Javascript ImageData 绘图缩放不正确

Question

该程序的想法是获取地图图像并在该地图上覆盖黑色 canvas。然后用户将点击 canvas 的某些部分，类似于喷漆工具，鼠标附近的像素将在 canvas 上变得透明。因此，将显示地图（如 war 类型特征的雾）。当我在 canvas 的左上角附近单击时，喷漆会按预期工作，但是当我进一步向右和向下单击 canvas 时，变得透明的像素会进一步向右和向下.. . 知道这里出了什么问题吗？这是代码：

// On document ready function.
$(document).ready(function() {
  canvas = document.getElementById("myImageDisplayerCanvas");
  drawingContext = canvas.getContext("2d");
  drawingContext.fillStyle = "#000000";
  drawingContext.fillRect(0, 0, 800, 554);
});

// Spray paint logic.
var _intervalId, // used to track the current interval ID
  _center, // the current center to spray
  density = 10,
  radius = 10,
  drawingCxt,
  leftClickPressed,
  drawingContext,
  canvas;

function getRandomOffset() {
  var randomAngle = Math.random() * 360;
  var randomRadius = Math.random() * radius;

  return {
    x: Math.cos(randomAngle) * randomRadius,
    y: Math.sin(randomAngle) * randomRadius
  };
}

this.startDrawing = function(position) {
  _center = position;
  leftClickPressed = true;
  // spray once every 10 milliseconds
  _intervalId = setInterval(this.spray, 10);
};

this.moveDrawing = function(position) {
  if (leftClickPressed) {
    clearInterval(_intervalId);
    _center = position;
    // spray once every 10 milliseconds
    _intervalId = setInterval(this.spray, 10);
  }
}

this.finishDrawing = function(position) {
  clearInterval(_intervalId);
  leftClickPressed = false;
};

this.spray = function() {
  var centerX = parseInt(_center.offsetX),
    centerY =
    parseInt(_center.offsetY),
    i;

  for (i = 0; i < density; i++) {
    var offset = getRandomOffset();
    var x = centerX + parseInt(offset.x) - 1,
      y = centerY +
      parseInt(offset.y) - 1;
    var dy = y * 800 * 4;
    var pos = dy + x * 4;
    var imageData = drawingContext.getImageData(0, 0, 800, 554);
    imageData.data[pos++] = 0;
    imageData.data[pos++] = 0;
    imageData.data[pos++] = 0;
    imageData.data[pos++] = 0;
    drawingContext.putImageData(imageData, 0, 0);
  }
};

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.1/jquery.min.js"></script>
<div id="myImageDisplayerDiv" style="position:relative; width:800px; height:554px">
  <img src="~/images/RedLarch.jpg" style="width:800px; height:554px; top: 0; left: 0; position: absolute; z-index: 0" />
  <canvas id="myImageDisplayerCanvas" onmousedown="startDrawing(event)" onmousemove="moveDrawing(event)" onmouseup="finishDrawing(event)" style="width:800px; height:554px; top: 0; left: 0; position: absolute; z-index: 1; opacity: 1; fill: black" />
</div>

Answer 1

设置 canvas 分辨率。

喷子偏移的原因是你没有设置canvas分辨率。您通过 canvas 元素宽度和高度属性

设置分辨率

<canvas id="canvas" width="800" height="554"></canvas>

设置样式宽度和高度会设置 canvas 显示大小而不是分辨率。如果您不设置 canvas 分辨率，它默认为 300 x 150。

您的代码还有许多其他问题。

获取整个 canvas 图像数据只是为了设置单个像素是一种矫枉过正的做法。只需创建一个像素 imageData 对象并将该像素放在需要的地方

使用requestAnimationFrame与显示器同步，切勿使用setInterval或setTimeout，因为它们未与显示器硬件同步，会给您带来无穷无尽的问题。

创建一个单一的鼠标事件函数来处理所有鼠标事件并只记录鼠标状态。使用requestAnimationFrame调用的主循环来处理绘图，从不从鼠标事件中绘制。

如果函数不是对象的一部分，则不需要使用 this.functionName = function(){} 定义函数。

Trig 函数使用弧度而不是度数。 360 度的弧度为 2 * Math.PI

下面是使用上述注释快速重写的代码。

const ctx = canvas.getContext("2d");
requestAnimationFrame(mainLoop); // start main loop when code below has run
ctx.fillStyle = "#000000";
ctx.fillRect(0, 0, 800, 554);

// create a pixel buffer for one pixel
const imageData = ctx.getImageData(0, 0, 1, 1);
const pixel32 = new Uint32Array(imageData.data.buffer);
pixel32[0] = 0;

// Spray paint logic.
const density = 10;
const  radius = 10;

// mouse handler
const mouse = {x : 0, y : 0, button : false};
function mouseEvents(e){
 const bounds = canvas.getBoundingClientRect();
 mouse.x = e.pageX - bounds.left - scrollX;
 mouse.y = e.pageY - bounds.top - scrollY;
 mouse.button = e.type === "mousedown" ? true : e.type === "mouseup" ? false : mouse.button;
}
["down","up","move"].forEach(name => document.addEventListener("mouse"+name,mouseEvents));

function getRandomOffset() {
  const angle = Math.random() * Math.PI * 2;
  const rad = Math.random() * radius;
  return { x: Math.cos(angle) * rad, y: Math.sin(angle) * rad};
}

function spray(pos) {
  var i;
  for (i = 0; i < density; i++) {
    const offset = getRandomOffset();
    ctx.putImageData(imageData, pos.x + offset.x, pos.y + offset.y);
  }
}

// main loop called 60 times a second
function mainLoop(){
  if (mouse.button) { spray(mouse) }
  requestAnimationFrame(mainLoop);
}

<canvas id="canvas" width="800" height="554"></canvas>

Javascript ImageData 绘图缩放不正确

Javascript ImageData Drawing Is Scaling Incorrectly

javascript

jquery

getimagedata

html5-canvas

drawingcontext

设置 canvas 分辨率。