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无法更改HTML 5画布背景

[英]Can't change html 5 canvas background

 原文  2014-03-26 16:10:09  7   1    javascript/ html5/ canvas/ background

问题描述

Im using this script to create canvas on which i can paint and then save the picture all together. 我使用此脚本来创建可以在其上绘画的画布，然后将所有图片保存在一起。 : ： 

<script type="text/javascript" src="processing.js"></script>
<script type="text/javascript">

/* @pjs preload="/example.jpg"; */

bg = loadImage("example.jpg");




var canvas, ctx, flag = false,
    prevX = 0,
    currX = 0,
    prevY = 0,
    currY = 0,
    dot_flag = false;

var x = "black",
    y = 2;

function init() {
    canvas = document.getElementById('can');
    ctx = canvas.getContext("2d");
    w = canvas.width;
    h = canvas.height;

    canvas.addEventListener("mousemove", function (e) {
        findxy('move', e)
    }, false);
    canvas.addEventListener("mousedown", function (e) {
        findxy('down', e)
    }, false);
    canvas.addEventListener("mouseup", function (e) {
        findxy('up', e)
    }, false);
    canvas.addEventListener("mouseout", function (e) {
        findxy('out', e)
    }, false);
}

function color(obj) {
    switch (obj.id) {
        case "green":
            x = "green";
            break;
        case "blue":
            x = "blue";
            break;
        case "red":
            x = "red";
            break;
        case "yellow":
            x = "yellow";
            break;
        case "orange":
            x = "orange";
            break;
        case "black":
            x = "black";
            break;
        case "white":
            x = "white";
            break;
    }
    if (x == "white") y = 14;
    else y = 2;

}

function draw() {
    ctx.beginPath();
    ctx.moveTo(prevX, prevY);
    ctx.lineTo(currX, currY);
    ctx.strokeStyle = x;
    ctx.lineWidth = y;
    ctx.stroke();
    ctx.closePath();
}

function erase() {
    var m = confirm("Want to clear");
    if (m) {
        ctx.clearRect(0, 0, w, h);
        document.getElementById("canvasimg").style.display = "none";
    }
}

function save() {
    document.getElementById("canvasimg").style.border = "2px solid";
    var dataURL = canvas.toDataURL();
    document.getElementById("canvasimg").src = dataURL;
    document.getElementById("canvasimg").style.display = "inline";
}

function findxy(res, e) {
    if (res == 'down') {
        prevX = currX;
        prevY = currY;
        currX = e.clientX - canvas.offsetLeft;
        currY = e.clientY - canvas.offsetTop;

        flag = true;
        dot_flag = true;
        if (dot_flag) {
            ctx.beginPath();
            ctx.fillStyle = x;
            ctx.fillRect(currX, currY, 2, 2);
            ctx.closePath();
            dot_flag = false;
        }
    }
    if (res == 'up' || res == "out") {
        flag = false;
    }
    if (res == 'move') {
        if (flag) {
            prevX = currX;
            prevY = currY;
            currX = e.clientX - canvas.offsetLeft;
            currY = e.clientY - canvas.offsetTop;
            draw();
        }
    }
}
</script>

But my background stays all white and doesn't change. 但是我的背景全是白色，没有变化。 When i open this website, it says : "loadImage is not defined". 当我打开此网站时，它说：“未定义loadImage”。 Any help will be very apreciated. 任何帮助将非常感激。

1 个解决方案

解决方案1
 1  2014-03-26 16:26:18

First make sure you define a canvas in your HTML file and define the size of your canvas: 首先，请确保您在HTML文件中定义了canvas ，并定义了画布的大小： 

<canvas id="myCanvas" width="500" height="500"></canvas>

So instead of defining canvas in your Javascript, it's much easier to define it in the HTML and grab it in your Javascript. 因此，与其在Javascript中定义画布，不如在HTML中定义画布并在Javascript中进行抓取要容易得多。 

var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');

The context is where you do all the drawing. 上下文是您绘制所有图形的地方。 Now, you can define your loadImage (or not use a method at all if you're only loading one image): 现在，您可以定义您的loadImage（如果只加载一个图像，则根本不使用任何方法）： 

function loadImage(imageSrc, x, y) {
    var image = new Image();
    image.src = imageSrc;
    image.onload = function() {
        context.drawImage(image, x, y);   
    }​
}

Then, you can call that function when the page loads. 然后，您可以在页面加载时调用该函数。

Live Demo Here 现场演示在这里

If you copy and paste the following code, you should get a working demo (however, there appears to be other issues with saving and clearing that you should look to fix): 如果复制并粘贴以下代码，则应该获得有效的演示（但是，保存和清除似乎还有其他问题，您应该寻求解决）： 

