超酷的HTML5 Canvas網路畫板教程

2014-09-06 分類：WEB開發、程式設計開發、首頁精華0人評論發表於2014-09-06

本文由碼農網 – 小峰原創，轉載請看清文末的轉載要求，歡迎參與我們的付費投稿計劃！

在今天的HTML教程裡，我們要學習一下如何使用HTML5 Canvas實現一個超酷而又簡單的網路畫板功能。在這個教程中，我們可以選擇筆刷的型別、筆刷的大小以及筆刷的顏色，當然作為畫板還需要很多功能，這裡只是最基本的繪製功能，大家可以自己實現類似矩形、橢圓等複雜元素的繪製。

你也可以再這裡檢視DEMO演示

下面我們來簡單地分析一下實現這個HTML5網頁畫板的原理及程式碼，程式碼由HTML以及Javascript組成，主要還是Javascript程式碼。

HTML程式碼：

<div style="width:530px;margin:10px auto">
    <div id="canvasDiv"></div>
</div>

HTML程式碼非常簡單，僅僅是構造了一個canvas容器，我們的畫板就在這個地方生成。

Javascript程式碼：

首先我們通過一組變數來定義畫板的樣式，以及一些資料的初始化：

var canvas;
var context;
var canvasWidth = 490;
var canvasHeight = 220;
var padding = 25;
var lineWidth = 8;
var colorPurple = "#cb3594";
var colorGreen = "#659b41";
var colorYellow = "#ffcf33";
var colorBrown = "#986928";
var outlineImage = new Image();
var crayonImage = new Image();
var markerImage = new Image();
var eraserImage = new Image();
var crayonBackgroundImage = new Image();
var markerBackgroundImage = new Image();
var eraserBackgroundImage = new Image();
var crayonTextureImage = new Image();
var clickX = new Array();
var clickY = new Array();
var clickColor = new Array();
var clickTool = new Array();
var clickSize = new Array();
var clickDrag = new Array();
var paint = false;
var curColor = colorPurple;
var curTool = "crayon";
var curSize = "normal";
var mediumStartX = 18;
var mediumStartY = 19;
var mediumImageWidth = 93;
var mediumImageHeight = 46;
var drawingAreaX = 111;
var drawingAreaY = 11;
var drawingAreaWidth = 267;
var drawingAreaHeight = 200;
var toolHotspotStartY = 23;
var toolHotspotHeight = 38;
var sizeHotspotStartY = 157;
var sizeHotspotHeight = 36;
var sizeHotspotWidthObject = new Object();
sizeHotspotWidthObject.huge = 39;
sizeHotspotWidthObject.large = 25;
sizeHotspotWidthObject.normal = 18;
sizeHotspotWidthObject.small = 16;
var totalLoadResources = 8;
var curLoadResNum = 0;

接下來開始準備畫布，也就是初始化Canvas物件：

function prepareCanvas()
{
    // Create the canvas (Neccessary for IE because it doesn't know what a canvas element is)
    var canvasDiv = document.getElementById('canvasDiv');
    canvas = document.createElement('canvas');
    canvas.setAttribute('width', canvasWidth);
    canvas.setAttribute('height', canvasHeight);
    canvas.setAttribute('id', 'canvas');
    canvasDiv.appendChild(canvas);
    if(typeof G_vmlCanvasManager != 'undefined') {
        canvas = G_vmlCanvasManager.initElement(canvas);
    }
    context = canvas.getContext("2d"); // Grab the 2d canvas context
    // Note: The above code is a workaround for IE 8 and lower. Otherwise we could have used:
    //     context = document.getElementById('canvas').getContext("2d");

