有没有办法在画布上绘制一些我部分超出屏幕边界的东西？

Question

我一直在考虑尝试用 JavaScript 创建一个游戏引擎来提高我的技能。 我正在考虑的一件事是游戏摄像机，它将具有一定的 x 和 y 范围以及全局位置。

其他所有东西也都有一个全局坐标。当需要绘制屏幕时，程序会检查哪些物体在相机的边界内，获取相对于相机的坐标，然后绘制它们。

我的问题是某些对象会在屏幕上显示一半，所以我想知道是否有办法在画布上绘制这些东西。

Answer 1

您可以完全绘制它们，但应该隐藏的部分不会显示，因为它将被绘制在画布之外。

或者您可以部分绘制它们。确定应该显示多少，然后绘制这部分。可能有用：

https://www.html5canvastutorials.com/advanced/html5-canvas-clipping-region-tutorial/

https://www.html5canvastutorials.com/tutorials/html5-canvas-image-crop/

编辑：我写了一些东西，只有当它们完全或部分在相机的视野范围内（红色）时才会显示灰色方块。

您可以更改模式（正常：显示所有方块，不包括相机外：仅显示相机范围内的方块），您可以更改相机范围的大小。

您也可以使用箭头键移动红色方块。在排除关闭相机模式下，如果它退出相机范围，它将消失。

let canvas, ctx;
let squares;
const color = {
	grey: '#aaa',
  black: '#000',
  red: '#f00'
};
const squareSize = 10;
const camera = {};
const mode = {
	NORMAL: 'NORMAL',
	EXCLUDE_OFF_CAMERA: 'EXCLUDE_OFF_CAMERA',
}
let currentMode;
let mainSquare;
let mainSquareSpeed;

const interval = 1000 / 30;
const KEY_LEFT = 37;
const KEY_RIGHT = 39;
const KEY_UP = 38;
const KEY_DOWN = 40;

let keyUpPressede;
let keyDownPressed;
let keyLeftPressed;
let keyRightPressed;

init();

function init() {
	canvas = document.getElementById("canvas");
  ctx = canvas.getContext("2d");
  
  camera.width =  camera.height = 70;
	camera.x = (canvas.width - camera.width) / 2;
	camera.y = (canvas.height - camera.height) / 2;
  
  currentMode = mode.EXCLUDE_OFF_CAMERA;
  
  keyUpPressed = false;
  keyDownPressed = false;
  keyLeftPressed = false;
  keyRightPressed = false;
  
  squares = createSquares(canvas, 20);
  mainSquare = squares[0];
  mainSquare.x = canvas.width / 2;
  mainSquare.y = canvas.height / 2;
  mainSquare.color = color.red;
  mainSquareSpeed = 3;
  
  window.addEventListener("keydown", onKeyDown, false);
	window.addEventListener("keyup", onKeyUp, false);
  document.getElementById("modeNormal").onclick = function() {
  	currentMode = mode.NORMAL;
  }
  document.getElementById("modeExcludeOffCamera").onclick = function() {
  	currentMode = mode.EXCLUDE_OFF_CAMERA;
  }
  document.getElementById("cameraSize").addEventListener("change", function(e) {
		camera.width =  camera.height = parseInt(e.currentTarget.value, 10);
    camera.x = (canvas.width - camera.width) / 2;
    camera.y = (canvas.height - camera.height) / 2;
  }, false);
  
    // requestAnim shim layer by Paul Irish
  window.requestAnimFrame = (function(){
    return  window.requestAnimationFrame       || 
      window.webkitRequestAnimationFrame || 
      window.mozRequestAnimationFrame    || 
      window.oRequestAnimationFrame      || 
      window.msRequestAnimationFrame     || 
      function(/* function */ callback, /* DOMElement */ element){
      window.setTimeout(callback, 1000 / 10);
    };
  })();

  step(canvas, ctx, camera, squares, mainSquare, squareSize, mainSquareSpeed);
}

function step(canvas, ctx, camera, squares, mainSquare, squareSize, squareSpeed) {
  requestAnimFrame(function() {
  	step(canvas, ctx, camera, squares, mainSquare, squareSize, squareSpeed);
  });
  update(canvas, mainSquare, squareSize, squareSpeed);
  clear(canvas, ctx);
  draw(canvas, ctx, camera, squares, squareSize);
}