<!DOCTYPE html>
<html>
  <head>
    <title>HTML5 Picture</title>
    <script>

    var canvas, ctx, flag = false,
      prevX = 0,
      currX = 0,
      prevY = 0,
      currY = 0,
      dot_flag = false;

    var x = "black",
    y = 2;

    function init() {
    canvas = document.getElementById('can');
    ctx = canvas.getContext('2d');

    var loadImage = function (imageSrc, x, y, width, height) {
    var image = new Image();
    image.src = imageSrc;
    image.onload = function() {
        ctx.drawImage(image, x, y, width, height);   
      }
    };

    loadImage("http://img4.wikia.nocookie.net/__cb20110526012910/pokemon/images/4/49/Ash_Pikachu.png", 0, 0, 500, 300);

    w = canvas.width;
    h = canvas.height;

    canvas.addEventListener("mousemove", function (e) {
        findxy('move', e)
    }, false);
    canvas.addEventListener("mousedown", function (e) {
        findxy('down', e)
    }, false);
    canvas.addEventListener("mouseup", function (e) {
        findxy('up', e)
    }, false);
    canvas.addEventListener("mouseout", function (e) {
        findxy('out', e)
    }, false);
    }

    function color(obj) {
    switch (obj.id) {
        case "green":
        x = "green";
        break;
        case "blue":
        x = "blue";
        break;
        case "red":
        x = "red";
        break;
        case "yellow":
        x = "yellow";
        break;
        case "orange":
        x = "orange";
        break;
        case "black":
        x = "black";
        break;
        case "white":
        x = "white";
        break;
    }
    if (x == "white") y = 14;
    else y = 2;

    }

    function draw() {
    ctx.beginPath();
    ctx.moveTo(prevX, prevY);
    ctx.lineTo(currX, currY);
    ctx.strokeStyle = x;
    ctx.lineWidth = y;
    ctx.stroke();
    ctx.closePath();
    }

    function erase() {
    var m = confirm("Want to clear");
    if (m) {
        ctx.clearRect(0, 0, w, h);
        document.getElementById("canvasimg").style.display = "none";
    }
    }

    function save() {
    document.getElementById("canvasimg").style.border = "2px solid";
    var dataURL = canvas.toDataURL();
    document.getElementById("canvasimg").src = dataURL;
    document.getElementById("canvasimg").style.display = "inline";
    }

    function findxy(res, e) {
    if (res == 'down') {
        prevX = currX;
        prevY = currY;
        currX = e.clientX - canvas.offsetLeft;
        currY = e.clientY - canvas.offsetTop;

        flag = true;
        dot_flag = true;
        if (dot_flag) {
        ctx.beginPath();
        ctx.fillStyle = x;
        ctx.fillRect(currX, currY, 2, 2);
        ctx.closePath();
        dot_flag = false;
        }
    }
    if (res == 'up' || res == "out") {
        flag = false;
    }
    if (res == 'move') {
        if (flag) {
        prevX = currX;
        prevY = currY;
        currX = e.clientX - canvas.offsetLeft;
        currY = e.clientY - canvas.offsetTop;
        draw();
        }
    }
    }
    </script>
  </head>
  <body onload="init();">
    <canvas id="can" width="500" height="300" style="position:absolute;top:10%;left:10%;border:2px solid;"></canvas>
    <div style="position:absolute;top:12%;left:43%;">Choose Color</div>
    <div style="position:absolute;top:15%;left:45%;width:10px;height:10px;background:green;" id="green" onclick="color(this)"></div>
    <div style="position:absolute;top:15%;left:46%;width:10px;height:10px;background:blue;" id="blue" onclick="color(this)"></div>
    <div style="position:absolute;top:15%;left:47%;width:10px;height:10px;background:red;" id="red" onclick="color(this)"></div>
    <div style="position:absolute;top:17%;left:45%;width:10px;height:10px;background:yellow;" id="yellow" onclick="color(this)"></div>
    <div style="position:absolute;top:17%;left:46%;width:10px;height:10px;background:orange;" id="orange" onclick="color(this)"></div>
    <div style="position:absolute;top:17%;left:47%;width:10px;height:10px;background:black;" id="black" onclick="color(this)"></div>
    <div style="position:absolute;top:20%;left:43%;">Eraser</div>
    <div style="position:absolute;top:22%;left:45%;width:15px;height:15px;background:white;border:2px solid;" id="white" onclick="color(this)"></div>
    <img id="canvasimg" style="position:absolute;top:10%;left:52%;" style="display:none;">
    <input type="button" value="save" id="btn" size="30" onclick="save()" style="position:absolute;top:55%;left:10%;">
    <input type="button" value="clear" id="clr" size="23" onclick="erase()" style="position:absolute;top:55%;left:15%;">
  </body>
</html>

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