使用canvas怎么自定义一个头像功能

使用canvas怎么自定义一个头像功能？很多新手对此不是很清楚，为了帮助大家解决这个难题，下面小编将为大家详细讲解，有这方面需求的人可以来学习下，希望你能有所收获。

实现细节：

因为你要对图片所在的区域进行截图，所以你得制作一张canvas，盖在图片所在的区域。这里，我们给出了一个函数，根据传入的DOM里面元素的类名创建相同位置的canvas，盖在原来的DOM元素上面：

function createCanvasByClassName(tag) {    var canvasInitialWidth = $('.' + tag).width();    var canvasInitialHeight = $('.' + tag).height();    var left = $('.' + tag).offset().left - $('.' + tag).parent('.portraitContainer').offset().left + 1;    var top = $('.' + tag).offset().top - $('.' + tag).parent('.portraitContainer').offset().top + 1;    //var left = $('.' + tag).offset().left + 1;    //var top = $('.' + tag).offset().top + 1;    clearCanvasObj.left = $('.' + tag).offset().left + 1;    clearCanvasObj.top = $('.' + tag).offset().top + 1;    // clearCanvasObj.left = left;    // clearCanvasObj.top = top;    var canvasElement = $('<canvas></canvas>');    var randomNum = Math.floor(getRandom(0, 10000));    clearCanvasObj.canvasId = randomNum;    canvasElement.attr({        id: 'canvas',        width: canvasInitialWidth,        height: canvasInitialHeight    });    canvasElement.css({        position: 'absolute',        top: top,         left: left    });    //$('body').append(canvasElement);    var appendEle = $('.portraitContainer').append(canvasElement);    var canvas = document.getElementById('canvas');    var ctx = canvas.getContext('2d');    //ctx.fillStyle = "rgba(211,211,216,0.5)";    ctx.clearRect(0, 0, canvasInitialWidth, canvasInitialHeight);    ctx.fillStyle = "rgba(0,0,0, 0.4)";    ctx.fillRect(0, 0, canvasInitialWidth, canvasInitialHeight);    return canvas;}

有了这张canvas你就可以在你图片所在区域肆意的操作了。首先，降整个区域画上一个浅黑色的阴影，然后再擦除初始小方框区域里面的颜色。然后给整个页面添加mousedown，mousemove，mouseup事件，他们所做的功能就跟你在页面中实现一个拖拽的功能类似，这里重点说下mousemove里面做的操作，代码如下：