    // Load images
    // -----------
    crayonImage.onload = function() { resourceLoaded(); 
    };
    crayonImage.src = "images/crayon-outline.png";
    //context.drawImage(crayonImage, 0, 0, 100, 100);

    markerImage.onload = function() { resourceLoaded(); 
    };
    markerImage.src = "images/marker-outline.png";

    eraserImage.onload = function() { resourceLoaded(); 
    };
    eraserImage.src = "images/eraser-outline.png";    

    crayonBackgroundImage.onload = function() { resourceLoaded(); 
    };
    crayonBackgroundImage.src = "images/crayon-background.png";

    markerBackgroundImage.onload = function() { resourceLoaded(); 
    };
    markerBackgroundImage.src = "images/marker-background.png";

    eraserBackgroundImage.onload = function() { resourceLoaded(); 
    };
    eraserBackgroundImage.src = "images/eraser-background.png";

    crayonTextureImage.onload = function() { resourceLoaded(); 
    };
    crayonTextureImage.src = "images/crayon-texture.png";

    outlineImage.onload = function() { resourceLoaded(); 
    };
    outlineImage.src = "images/watermelon-duck-outline.png";

    // Add mouse events
    // ----------------
    $('#canvas').mousedown(function(e)
    {
        // Mouse down location
        var mouseX = e.pageX - this.offsetLeft;
        var mouseY = e.pageY - this.offsetTop;

        if(mouseX < drawingAreaX) // Left of the drawing area
        {
            if(mouseX > mediumStartX)
            {
                if(mouseY > mediumStartY && mouseY < mediumStartY + mediumImageHeight){
                    curColor = colorPurple;
                }else if(mouseY > mediumStartY + mediumImageHeight && mouseY < mediumStartY + mediumImageHeight * 2){
                    curColor = colorGreen;
                }else if(mouseY > mediumStartY + mediumImageHeight * 2 && mouseY < mediumStartY + mediumImageHeight * 3){
                    curColor = colorYellow;
                }else if(mouseY > mediumStartY + mediumImageHeight * 3 && mouseY < mediumStartY + mediumImageHeight * 4){
                    curColor = colorBrown;
                }
            }
        }
        else if(mouseX > drawingAreaX + drawingAreaWidth) // Right of the drawing area
        {
            if(mouseY > toolHotspotStartY)
            {
                if(mouseY > sizeHotspotStartY)
                {
                    var sizeHotspotStartX = drawingAreaX + drawingAreaWidth;
                    if(mouseY < sizeHotspotStartY + sizeHotspotHeight && mouseX > sizeHotspotStartX)
                    {
                        if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.huge){
                            curSize = "huge";
                        }else if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.large + sizeHotspotWidthObject.huge){
                            curSize = "large";
                        }else if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.normal + sizeHotspotWidthObject.large + sizeHotspotWidthObject.huge){
                            curSize = "normal";
                        }else if(mouseX < sizeHotspotStartX + sizeHotspotWidthObject.small + sizeHotspotWidthObject.normal + sizeHotspotWidthObject.large + sizeHotspotWidthObject.huge){
                            curSize = "small";                        
                        }
                    }
                }
                else
                {
                    if(mouseY < toolHotspotStartY + toolHotspotHeight){
                        curTool = "crayon";
                    }else if(mouseY < toolHotspotStartY + toolHotspotHeight * 2){
                        curTool = "marker";
                    }else if(mouseY < toolHotspotStartY + toolHotspotHeight * 3){
                        curTool = "eraser";
                    }
                }
            }
        }
        else if(mouseY > drawingAreaY && mouseY < drawingAreaY + drawingAreaHeight)
        {
            // Mouse click location on drawing area
        }
        paint = true;
        addClick(mouseX, mouseY, false);
        redraw();
    });

    $('#canvas').mousemove(function(e){
        if(paint==true){
            addClick(e.pageX - this.offsetLeft, e.pageY - this.offsetTop, true);
            redraw();
        }
    });

    $('#canvas').mouseup(function(e){
        paint = false;
          redraw();
    });

    $('#canvas').mouseleave(function(e){
        paint = false;
    });
}

看起來很複雜，前面主要是初始化canvas的背景圖片，後面主要是初始化畫筆事件，像click、mouseup、mouseleave等滑鼠事件。

下面是draw的主要方法：

function redraw()
{
    // Make sure required resources are loaded before redrawing
    if(curLoadResNum < totalLoadResources){ return; }

    clearCanvas();

    var locX;
    var locY;
    if(curTool == "crayon")
    {
        // Draw the crayon tool background
        context.drawImage(crayonBackgroundImage, 0, 0, canvasWidth, canvasHeight);

