JavaScript canvas.drawImage alpha损失
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javascript的新手。
我正在构建一个网站，将模板图像与用户的输入进行比较，并构建一些图像处理指令。模板图像存储在站点的文件夹中。
$(function() {
$("#btnDirective").click(generateEmoteDirectiveFile);
});
function generateEmoteDirectiveFile() {
getImageData('imgs/templates/human.png');
}
function getImageData(source) {
var image = new Image();
image.onload = function () {
var canvas = document.createElement('canvas');
canvas.width = this.naturalWidth;
canvas.height = this.naturalHeight;
var ctx = canvas.getContext('2d');
ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
var data = ctx.getImageData(0, 0, canvas.width, canvas.height).data;
for (var px = 0, cx = canvas.width * canvas.height * 4; px < cx; px += 4)
if (data[px+3] != 255 && data[px+3] != 0)
console.log("Alpha: " + data[px+3]);
};
image.src = source;
}<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<script src="scripts/jquery-1.11.3.min.js"></script>
<script src="scripts/script.js"></script>
</head>
<body>
<button id="btnDirective">Generate</button>
</body>
</html>'imgs/templates/human.png'包含许多半透明值，从不提取：
答案
所以你试图得到每个像素的整数值，它的alpha值既不是0也不是255？
那么，您的图像具有完全透明的像素，或者它们是纯白色/黑色。所以你没有部分透明的像素。
请参阅下面使用的Emojipedia的辣椒表情符号图像。大约约6％的像素是部分透明的（围绕边界）。
$(function() {
$('#btnDirective').click(generateEmoteDirectiveFile);
});
function generateEmoteDirectiveFile() {
getImageData('https://i.imgur.com/4LzuX3G.png');
getImageData('https://i.imgur.com/R7g697w.png'); // Using a testable image
}
function getImageData(source) {
var image = new Image();
image.crossOrigin = 'Anonymous'; // This allows Imgur CORS...
image.onload = function() {
var canvas = document.createElement('canvas');
canvas.width = this.naturalWidth;
canvas.height = this.naturalHeight;
document.body.appendChild(canvas); // Add canvas to body...
var ctx = canvas.getContext('2d');
ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
var data = ctx.getImageData(0, 0, canvas.width, canvas.height).data;
var pixels = canvas.width * canvas.height;
var alphaValues = [];
for (var px = 0, cx = pixels * 4; px < cx; px += 4) {
if (data[px + 3] != 255 && data[px + 3] != 0) {
alphaValues.push(data[px + 3]);
}
}
var nonAlphaPixels = (alphaValues.length / pixels * 100).toFixed(2);
console.log('Alpha (' + nonAlphaPixels + '%): ' + alphaValues.join(','));
};
image.src = source;
}
function drawPartialPixels(imageData) {
}body { background: #444; }
#btnDirective { display: block; }<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
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