本文基于腾讯课堂老胡的课《跟我学Openlayers--基础实例详解》做的学习笔记，使用的openlayers 5.3.x api。
源码 见 1081.html ，对应的 官网示例
image.png
核心技术点：
layer.on('postcompose', function(event) {})
绑定了layer的'postcompose'事件，并在渲染后进行图像的处理
image.png
<!DOCTYPE html>
<html>
<head>
<title>图像滤镜</title>
<link rel="stylesheet" href="../include/ol.css" type="text/css" />
<script src="../include/ol.js"></script>
</head>
<style></style>
<body>
<select id="kernel" name="kernel">
<option>none</option>
<option selected>sharpen锐化</option>
<option value="sharpenless">sharpen less轻度锐化</option>
<option>blur模糊</option>
<option>shadow阴影</option>
<option>emboss浮雕</option>
<option value="edge">edge detect边缘查找</option>
</select>
<div id="map" class="map"></div>
<div id="info"> </div>
<script>
var key = 'As1HiMj1PvLPlqc_gtM7AqZfBL8ZL3VrjaS3zIb22Uvb9WKhuJObROC-qUpa81U5';
var imagery = new ol.layer.Tile({
source: new ol.source.OSM()
});
var map = new ol.Map({
layers: [imagery],
target: 'map',
view: new ol.View({
center: ol.proj.fromLonLat([-120, 50]),
zoom: 6
})
});
var kernels = {
none: [
0, 0, 0,
0, 1, 0,
0, 0, 0
],
sharpen: [
0, -1, 0,
-1, 5, -1,
0, -1, 0
],
sharpenless: [
0, -1, 0,
-1, 10, -1,
0, -1, 0
],
blur: [
1, 1, 1,
1, 1, 1,
1, 1, 1
],
shadow: [
1, 2, 1,
0, 1, 0,
-1, -2, -1
],
emboss: [
-2, 1, 0,
-1, 1, 1,
0, 1, 2
],
edge: [
0, 1, 0,
1, -4, 1,
0, 1, 0
]
};
// 图像处理的方法，归一化
function normalize(kernel) {
var len = kernel.length;
var normal = new Array(len);
var i, sum = 0;
for (i = 0; i < len; ++i) {
sum += kernel[i];
}
if (sum <= 0) {
normal.normalized = false;
sum = 1;
} else {
normal.normalized = true;
}
for (i = 0; i < len; ++i) {
normal[i] = kernel[i] / sum;
}
return normal;
}
var select = document.getElementById('kernel');
var selectedKernel = normalize(kernels[select.value]);
// map 和layers 都可以触发postcompose
/**
* 更新kernel并且在选项更改时重新归一化.
*/
select.onchange = function () {
selectedKernel = normalize(kernels[select.value]);
map.render();//归一化之后，重新渲染地图
};
/**
*在"postcompose" 触发之后应用滤镜.
*/
imagery.on('postcompose', function (event) {
convolve(event.context, selectedKernel);//卷积操作，应用滤镜效果
});
/**
* Apply a convolution kernel to canvas. This works for any size kernel, but
* performance starts degrading above 3 x 3.
* @param {CanvasRenderingContext2D} context Canvas 2d context.
* @param {Array<number>} kernel Kernel.
* 此处是卷积操作
*/
function convolve(context, kernel) {
var canvas = context.canvas;
var width = canvas.width;
var height = canvas.height;
var size = Math.sqrt(kernel.length);
var half = Math.floor(size / 2);
var inputData = context.getImageData(0, 0, width, height).data;
var output = context.createImageData(width, height);
var outputData = output.data;
for (var pixelY = 0; pixelY < height; ++pixelY) {
var pixelsAbove = pixelY * width;
for (var pixelX = 0; pixelX < width; ++pixelX) {
var r = 0, g = 0, b = 0, a = 0;
for (var kernelY = 0; kernelY < size; ++kernelY) {
for (var kernelX = 0; kernelX < size; ++kernelX) {
var weight = kernel[kernelY * size + kernelX];
var neighborY = Math.min(
height - 1, Math.max(0, pixelY + kernelY - half));
var neighborX = Math.min(
width - 1, Math.max(0, pixelX + kernelX - half));
var inputIndex = (neighborY * width + neighborX) * 4;
r += inputData[inputIndex] * weight;
g += inputData[inputIndex + 1] * weight;
b += inputData[inputIndex + 2] * weight;
a += inputData[inputIndex + 3] * weight;
}
}
var outputIndex = (pixelsAbove + pixelX) * 4;
outputData[outputIndex] = r;
outputData[outputIndex + 1] = g;
outputData[outputIndex + 2] = b;
outputData[outputIndex + 3] = kernel.normalized ? a : 255;
}
}
context.putImageData(output, 0, 0);
}
</script>
</body>
</html>
网友评论