我尝试用javascript制作绘画应用程序。我需要通过按下按钮来制作方形网格。我做了这样的网格，但它不在背景上。我应该如何在后台传递js制作的网格？
function print_grid()
{
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillRect(0, 0, 5100, 5100);
ctx.clearRect(0, 0, 5100, 5100);
ctx.beginPath();
for (let i = 0; i < 39; i++)
{
ctx.lineWidth = 1;
ctx.moveTo(50*i, 0);
ctx.lineTo(50*i, 5100);
ctx.moveTo(0, 50*i);
ctx.lineTo(5100, 50*i);
}
ctx.stroke();
}<!DOCTYPE html>
<html>
<body>
<h1>Board</h1>
<button onclick="print_grid()">square</button>
<p >draw!!!</p>
<canvas id="myCanvas" width="1000" height="1000" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.
</canvas>
</body>
</html>一个简单的解决方案是使用两个层叠的画布。通过这种方式，您可以在背景层上绘制网格，而不依赖于前景中的画布。
这里有一个例子：
const canvas = document.getElementById('canvas2');
const ctx = canvas.getContext('2d');
let coordinates = {
x: 0,
y: 0
};
let painting = false;
function getPosition(event) {
coordinates.x = event.clientX - canvas.offsetLeft;
coordinates.y = event.clientY - canvas.offsetTop;
}
function startPainting(event) {
painting = true;
getPosition(event);
}
function stopPainting() {
painting = false;
}
function draw(event) {
if (!painting) return;
ctx.beginPath();
ctx.lineWidth = 5;
ctx.lineCap = 'round';
ctx.strokeStyle = 'red';
ctx.moveTo(coordinates.x, coordinates.y);
getPosition(event);
ctx.lineTo(coordinates.x, coordinates.y);
ctx.stroke();
}
function clearCanvas() {
ctx.clearRect(0, 0, canvas.width, canvas.height);
}
function printGrid() {
let backgroundCanvas = document.getElementById('canvas');
let context = backgroundCanvas.getContext("2d");
context.beginPath();
for (let a = 0; a < 10; a++) {
context.moveTo(0, parseInt(a * (backgroundCanvas.height / 9)));
context.lineTo(backgroundCanvas.width, parseInt(a * (backgroundCanvas.height / 9)));
context.moveTo(parseInt(a * (backgroundCanvas.width / 9)), 0);
context.lineTo(parseInt(a * (backgroundCanvas.width / 9)), backgroundCanvas.height);
}
context.stroke();
context.closePath();
}
printGrid();
document.addEventListener('mousedown', startPainting);
document.addEventListener('mouseup', stopPainting);
document.addEventListener('mousemove', draw);<button onclick='clearCanvas();'>Clear</button>
<div>
<canvas id='canvas' style='position: absolute'></canvas>
<canvas id='canvas2' style='position: absolute'></canvas>
</div>