如何实现 SVG 的 preserveAspectRatio="xMinYMin slice";用于

本文介绍了如何实现 SVG 的 preserveAspectRatio="xMinYMin slice";用于画布的 drawImage 方法.的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着跟版网的小编来一起学习吧！

问题描述

是否有在画布中为 drawImage() 方法实现 SVG 的 preserveAspectRatio="xMinYMin slice" 的解决方法?

Is there a workaround for implement SVG's preserveAspectRatio="xMinYMin slice" for drawImage() method in canvas?

推荐答案

肯的回答在标记为重复的答案中确实与 xMinYMin slice 执行相同的操作，如果您将其称为 drawImageProp(ctx, img, 0, 0, canvas.width, canvas.height,0, 0).

Ken's answer in the marked as duplicate answer indeed does the same operation as xMinYMin slice if you call it like drawImageProp(ctx, img, 0, 0, canvas.width, canvas.height, 0, 0).

但是对于那些想要完整实现 svg 的 preserveAspectRatio 属性的人来说，这里有一个:

But for the ones who want the full implementation of the svg's preserveAspectRatio attribute, here is one :

根据 drawImage() 方法的要求，它确实返回了一个带有 sx,sy,sw,sh,dx,dy,dw,dh 的对象.

It does return an object with sx,sy,sw,sh,dx,dy,dw,dh as requested by the drawImage() method.

	var preserveAspectRatio = function(source, destination, userString) {

	  var srcWidth = source.width,
	    srcHeight = source.height,
	    destinationW = destination.width,
	    destinationH = destination.height;

	  // we should keep the whole source
	  var aRMeet = function(args) {

	    var srcRatio = (srcHeight / srcWidth),
	      destRatio = (destinationH / destinationW),

	      resultWidth = destRatio > srcRatio ? destinationW : destinationH / srcRatio,
	      resultHeight = destRatio > srcRatio ? destinationW * srcRatio : destinationH;

	    var getPos = function(arg, res, dest) {

	      var max = Math.max(res, dest),
	        min = Math.min(res, dest);

	      switch (arg) {
	        case 'Min': return 0;
	        case 'Mid': return (max - min) / 2;
	        case 'Max': return max - min;
	        default:    return 'invalid';
	      }
	    };

	    var obj = {
	      img: returnedImg,
	      sx: 0,
	      sy: 0,
	      swidth: srcWidth,
	      sheight: srcHeight,
	      dx: getPos(args[0], resultWidth, destinationW),
	      dy: getPos(args[1], resultHeight, destinationH),
	      dwidth: resultWidth,
	      dheight: resultHeight
	    };

	    if (obj[5] === 'invalid' || obj[6] === 'invalid') {
	      return default_obj;
	    }

	    return obj;
	  };

	  // we should slice the larger part
	  var aRSlice = function(args) {

	    var resultWidth, resultHeight;

	    var a = function() {
	      resultWidth = destinationW;
	      resultHeight = srcHeight * destinationW / srcWidth;
	    };

	    var b = function() {
	      resultWidth = srcWidth * destinationH / srcHeight;
	      resultHeight = destinationH;
	    };

	    if (destinationW > destinationH) {
	      a();
	      if (destinationH > resultHeight) {
	        b();
	      }
	    } else if (destinationW === destinationH) {
	      if (srcWidth > srcHeight) {
	        b();
	      } else {
	        a();
	      }
	    } else {
	      b();
	      if (destinationW > resultWidth) {
	        a();
	      }
	    }

	    var getPos = function(arg, res, dest, src) {
	      switch (arg) {
	        case 'Min': return 0;
	        case 'Mid': return (res - dest) / 2 * src / res;
	        case 'Max': return (res - dest) * src / res;
	        default: return 'invalid';
	      }
	    };

	    var x = getPos(args[0], resultWidth, destinationW, srcWidth);
	    var y = getPos(args[1], resultHeight, destinationH, srcHeight);

	    var obj = {
	      img: returnedImg,
	      sx: x,
	      sy: y,
	      swidth: srcWidth - x,
	      sheight: srcHeight - y,
	      dx: 0,
	      dy: 0,
	      dwidth: resultWidth - (x * (resultWidth / srcWidth)),
	      dheight: resultHeight - (y * (resultHeight / srcHeight)),
	    };

	    if (obj[1] === 'invalid' || obj[2] === 'invalid') {
	      return default_obj;
	    }

	    return obj;
	  };

	  // check if the object passed was drawable over a canvas
	  var returnedImg = source.nodeName === 'IMG' || source.nodeName === 'VIDEO' || source.nodeName === 'CANVAS' ? source : null;

