javascript - HTML5 canvas 多重效果 - 锯齿状边缘

Question

我们公司的网站有一个内置于 Flash 中的“随机碎片生成器”，它会在网站标题下方随机创建许多重叠的彩色碎片图形。

http://www.clarendonmarketing.com

我正在尝试使用 HTML5 复制这种效果，虽然我可以很容易地生成随机碎片，但混合重叠(在 Adob​​e 术语中是乘法)证明是一个挑战。

我有一个解决方案，基本上是在绘制每个分片之前创建一个包含所有 Canvas 像素数据的数组，然后在绘制每个分片之后创建另一个包含 Canvas 像素数据的数组。然后它比较两者，并在第一个数组中找到一个非透明像素，该像素在第二个数组中的对应像素与当前选择的填充颜色匹配，它使用由“乘法”函数(topValue * bottomValue)确定的新颜色值重新绘制它/255).

通常这很好用并达到了预期的效果，除了在重叠碎片的边缘周围会产生锯齿状效果。

我相信这与浏览器的抗锯齿有关。我已尝试为计算出的像素复制原始像素的 alpha channel 值，但这似乎没有帮助。

Javascript:

// Random Shard Generator v2 (HTML5)

var theCanvas;
var ctx;

var maxShards = 6;
var minShards = 3;

var fillArray = new Array(
    [180,181,171,255], 
    [162,202,28,255], 
    [192,15,44,255], 
    [222,23,112,255], 
    [63,185,127,255], 
    [152,103,158,255], 
    [251,216,45,255], 
    [249,147,0,255], 
    [0,151,204,255]
);

var selectedFill;

window.onload = function() {

    theCanvas = document.getElementById('shards');
    ctx = theCanvas.getContext('2d');
    //ctx.translate(-0.5, -0.5)

    var totalShards = getRandom(maxShards, minShards);

    for(i=0; i<=totalShards; i++) {
        //get snapshot of current canvas
        imgData = ctx.getImageData(0,0,theCanvas.width,theCanvas.height);
        currentPix = imgData.data
        //draw a shard
        drawRandomShard();
        //get snapshot of new canvas
        imgData = ctx.getImageData(0,0,theCanvas.width,theCanvas.height);
        pix = imgData.data;
        //console.log(selectedFill[0]+','+selectedFill[1]+','+selectedFill[2]);
        //alert('break')

        //CALCULATE THE MULTIPLIED RGB VALUES FOR OVERLAPPING PIXELS

        for (var j = 0, n = currentPix.length; j < n; j += 4) {
            if (    
                //the current pixel is not blank (alpha 0)
                (currentPix[j+3]>0)
                    && //and the new pixel matches the currently selected fill colour
                (pix[j]==selectedFill[0] && pix[j+1]==selectedFill[1] && pix[j+2]==selectedFill[2])
            ) { //multiply the current pixel by the selected fill colour
                //console.log('old: '+currentPix[j]+','+currentPix[j+1]+','+currentPix[j+2]+','+currentPix[j+3]+'\n'+'new: '+pix[j]+','+pix[j+1]+','+pix[j+2]+','+pix[j+3]);
                pix[j] = multiply(selectedFill[0], currentPix[j]); // red
                pix[j+1] = multiply(selectedFill[1], currentPix[j+1]); // green
                pix[j+2] = multiply(selectedFill[2], currentPix[j+2]); // blue
            }
        }
        //update the canvas
        ctx.putImageData(imgData, 0, 0);        
    }


};

function drawRandomShard() {

    var maxShardWidth = 200;
    var minShardWidth = 30;
    var maxShardHeight = 16;
    var minShardHeight = 10;
    var minIndent = 4;
    var maxRight = theCanvas.width-maxShardWidth;
    //generate a random start point
    var randomLeftAnchor = getRandom(maxRight, 0);
    //generate a random right anchor point
    var randomRightAnchor = getRandom((randomLeftAnchor+maxShardWidth),(randomLeftAnchor+minShardWidth));
    //generate a random number between the min and max limits for the lower point
    var randomLowerAnchorX = getRandom((randomRightAnchor - minIndent),(randomLeftAnchor + minIndent));
    //generate a random height for the shard
    var randomLowerAnchorY = getRandom(maxShardHeight, minShardHeight);
    //select a fill colour from an array
    var fillSelector = getRandom(fillArray.length-1,0);
    //console.log(fillSelector);
    selectedFill = fillArray[fillSelector];

    drawShard(randomLeftAnchor, randomLowerAnchorX, randomLowerAnchorY, randomRightAnchor, selectedFill);

}

function drawShard(leftAnchor, lowerAnchorX, lowerAnchorY, rightAnchor, selectedFill) {

    ctx.beginPath();
    ctx.moveTo(leftAnchor,0);
    ctx.lineTo(lowerAnchorX,lowerAnchorY);
    ctx.lineTo(rightAnchor,0);
    ctx.closePath();
    fillColour = 'rgb('+selectedFill[0]+','+selectedFill[1]+','+selectedFill[2]+')';
    ctx.fillStyle=fillColour;
    ctx.fill();

};

function getRandom(high, low) {
    return Math.floor(Math.random() * (high-low)+1) + low;
}

function multiply(topValue, bottomValue){
    return topValue * bottomValue / 255;
};

工作演示: http://www.clarendonmarketing.com/html5shards.html

Answer 1

你真的需要乘法吗？为什么不只使用较低的不透明度混合？

演示 http://jsfiddle.net/wk3eE/

ctx.globalAlpha = 0.6;
for(var i=totalShards;i--;) drawRandomShard();

编辑:如果你真的需要乘法，那么leave it to the professionals ，因为带有 alpha 值的乘法模式有点棘手:

演示 2:http://jsfiddle.net/wk3eE/2/

<script type="text/javascript" src="context_blender.js"></script>
<script type="text/javascript">
  var ctx = document.querySelector('canvas').getContext('2d');

  // Create an off-screen canvas to draw shards to first
  var off = ctx.canvas.cloneNode(true).getContext('2d');

  var w = ctx.canvas.width, h = ctx.canvas.height;
  for(var i=totalShards;i--;){
    off.clearRect(0,0,w,h);        // clear the offscreen context first
    drawRandomShard(off);          // modify to draw to the offscreen context
    off.blendOnto(ctx,'multiply'); // multiply onto the main context
  }
</script>