如何实现SVG的preserveAspectRatio=“xMinYMin slice"用于画布 drawImage 方法. [英] How to implement SVG's preserveAspectRatio="xMinYMin slice" for canvas drawImage method.

问题描述

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

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

推荐答案

Ken 的回答 在标记为重复的答案中确实执行与 xMinYMin slice 相同的操作，如果你像 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 :

它确实按照 drawImage() 方法的要求返回了一个带有 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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