JavaScript裁剪圖片不變形的實現方法
最近瀏覽了不少網站的圖片裁切效果，大部分的做法如下圖所示（借用一張腳本之家的圖片），通過改變裁切框的大小來選取合適的位置。
但本文介紹的是另外一種裁切方式，裁切框由開發者決定，圖片大小由用戶決定，通過縮放、拖動圖片來選取合適位置，并且在這一過程中始終保持圖片寬高比，如右上圖。
這樣做法主要有以下優點：
- 裁切框的寬高與跟實際使用的處寬高比一致，防止出現圖片變形問題
- 不限制圖片的顯示大小，保證圖片原始比例，通過縮放可得到原始尺寸
- 對于局部的裁切更加友好，比如截取一張高清圖片中很小的一個部位，我們只需將圖片放大并拖動到裁切框內即可，而其他方式需要將裁切框調整的非常小，不利于用戶操作 </ol>
- 等待圖片加載完畢，獲取圖片的原始尺寸，后續計算縮放因子時會用到
- 在保證裁切區域寬高比的情況下，讓裁切區域盡量的填滿工作區。這里裁切工作最重要的就是防止圖片變形，所以只要保證寬高比一致可以將裁切區域適當放大。
- 保持圖片原始寬高比的前提下，讓圖片盡量接近裁切框
- 機上計算完成后設置圖片初始位置，讓裁切框相對圖片居中 </ol>
說完了有點也該說說缺點，缺點就是難度增大了一個數量級。。。。
主體思路是利用兩張圖片，將他們絕對定位，一張放在裁切框內一張放在裁切框外并設置透明效果，裁切框overflow為hidden，時刻保持兩張圖片的絕對同步。
<div class="jimu-crop-image" data-dojo-attach-point="cropSection">
<div class="viewer-box" data-dojo-attach-point="viewerBox">
<div class="viewer-content" data-dojo-attach-point="viewerContent">
<img class="viewer-image hide-image" data-dojo-attach-point="viewerImage" src="">
</div>
<img class="base-image hide-image" data-dojo-attach-point="baseImage" data-dojo-attach-event="mousedown:_onViewerMouseDown,mouseup:_onViewerMouseUp">
<div class="controller">
<div class="zoom-out" data-dojo-attach-event="click:_onZoomOutClick">-</div>
<div class="slider" data-dojo-attach-point="sliderNode">
<div class="button" data-dojo-attach-point="sliderButton" data-dojo-attach-event="mousedown:_onSliderMouseDown,mouseup:_onSliderMouseUp"></div>
<div class="horizontal"></div>
</div>
<div class="zoom-in" data-dojo-attach-event="click:_onZoomInClick">+</div>
</div>
</div>
</div></pre>
首先在postCreate中綁定document的mousemove跟mousedown事件，在鼠標離開工作區后仍可以繼續拖動或縮放。接下來的主要工作在startup跟_init函數中。不熟悉dojo的道友只要知道postCreate會在startup之前執行即可。
startup: function() {
var timeOut = /data:image\/(.*);base64/.test(this.imageSrc) ? 50 : 500;
var tic = lang.hitch(this, function() {
var imageStyle = html.getComputedStyle(this.baseImage);
var imageWidth = parseFloat(imageStyle.width);
console.log('image width', imageWidth);
if (isFinite(imageWidth) && imageWidth > 0) {
this._init();
} else {
setTimeout(tic, timeOut);
}
});
setTimeout(tic, timeOut);
},
_init: function() {
debugger;
var cropSectionStyle = html.getComputedStyle(this.cropSection);
var cropSectionContentBox = html.getContentBox(this.cropSection);
var imageStyle = html.getComputedStyle(this.baseImage);
var imageWidth = parseFloat(imageStyle.width);
var imageHeight = parseFloat(imageStyle.height);
var imageRadio = imageWidth / imageHeight;
this._maxImageWidth = imageWidth;
this._maxImageHeight = imageHeight;
if (imageHeight < this.realHeight && imageWidth < this.realWidth) {
alert('image is too smaller to display');
return;
}
//create a box which keep the ratio of width and height to full fill the content of popup
this.idealWidth = this.realWidth;
this.idealHeight = this.realHeight;
this.ratio = this.ratio ? this.ratio : this.realWidth / this.realHeight;
if (this.ratio >= 1) {
if (this.realWidth <= cropSectionContentBox.w) {
this.idealWidth += (cropSectionContentBox.w - this.realWidth) / 2;
} else {
this.idealWidth = cropSectionContentBox.w;
}
this.idealHeight = this.idealWidth / this.ratio;
} else {
if (this.realHeight <= cropSectionContentBox.h) {
this.idealHeight += (cropSectionContentBox.h - this.idealHeight) / 2;
} else {
this.idealHeight = cropSectionContentBox.h;
}
this.idealWidth = this.idealHeight * this.ratio;
}
html.setStyle(this.viewerBox, {
width: this.idealWidth + 'px',
height: this.idealHeight + 'px'
});
var paddingTop = Math.abs((parseFloat(cropSectionStyle.height) - this.idealHeight) / 2);
html.setStyle(this.cropSection, {
'paddingTop': paddingTop + 'px',
'paddingBottom': paddingTop + 'px'