        // Purple
        locX = (curColor == colorPurple) ? 18 : 52;
        locY = 19;

        context.beginPath();
        context.moveTo(locX + 41, locY + 11);
        context.lineTo(locX + 41, locY + 35);
        context.lineTo(locX + 29, locY + 35);
        context.lineTo(locX + 29, locY + 33);
        context.lineTo(locX + 11, locY + 27);
        context.lineTo(locX + 11, locY + 19);
        context.lineTo(locX + 29, locY + 13);
        context.lineTo(locX + 29, locY + 11);
        context.lineTo(locX + 41, locY + 11);
        context.closePath();
        context.fillStyle = colorPurple;
        context.fill();    

        if(curColor == colorPurple){
            context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }

        // Green
        locX = (curColor == colorGreen) ? 18 : 52;
        locY += 46;

        context.beginPath();
        context.moveTo(locX + 41, locY + 11);
        context.lineTo(locX + 41, locY + 35);
        context.lineTo(locX + 29, locY + 35);
        context.lineTo(locX + 29, locY + 33);
        context.lineTo(locX + 11, locY + 27);
        context.lineTo(locX + 11, locY + 19);
        context.lineTo(locX + 29, locY + 13);
        context.lineTo(locX + 29, locY + 11);
        context.lineTo(locX + 41, locY + 11);
        context.closePath();
        context.fillStyle = colorGreen;
        context.fill();    

        if(curColor == colorGreen){
            context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }

        // Yellow
        locX = (curColor == colorYellow) ? 18 : 52;
        locY += 46;

        context.beginPath();
        context.moveTo(locX + 41, locY + 11);
        context.lineTo(locX + 41, locY + 35);
        context.lineTo(locX + 29, locY + 35);
        context.lineTo(locX + 29, locY + 33);
        context.lineTo(locX + 11, locY + 27);
        context.lineTo(locX + 11, locY + 19);
        context.lineTo(locX + 29, locY + 13);
        context.lineTo(locX + 29, locY + 11);
        context.lineTo(locX + 41, locY + 11);
        context.closePath();
        context.fillStyle = colorYellow;
        context.fill();    

        if(curColor == colorYellow){
            context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }

        // Yellow
        locX = (curColor == colorBrown) ? 18 : 52;
        locY += 46;

        context.beginPath();
        context.moveTo(locX + 41, locY + 11);
        context.lineTo(locX + 41, locY + 35);
        context.lineTo(locX + 29, locY + 35);
        context.lineTo(locX + 29, locY + 33);
        context.lineTo(locX + 11, locY + 27);
        context.lineTo(locX + 11, locY + 19);
        context.lineTo(locX + 29, locY + 13);
        context.lineTo(locX + 29, locY + 11);
        context.lineTo(locX + 41, locY + 11);
        context.closePath();
        context.fillStyle = colorBrown;
        context.fill();    

        if(curColor == colorBrown){
            context.drawImage(crayonImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(crayonImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }
    }
    else if(curTool == "marker")
    {
        // Draw the marker tool background
        context.drawImage(markerBackgroundImage, 0, 0, canvasWidth, canvasHeight);

        // Purple
        locX = (curColor == colorPurple) ? 18 : 52;
        locY = 19;

        context.beginPath();
        context.moveTo(locX + 10, locY + 24);
        context.lineTo(locX + 10, locY + 24);
        context.lineTo(locX + 22, locY + 16);
        context.lineTo(locX + 22, locY + 31);
        context.closePath();
        context.fillStyle = colorPurple;
        context.fill();    

        if(curColor == colorPurple){
            context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }

        // Green
        locX = (curColor == colorGreen) ? 18 : 52;
        locY += 46;

        context.beginPath();
        context.moveTo(locX + 10, locY + 24);
        context.lineTo(locX + 10, locY + 24);
        context.lineTo(locX + 22, locY + 16);
        context.lineTo(locX + 22, locY + 31);
        context.closePath();
        context.fillStyle = colorGreen;
        context.fill();    

        if(curColor == colorGreen){
            context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }

