[英]What is wrong with this method for counting non-white pixels in a canvas element?
[英]Counting pixels value in Canvas
我對畫布中圖像的像素計數有疑問,請參見下面的代碼:
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Pixel Counting</title>
<script type="text/javascript">
window.onload = function() {
var img = new Image();
img.src="lena.jpg";
img.onload = function() {
countPixel(img)
};
}
function countPixel(img) {
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
// Draw the image to canvas.
context.drawImage(img, 0, 0);
// Now we can get the image data from the canvas.
var imageData = context.getImageData(0, 0, img.width, img.height);
var data = imageData.data;
// Do the pixel counting.
var redCount = new Array(256);
var greenCount = new Array(256);
var blueCount = new Array(256);
for (var i = 0; i < 256; i++) {
redCount[i] = 0;
greenCount[i] = 0;
blueCount[i] = 0;
}
for (var i = 0; i < data.length; i += 4) {
redCount[data[i]]++; // red
greenCount[data[i + 1]]++; // green
blueCount[data[i + 2]]++; // blue
}
// Write the result to table.
var pixelTable = document.getElementById('pixel_table');
for (var i = 0; i < 256; i++) {
var row = pixelTable.insertRow(-1);
row.insertCell(-1).innerHTML = i;
row.insertCell(-1).innerHTML = redCount[i];
row.insertCell(-1).innerHTML = greenCount[i];
row.insertCell(-1).innerHTML = blueCount[i];
}
}
</script>
</head>
<body>
<div>
<canvas id="canvas" width="500" height="500">
</canvas>
</div>
<div>
<table id="pixel_table" border="1">
<caption style="font-size:25px;font-weight:bold;">Pixel Count</caption>
<tr id="header"><th>Intensity</th><th>Red</th><th>Green</th><th>Blue</th></tr>
</table>
</div>
</body>
</html>
我不明白這個for循環:
for (var i = 0; i < 256; i++) {
redCount[i] = 0;
greenCount[i] = 0;
blueCount[i] = 0;
}
這里的循環是什么意思? 這是計數的開始部分,但是為什么要將所有值都設為零?
如果不這樣做,數組將被未定義的值填充。
new Array(5);
將導致:
[undefined, undefined, undefined, undefined, undefined]
這是必需的,因為此時尚未定義聲明的數組中的任何元素。 循環從數組中的第一個元素開始,然后遍歷每個單個元素將初始值設置為0(否則它將是未定義的,這會在以后嘗試向其添加數字時給您帶來麻煩)。
但是,在這種情況下,更好的選擇是替換此塊:
var redCount = new Array(256);
var greenCount = new Array(256);
var blueCount = new Array(256);
for (var i = 0; i < 256; i++) {
redCount[i] = 0;
greenCount[i] = 0;
blueCount[i] = 0;
}
使用類型化數組,類型化數組的所有值都初始化為0,並且比基於節點的數組要快:
var redCount = new Uint32Array(256);
var greenCount = new Uint32Array(256);
var blueCount = new Uint32Array(256);
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