moveTo(startX, startY);
    ctx.lineTo(endAx, endAy);
    ctx.stroke();
    if (arrowhead) {
      this.drawArrowHead(endX, endY, arrowDirection, {style, length: arrowheadLength});
    }
  }

  drawArrowHead(x, y, direction, {style, length}) {
    const arrowheadLength = length ?? ARROWHEAD_LENGTH;
    const arrowheadWidth = arrowheadLength / ARROWHEAD_LENGTH * ARROWHEAD_WIDTH;
    const ctx = this.ctx;
    ctx.fillStyle = style;
    // To make this simple, draw the arrowhead and then rotate and translate it as needed.
    // Keep arrows at normal scale
    const scaledArrowheadLength = arrowheadLength / this.scale;
    const scaledArrowheadWidth = arrowheadWidth / this.scale;
    ctx.beginPath();
    ctx.moveTo(x, y);
    ctx.translate(x, y);
    ctx.rotate(direction);
    ctx.lineTo(-scaledArrowheadLength, -scaledArrowheadWidth / 2);
    ctx.lineTo(-scaledArrowheadLength, scaledArrowheadWidth / 2);
    ctx.closePath();
    ctx.fill();
    ctx.resetTransform();
  }
}