	  // if an invalid string or none is set as the preserveAspectRatio, this should be considered as "xMidYMid meet"
	  var default_obj = aRMeet(['Mid', 'Mid']);

	  if (!userString) {
	    return default_obj;
	  } else {

	    var args = userString.trim().split(' '),
	      minMidMax = args[0].replace('x', '').split('Y');

	    switch (args[args.length - 1]) {
	      case "meet":  return aRMeet(minMidMax);
	      case "slice": return aRSlice(minMidMax);
	      default:      return default_obj;
	    }
	  }
	};

	 //
	 //  Snippet code 
	 //_______________

	var images = [new Image(), new Image(), new Image()];
	var img = images[0];
	var selects = document.querySelectorAll('select');

	var ctxL = outputLandscape.getContext('2d');
	var ctxP = outputPortrait.getContext('2d');
	var ctxS = outputSquare.getContext('2d');

	var update = function() {
	  ctxL.clearRect(0, 0, outputLandscape.width, outputLandscape.height);
	  ctxP.clearRect(0, 0, outputPortrait.width, outputPortrait.height);
	  ctxS.clearRect(0, 0, outputSquare.width, outputSquare.height);

	  var aspectRatioStr = 'x' + selects[0].value + 'Y' + selects[1].value + ' ' + selects[2].value;

	  var p = preserveAspectRatio(img, outputPortrait, aspectRatioStr);
	  ctxP.drawImage(p.img, p.sx, p.sy, p.swidth, p.sheight, p.dx, p.dy, p.dwidth, p.dheight);

	  var l = preserveAspectRatio(img, outputLandscape, aspectRatioStr);
	  ctxL.drawImage(l.img, l.sx, l.sy, l.swidth, l.sheight, l.dx, l.dy, l.dwidth, l.dheight);

	  var s = preserveAspectRatio(img, outputSquare, aspectRatioStr);
	  ctxS.drawImage(s.img, s.sx, s.sy, s.swidth, s.sheight, s.dx, s.dy, s.dwidth, s.dheight);

	  svgImagePortrait.setAttribute('preserveAspectRatio', aspectRatioStr);
	  svgImageLandscape.setAttribute('preserveAspectRatio', aspectRatioStr);
	  svgImageSquare.setAttribute('preserveAspectRatio', aspectRatioStr);
	};

	for (var i = 0; i < selects.length - 1; i++) {
	  selects[i].onchange = update;
	}
	selects[3].onchange = function() {
	  img = images[+this.value];
	  update();
	  svgImagePortrait.setAttributeNS('http://www.w3.org/1999/xlink', 'href', img.src);
	  svgImageLandscape.setAttributeNS('http://www.w3.org/1999/xlink', 'href', img.src);
	  svgImageSquare.setAttributeNS('http://www.w3.org/1999/xlink', 'href', img.src);

	}

	img.onload = function() {
	  svgImagePortrait.setAttributeNS('http://www.w3.org/1999/xlink', 'href', this.src);
	  svgImageLandscape.setAttributeNS('http://www.w3.org/1999/xlink', 'href', this.src);
	  svgImageSquare.setAttributeNS('http://www.w3.org/1999/xlink', 'href', this.src);

	  update();
	};


	images[0].src = "http://lorempixel.com/100/200";
	images[1].src = "http://lorempixel.com/200/100";
	images[2].src = "http://lorempixel.com/200/200";

html,body,canvas {
  margin: 0;
  display: block;
  font-size: .9em;
}
svg,canvas {
  display: inline-block;
}
canvas {
  border: 1px solid green;
}
svg {
  border: 1px solid blue;
}

x
<select name="x">
  <option value="Min">Min</option>
  <option value="Mid">Mid</option>
  <option value="Max">Max</option>
</select>
Y
<select name="Y">
  <option value="Min">Min</option>
  <option value="Mid">Mid</option>
  <option value="Max">Max</option>
</select>

<select name="slice_meet">
  <option value="slice">slice</option>
  <option value="meet">meet</option>
</select>
image format :
<select name="format">
  <option value="0">portrait</option>
  <option value="1">landscape</option>
  <option value="2">square</option>
</select>
<br>

<canvas id="outputLandscape" width="280" height="200"></canvas>
<svg width="280" height="200" viewBox="0 0 280 200">
  <image id="svgImageLandscape" x="0" y="0" width="100%" height="100%"></image>
</svg>
<br>
<canvas id="outputPortrait" width="200" height="280"></canvas>
<svg width="200" height="280" viewBox="0 0 200 280">
  <image id="svgImagePortrait" x="0" y="0" width="100%" height="100%"></image>
</svg>
<br>
<canvas id="outputSquare" width="200" height="200"></canvas>
<svg width="200" height="200" viewBox="0 0 200 200">
  <image id="svgImageSquare" x="0" y="0" width="100%" height="100%"></image>
</svg>

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如何实现 SVG 的 preserveAspectRatio="xMinYMin slice";用于

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