        // Yellow
        locX = (curColor == colorYellow) ? 18 : 52;
        locY += 46;

        context.beginPath();
        context.moveTo(locX + 10, locY + 24);
        context.lineTo(locX + 10, locY + 24);
        context.lineTo(locX + 22, locY + 16);
        context.lineTo(locX + 22, locY + 31);
        context.closePath();
        context.fillStyle = colorYellow;
        context.fill();    

        if(curColor == colorYellow){
            context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }

        // Yellow
        locX = (curColor == colorBrown) ? 18 : 52;
        locY += 46;

        context.beginPath();
        context.moveTo(locX + 10, locY + 24);
        context.lineTo(locX + 10, locY + 24);
        context.lineTo(locX + 22, locY + 16);
        context.lineTo(locX + 22, locY + 31);
        context.closePath();
        context.fillStyle = colorBrown;
        context.fill();    

        if(curColor == colorBrown){
            context.drawImage(markerImage, locX, locY, mediumImageWidth, mediumImageHeight);
        }else{
            context.drawImage(markerImage, 0, 0, 59, mediumImageHeight, locX, locY, 59, mediumImageHeight);
        }
    }
    else if(curTool == "eraser")
    {
        context.drawImage(eraserBackgroundImage, 0, 0, canvasWidth, canvasHeight);
        context.drawImage(eraserImage, 18, 19, mediumImageWidth, mediumImageHeight);    
    }else{
        alert("Error: Current Tool is undefined");
    }

    if(curSize == "small"){
        locX = 467;
    }else if(curSize == "normal"){
        locX = 450;
    }else if(curSize == "large"){
        locX = 428;
    }else if(curSize == "huge"){
        locX = 399;
    }
    locY = 189;
    context.beginPath();
    context.rect(locX, locY, 2, 12);
    context.closePath();
    context.fillStyle = '#333333';
    context.fill();    

    // Keep the drawing in the drawing area
    context.save();
    context.beginPath();
    context.rect(drawingAreaX, drawingAreaY, drawingAreaWidth, drawingAreaHeight);
    context.clip();

    var radius;
    var i = 0;
    for(; i < clickX.length; i++)
    {        
        if(clickSize[i] == "small"){
            radius = 2;
        }else if(clickSize[i] == "normal"){
            radius = 5;
        }else if(clickSize[i] == "large"){
            radius = 10;
        }else if(clickSize[i] == "huge"){
            radius = 20;
        }else{
            alert("Error: Radius is zero for click " + i);
            radius = 0;    
        }

        context.beginPath();
        if(clickDrag[i] && i){
            context.moveTo(clickX[i-1], clickY[i-1]);
        }else{
            context.moveTo(clickX[i], clickY[i]);
        }
        context.lineTo(clickX[i], clickY[i]);
        context.closePath();

        if(clickTool[i] == "eraser"){
            //context.globalCompositeOperation = "destination-out"; // To erase instead of draw over with white
            context.strokeStyle = 'white';
        }else{
            //context.globalCompositeOperation = "source-over";    // To erase instead of draw over with white
            context.strokeStyle = clickColor[i];
        }
        context.lineJoin = "round";
        context.lineWidth = radius;
        context.stroke();

    }
    //context.globalCompositeOperation = "source-over";// To erase instead of draw over with white
    context.restore();

    // Overlay a crayon texture (if the current tool is crayon)
    if(curTool == "crayon"){
        context.globalAlpha = 0.4; // No IE support
        context.drawImage(crayonTextureImage, 0, 0, canvasWidth, canvasHeight);
    }
    context.globalAlpha = 1; // No IE support

    // Draw the outline image
    context.drawImage(outlineImage, drawingAreaX, drawingAreaY, drawingAreaWidth, drawingAreaHeight);
}

其實HTML5說白了還是需要很多Javascript支援，不過Canvas非常不錯，可以讓你在上面自由地繪製圖形和動畫。這款基於HTML5 Canvas的網頁畫板就是一個很好的例子。原始碼下載>>

本文連結：http://www.codeceo.com/article/cool-html5-canvas-drawing.html
本文作者：碼農網 – 小峰
[ 原創作品，轉載必須在正文中標註並保留原文連結和作者等資訊。]

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