});
// keep original ratio of image
if (imageRadio >= 1) {
if (this.idealHeight * imageRadio >= this.idealWidth) {
html.setStyle(this.viewerImage, 'height', this.idealHeight + 'px');
html.setStyle(this.baseImage, 'height', this.idealHeight + 'px');
} else {
var properlyHeight = this._findProperlyValue(0, this.idealWidth, this.idealWidth, function(p) {
return p * imageRadio;
});
html.setStyle(this.viewerImage, 'height', properlyHeight + 'px');
html.setStyle(this.baseImage, 'height', properlyHeight + 'px');
}
} else {
if (this.idealWidth / imageRadio >= this.idealHeight) {
html.setStyle(this.viewerImage, 'width', this.idealWidth + 'px');
html.setStyle(this.baseImage, 'width', this.idealWidth + 'px');
} else {
var properlyWidth = this._findProperlyValue(0, this.idealHeight, this.idealHeight, function(p) {
return p / imageRadio;
});
html.setStyle(this.viewerImage, 'width', properlyWidth + 'px');
html.setStyle(this.baseImage, 'width', properlyWidth + 'px');
}
}
query('.hide-image', this.domNode).removeClass('hide-image');
imageStyle = html.getComputedStyle(this.baseImage);
imageWidth = parseFloat(imageStyle.width);
imageHeight = parseFloat(imageStyle.height);
this._minImageWidth = imageWidth;
this._minImageHeight = imageHeight;
this._currentImageWidth = imageWidth;
this._currentImageHeight = imageHeight;
this._currentTop = -(imageHeight - this.idealHeight) / 2;
this._currentLeft = -(imageWidth - this.idealWidth) / 2;
html.setStyle(this.baseImage, {
top: this._currentTop + 'px',
left: this._currentLeft + 'px'
});
html.setStyle(this.viewerImage, {
top: this._currentTop + 'px',
left: this._currentLeft + 'px'
});
//sometimes zoomratio < 1; it's should be not allowed to zoom
this._zoomRatio = this._maxImageWidth / this._minImageWidth;
if (!this._latestPercentage) {
this._latestPercentage = 0;
}
},</pre> <p>這里面做了以下幾件事： </p>
平移的過程比較簡單，只需要記錄移動過程中鼠標的相對位置變化，不斷改變圖片左上角的left跟top即可，在dragstart跟selectstart事件中preventDefault防止出現元素被選中變藍。
_resetImagePosition: function(clientX, clientY) {
var delX = clientX - this._currentX;
var delY = clientY - this._currentY;
if (this._currentTop + delY >= 0) {
html.setStyle(this.baseImage, 'top', 0);
html.setStyle(this.viewerImage, 'top', 0);
this._currentY = clientY;
this._currentTop = 0;
} else if (this._currentTop + delY <= this._maxOffsetTop) {
html.setStyle(this.baseImage, 'top', this._maxOffsetTop + 'px');
html.setStyle(this.viewerImage, 'top', this._maxOffsetTop + 'px');
this._currentY = clientY;
this._currentTop = this._maxOffsetTop;
} else {
html.setStyle(this.baseImage, 'top', this._currentTop + delY + 'px');
html.setStyle(this.viewerImage, 'top', this._currentTop + delY + 'px');
this._currentY = clientY;
this._currentTop += delY;
}
if (this._currentLeft + delX >= 0) {
html.setStyle(this.baseImage, 'left', 0);
html.setStyle(this.viewerImage, 'left', 0);
this._currentX = clientX;
this._currentLeft = 0;
} else if (this._currentLeft + delX <= this._maxOffsetLeft) {
html.setStyle(this.baseImage, 'left', this._maxOffsetLeft + 'px');
html.setStyle(this.viewerImage, 'left', this._maxOffsetLeft + 'px');
this._currentX = clientX;
this._currentLeft = this._maxOffsetLeft;
} else {
html.setStyle(this.baseImage, 'left', this._currentLeft + delX + 'px');
html.setStyle(this.viewerImage, 'left', this._currentLeft + delX + 'px');
this._currentX = clientX;
this._currentLeft += delX;
}
},</pre> <p>縮放的主要原則就是<strong>保持裁剪框的中心點在縮放前后的相對位置不變</strong>。 </p>
為了將縮放后的原裁切框的中心點移回原位，我們需要計算兩中值：圖片大小變化量，圖片左上角移動量。
var delImageWidth = this._minImageWidth (this._zoomRatio - 1) leftPercentage / 100;