function update(canvas, mainSquare, squareSize, squareSpeed) {
	if (keyLeftPressed) {
  	mainSquare.x -= squareSpeed;
  } else if (keyRightPressed) {
  	mainSquare.x += squareSpeed;
  }
	if (keyUpPressed) {
  	mainSquare.y -= squareSpeed;
  } else if (keyDownPressed) {
  	mainSquare.y += squareSpeed;
  }
  
  if (mainSquare.x < 0) {
  	mainSquare.x = canvas.width - squareSize;
  } else if (mainSquare.x > canvas.width - squareSize) {
  	mainSquare.x = 0;
  }
  if (mainSquare.y < 0) {
  	mainSquare.y = canvas.height - squareSize;
  } else if (mainSquare.y > canvas.height - squareSize) {
  	mainSquare.y = 0;
  }
}

function clear(canvas, ctx) {
	ctx.clearRect(0, 0, canvas.width, canvas.height);
}

function draw(canvas, ctx, camera, squares, size) {
  drawSquares(ctx, squares, size, camera);
	drawCamera(canvas, ctx, camera);
}

function drawSquares(ctx, squares, size, camera) {
	for (let i = squares.length - 1; i >= 0; i -= 1) {
  	if (inViewRange(squares[i], camera, size) || currentMode === mode.NORMAL) {
      drawSquare(ctx, squares[i], size);  		
    }
  }
}

function drawSquare(ctx, square, size) {
	ctx.fillStyle = square.color ? square.color : color.grey;
	ctx.strokeStyle = color.black;
  ctx.fillRect(square.x, square.y, size, size);
  ctx.strokeRect(square.x, square.y, size, size);
}

function drawCamera(canvas, ctx, camera) {
	ctx.fillStyle = color.black;
	ctx.strokeStyle = color.red;
  ctx.lineWidth = 3;
  
  ctx.globalAlpha = 0.3;
  ctx.fillRect(camera.x, camera.y, camera.width, camera.height);
  ctx.globalAlpha = 1;
  ctx.strokeRect(camera.x, camera.y, camera.width, camera.height);
}

/**
 * Found here: https://stackoverflow.com/questions/1527803/generating-random-whole-numbers-in-javascript-in-a-specific-range
 *
 * Returns a random integer between min (inclusive) and max (inclusive).
 * The value is no lower than min (or the next integer greater than min
 * if min isn't an integer) and no greater than max (or the next integer
 * lower than max if max isn't an integer).
 * Using Math.round() will give you a non-uniform distribution!
 */
function getRandomInt(min, max) {
    min = Math.ceil(min);
    max = Math.floor(max);
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

function createSquares(canvas, number) {
	const squares = [];
  
  for (let i = 0; i < number; i += 1) {
  	
  	squares.push({ 
      x: getRandomInt(0, canvas.width), 
      y: getRandomInt(0, canvas.height) 
    });
  }
  
  return squares;
}

function inViewRange(square, camera, squareSize) {
	if (square.x + squareSize > camera.x && camera.x + camera.width > square.x) {
		if (square.y + squareSize > camera.y && camera.y + camera.height > square.y) {
  		return true;
  	}
  }
  return false;
}

function onKeyDown(e) {
	switch (e.keyCode) {
  	case KEY_LEFT: keyLeftPressed = true; break;
  	case KEY_RIGHT: keyRightPressed = true; break;
  	case KEY_UP: keyUpPressed = true; break;
  	case KEY_DOWN: keyDownPressed = true; break;l
  }
}

function onKeyUp(e) {
	switch (e.keyCode) {
  	case KEY_LEFT: keyLeftPressed = false; break;
  	case KEY_RIGHT: keyRightPressed = false; break;
  	case KEY_UP: keyUpPressed = false; break;
  	case KEY_DOWN: keyDownPressed = false; break;
  }
}

canvas {
  border: 1px solid black;
  margin: 1px;
}

<canvas id="canvas" width=100 height=100></canvas>
<br>
<button id="modeNormal">modeNormal</button>
<button id="modeExcludeOffCamera">modeExcludeOffCamera</button>
Camera size: <input id="cameraSize" type="range" min="10" max="100" value="70">