globalCompositeOperation在背景上绘制白色

Question

我在globalCompositeOperation画布的javascript中遇到了问题。

https: //jsfiddle.net/6j51kxeh/

我使用的是“目的地”，在心灵的精灵中画一个黑色的正方形，所以当玩家错过了那颗心时，它会显示出来。它把黑色的心画得很好，但它画的比其他的东西都要白。如果您将globalCompositeOperation行注释掉，您将看到它是在其他事物之上绘制的(只是来自同一雪碧表的一组随机的精灵)。谢谢你的帮助。

我也有困难，只是链接到jsfiddle，我已经玩够了。

var canvas = document.createElement("canvas");    

canvas.height = window.innerHeight;
canvas.width  = window.innerWidth;

var scale = 4;

document.body.appendChild(canvas);

var draw = canvas.getContext("2d");

draw.imageSmoothingEnabled = false;
draw.mozImageSmoothingEnabled = false;
draw.oImageSmoothingEnabled = false;
draw.webkitImageSmoothingEnabled = false;

var assetsToLoad = [];
var assetsLoaded = 0;

var sprites = new Image();
sprites.onload = loadHandler;
sprites.src = "https://dl.dropboxusercontent.com/u/38825680/Site/images/sprites.png";
assetsToLoad.push(sprites);


function loadHandler() {
    assetsLoaded++;
    if(assetsLoaded === assetsToLoad.length) {
        console.log(assetsLoaded + " asset[s] loaded");

        Draw();
    }
};

function Draw() {
    draw.save();
    draw.scale(scale,scale);

    draw.drawImage(sprites,0,64,64,64,0,0,64,64);
    // draw.drawImage(sprites,112,80,16,16,0,0,16,16);
    draw.fillStyle = "black";
    draw.fillRect(0,0,16,16);

    //draw.globalCompositeOperation = "destination-in";

    draw.drawImage(sprites,96,48,16,16,0,0,16,16);


    draw.restore();
}

Answer 1

destination-in合成具有破坏性，因为它将擦除画布上的所有内容，除非新的(心脏)与任何现有的不透明像素重叠。所以这种形式的合成不太可能是你所需要的。

澄清:你想要在另一个精灵图像上画出心脏精灵，然后从心脏被添加到的另一个精灵图像中清除那个精灵？

如果是，则：

• 若要添加心脏，请将心脏精灵画在另一个精灵之上。
• 要清除心灵，只需重新画另一个精灵。

示例代码和演示：

​

var canvas = document.createElement("canvas");    

canvas.height = 640;//window.innerHeight;
canvas.width  = 960;//window.innerWidth;

var scale = 4;

document.body.appendChild(canvas);

var draw = canvas.getContext("2d");

draw.imageSmoothingEnabled = false;
draw.mozImageSmoothingEnabled = false;
draw.oImageSmoothingEnabled = false;
draw.webkitImageSmoothingEnabled = false;

var assetsToLoad = [];
var assetsLoaded = 0;

var sprites = new Image();
sprites.onload = loadHandler;

sprites.src = "https://dl.dropboxusercontent.com/u/38825680/Site/images/sprites.png";
assetsToLoad.push(sprites);


function loadHandler() {
  assetsLoaded++;
  if(assetsLoaded === assetsToLoad.length) {
    console.log(assetsLoaded + " asset[s] loaded");

    draw.scale(scale,scale);
    Draw();
  }
};

function Draw() {
  draw.drawImage(sprites,0,64,64,64,0,0,64,64);
}



function addHeart(x,y){
  draw.fillStyle = "black";
  draw.fillRect(x,y,16,16);
  draw.drawImage(sprites,96,48,16,16,x,y,16,16);
}

function removeHeart(x,y,spriteX,spriteY){
  draw.clearRect(x,y,16,16);
  draw.drawImage(sprites,spriteX,spriteY+64,16,16,x,y,16,16)
}

$('#add').click(function(){
  addHeart(0,0);
});

$('#remove').click(function(){
  removeHeart(0,0,0,0);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<button id=add>Add heart</button>
<button id=remove>Remove heart</button>

​