var delImageHeight = this._minImageHeight (this._zoomRatio - 1) leftPercentage / 100;
var imageStyle = html.getComputedStyle(this.baseImage);
this._currentLeft = parseFloat(imageStyle.left);
this._currentTop = parseFloat(imageStyle.top);
var delImageLeft = (Math.abs(this._currentLeft) + this.idealWidth / 2)
((this._minImageWidth + delImageWidth) / this._currentImageWidth - 1);
var delImageTop = (Math.abs(this._currentTop) + this.idealHeight / 2)
((this._minImageWidth + delImageWidth) / this._currentImageWidth - 1);</pre></div>
其中_zoomRatio = _maxImageWidth / _minImageWidth; _maxImageWidth為圖片原始大小，_minImageWidth是讓圖片接近裁切框的最小寬度。
leftPercentage為滑動按鈕相對滑動條的位移百分比。
_currentLeft、_currentTop是本次縮放前圖片相對裁切框的絕對位置(position:absolute)。
_currentImageWidth、_currentImageHeight是本次縮放前圖片的大小。
剩下要做的是防止裁切框內出現空白現象，假設用戶放大圖片，將圖片拖放到邊界與裁切框邊界重合，這時縮小圖片的話裁切框內便會出現空白。為了防止這種情況我們也需要做相應處理。
當圖片左上邊界與裁切框左上邊界重合時，無論如何縮小，image的left、top始終為零，只改變圖片大小。
當圖片右下邊界與裁切框右下邊界重合時，根據圖片大小與裁切框大小可以計算出合適的left跟top
//prevent image out the crop box
if (leftPercentage - _latestPercentage >= 0) {
console.log('zoomin');
html.setStyle(this.baseImage, {
top: this._currentTop -delImageTop + 'px',
left: this._currentLeft -delImageLeft + 'px'
});
html.setStyle(this.viewerImage, {
top: this._currentTop -delImageTop + 'px',
left: this._currentLeft -delImageLeft + 'px'
});
} else {
console.log('zoomout');
var top = 0;
var left = 0;
if (this._currentTop - delImageTop >= 0) {
top = 0;
} else if (this._currentTop - delImageTop +
this._minImageHeight + delImageHeight <=
this.idealHeight) {
top = this.idealHeight - this._minImageHeight - delImageHeight;
} else {
top = this._currentTop - delImageTop;
}
console.log(this._currentLeft, delImageLeft);
if (this._currentLeft - delImageLeft >= 0) {
left = 0;
} else if (this._currentLeft - delImageLeft +
this._minImageWidth + delImageWidth <=
this.idealWidth) {
left =this.idealWidth - this._minImageWidth - delImageWidth;
} else {
left = this._currentLeft - delImageLeft;
}
html.setStyle(this.baseImage, {
top: top + 'px',
left: left + 'px'
});
html.setStyle(this.viewerImage, {
top: top + 'px',
left: left + 'px'
});
}</pre> <p>以上便是客戶端的實現思路。<a href="/misc/goto?guid=4959630893966408891" target="_blank">全部代碼</a>，瀏覽器支持：現代瀏覽器和ie9+，稍后會將ie8也支持上。 </p>
服務器端使用nodejs+express框架，主要代碼如下：
/**
body: {
imageString: base64 code
maxSize: w,h
cropOptions: w,h,t,l
}
**/
exports.cropImage = function(req, res) {
var base64Img = req.body.imageString;
if(!/^data:image\/.;base64,/.test(base64Img)){
res.send({
success: false,
message: 'Bad base64 code format'
});
}
var fileFormat = base64Img.match(/^data:image\/(.);base64,/)[1];
var base64Data = base64Img.replace(/^data:image\/.*;base64,/, "");
var maxSize = req.body.maxSize;
maxSize = maxSize.split(',');
var cropOptions = req.body.cropOptions;
cropOptions = cropOptions.split(',');
try{
var buf = new Buffer(base64Data, 'base64');
var jimp = new Jimp(buf, 'image/' + fileFormat, function() {
var maxW = parseInt(maxSize[0], 10);
var maxH = parseInt(maxSize[1], 10);
var cropW = parseInt(cropOptions[0], 10);
var cropH = parseInt(cropOptions[1], 10);
var cropT = parseInt(cropOptions[2], 10);
var cropL = parseInt(cropOptions[3], 10);
this.resize(maxW, maxH)
.crop(cropT, cropL, cropW, cropH);
});
jimp.getBuffer('image/' + fileFormat, function(b) {
var base64String = "data:image/" + fileFormat + ";base64," + b.toString('base64');
res.send({
success: true,
source: base64String
});
});
}catch(err) {
logger.error(err);
res.send({
success: false,
message: 'unable to complete operations'
});
}
};</pre>來源：我的小樹林
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