function mousemoveFunc(event) {        var nowMouseX = event.clientX - clearCanvasObj.left;    var nowMouseY = event.clientY - clearCanvasObj.top;    if (nowMouseX >= clearCanvasObj.xStart && nowMouseX <= clearCanvasObj.xStart + clearCanvasObj.width && nowMouseY >= clearCanvasObj.yStart && nowMouseY <= clearCanvasObj.yStart + clearCanvasObj.height) {        clearCanvasObj.isCanvasArea = true;        //clearCanvasObj.isRightCorner = false;        imgContainerCanvas.style.cursor = 'move';    } else if ((nowMouseX >= clearCanvasObj.xStart + clearCanvasObj.width - 10) && (nowMouseX <= clearCanvasObj.xStart+ clearCanvasObj.width + 10)         && (nowMouseY >= clearCanvasObj.yStart + clearCanvasObj.height - 10) && (nowMouseY <= clearCanvasObj.yStart + clearCanvasObj.height + 10)) {        clearCanvasObj.isCanvasArea = true;        //clearCanvasObj.beginDraw = false;        imgContainerCanvas.style.cursor = 'se-resize';    }     else {        clearCanvasObj.isCanvasArea = false;        //clearCanvasObj.isRightCorner = false;        imgContainerCanvas.style.cursor = 'default';    }    var outerDomWidth = $(".imgContainer").width();    var outerDomHeight = $(".imgContainer").height();    var xDistance = event.clientX - clearCanvasObj.mouseX;    var yDistance = event.clientY - clearCanvasObj.mouseY;    //var outerCTX = canvas.getContext('2d');    //移动小方框    if (clearCanvasObj.beginDraw && clearCanvasObj.isCanvasArea && !clearCanvasObj.isRightCorner) {        ry_CTX.fillStyle = clearCanvasObj.color;        // console.log('1', clearCanvasObj.xStart, clearCanvasObj.yStart)        ry_CTX.fillRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);        //outerCTX.fillRect(0, 0, canvas.width, canvas.height);        clearCanvasObj.xStart += xDistance;        clearCanvasObj.yStart += yDistance;        //判断方框是否达到边界        if (clearCanvasObj.xStart <= 0) {            clearCanvasObj.xStart = 0;        }        if (clearCanvasObj.yStart <= 0) {            clearCanvasObj.yStart = 0;        }        if ((clearCanvasObj.xStart + clearCanvasObj.width) >= outerDomWidth) {            clearCanvasObj.xStart = outerDomWidth - clearCanvasObj.width;        }        if ((clearCanvasObj.yStart + clearCanvasObj.height) >= outerDomHeight) {            clearCanvasObj.yStart = outerDomHeight - clearCanvasObj.height;        }        // console.log('2', clearCanvasObj.xStart, clearCanvasObj.yStart)        ry_CTX.clearRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);        produceSmallPic(clearCanvasObj.xStart+clearCanvasObj.left, clearCanvasObj.yStart+clearCanvasObj.top, clearCanvasObj.width, clearCanvasObj.height, imageURL)        clearCanvasObj.mouseX = event.clientX;        clearCanvasObj.mouseY = event.clientY;    }    //拖拽小方框    if (clearCanvasObj.isRightCorner) {        ry_CTX.fillStyle = clearCanvasObj.color;        ry_CTX.fillRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);        var realDistance = Math.min(xDistance, yDistance)        clearCanvasObj.width +=  realDistance;        clearCanvasObj.height += realDistance;        //拖动时边界条件的判断        if (clearCanvasObj.xStart + clearCanvasObj.width >= outerDomWidth) {            clearCanvasObj.width = outerDomWidth - clearCanvasObj.xStart;            clearCanvasObj.height = outerDomWidth - clearCanvasObj.xStart;        }        if (clearCanvasObj.yStart + clearCanvasObj.height >= outerDomHeight) {            clearCanvasObj.width = outerDomHeight - clearCanvasObj.yStart;            clearCanvasObj.height = outerDomHeight - clearCanvasObj.yStart;        }        if (clearCanvasObj.width <= 10) {            clearCanvasObj.width = 10;        }        if (clearCanvasObj.height <= 10) {            clearCanvasObj.height = 10;        }        ry_CTX.clearRect(clearCanvasObj.xStart, clearCanvasObj.yStart, clearCanvasObj.width, clearCanvasObj.height);        produceSmallPic(clearCanvasObj.xStart+clearCanvasObj.left, clearCanvasObj.yStart+clearCanvasObj.top, clearCanvasObj.width, clearCanvasObj.height, imageURL);        clearCanvasObj.mouseX = event.clientX;        clearCanvasObj.mouseY = event.clientY;    }                            }

函数里面，你需要注意拖拽的边界条件，一个是方框不能拖到图片所在DOM外的边界；另外一个就是当你鼠标放在小方框所在的区域改变鼠标的样式。方框在拖动的过程中，我们不断重绘方框移动的区域（也就是不断的画上阴影），然后在新的位置调用clearRect函数，重新擦出一个小方框出来。在拖拽或是拉伸的过程中，我们会不断调用produceSmallPic函数，在右边的容器（每个容器都是一个canvas）里面不断根据容器大小重绘出所需的头像。代码如下：

function produceSmallPic(imageURL,left, top, width, height) {    var img = new Image();    img.src = imageURL;    var targetCtx = new Array();    var targetCanvas = null;    img.onload = function() {        portraitGroupsArr.forEach(function(item, index) {            targetCanvas = document.getElementById(item.class);            targetCtx.push(targetCanvas.getContext('2d'));            targetCtx[index].clearRect(0,0, item.width, item.height);            targetCtx[index].drawImage(img, left - clearCanvasObj.left, top - clearCanvasObj.top, width, height, 0, 0 , item.width, item.height);        })    }}

我们说下这个函数的作用，这里我们要注意一个参数imageURL，这个URL是由图片所在的DOM转化来的。因为你要把DOM所在的区域变成一张图片，这样你才能在利用drawImage函数截取你所需要的区域。所以我们先利用html2canvas库函数讲图片所在的DOM转化为canvas，这张canvas的内容是包含你所要截取的图片的，然后把这张canvas转化为图片取得图片地址imageURL，代码如下：

html2canvas(document.getElementById('imgContainer'), {        onrendered: function(canvas) {            var imageURL = canvasTransToImage(canavs);            ...        }})function canvasTransToImage(canvas) {    var imageURL = canvas.toDataURL('image/png');    return imageURL;}

接着，你就可以便利右侧的canvas容器，讲图片重回到里面了，整个过程就这样结束，回头看来是不是很简单。

相关依赖：

代码如下:

<script src="<a href="https://cdn.bootcss.com/html2canvas/0.5.0-beta4/html2canvas.min.js"></script">https://cdn.bootcss.com/html2canvas/0.5.0-beta4/html2canvas.min.js"></script</a>>

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