我们如何使用样式而不使用地图将 OpenLayers 3 功能渲染为 canvas？

Question

问题：

我们如何使用样式而不是使用地图将要素呈现为 canvas？

背景:

我有一个示例，它将几何图形渲染为 canvas 尊重 ol3 风格，但它只与未构建版本的 openlayers (ol-debug.js) 一起运行，因为它使用私有函数 (ol.vec.Mat4).

works when using ol-debug.js

fails when using ol.js

一种替代方法是创建一个地图，添加一个矢量图层，设置要素的样式，将要素添加到图层并从地图中删除所有 events/controls，使其看起来像 canvas.

第二种是使用goog.vec.Mat4。

let scale = Math.min(canvas.width / ol.extent.getWidth(extent), canvas.height / ol.extent.getHeight(extent));
console.log("scale", scale);

let transform = Mat4.makeTransform2D(identity,
    canvas.width / 2, canvas.height / 2, // translate to origin
    scale, -scale, //scale
    0, // rotation
    -center[0], -center[1] // translate back
);
console.log("transform", transform);

let renderer = new ol.render.canvas.Immediate(ctx, 1, extent, transform, 1);
renderer.drawFeature(feature, style);

第三个与第二个类似，我负责在使用 ol.render.toContext 之前将几何图形转换为像素坐标，如本 example 中所示。

我觉得差不多用完了吗？或者有别的办法吗？

Answer 1

呸！在 openlayers 网站上找到了 example！

其中样本坐标已经是像素：

<!DOCTYPE html>
<html>
  <head>
    <title>Render geometries to a canvas</title>
    <link rel="stylesheet" href="http://openlayers.org/en/v3.17.1/css/ol.css" type="text/css">
    <script src="http://openlayers.org/en/v3.17.1/build/ol.js"></script>
  </head>
  <body>
    <canvas id="canvas"></canvas>
    <script>
      var canvas = document.getElementById('canvas');
      var vectorContext = ol.render.toContext(canvas.getContext('2d'), {size: [100, 100]});

      var fill = new ol.style.Fill({color: 'blue'});
      var stroke = new ol.style.Stroke({color: 'black'});
      var style = new ol.style.Style({
        fill: fill,
        stroke: stroke,
        image: new ol.style.Circle({
          radius: 10,
          fill: fill,
          stroke: stroke
        })
      });
      vectorContext.setStyle(style);

      vectorContext.drawGeometry(new ol.geom.LineString([[10, 10], [90, 90]]));
      vectorContext.drawGeometry(new ol.geom.Polygon([[[2, 2], [98, 2], [2, 98], [2, 2]]]));
      vectorContext.drawGeometry(new ol.geom.Point([88, 88]));
    </script>
  </body>
</html>

但如问题所示，平移 -> 缩放 -> 平移转换数据：

function render(canvas: HTMLCanvasElement, line: ol.Coordinate[], style: ol.style.Style) {
    let extent = ol.extent.boundingExtent(line);
    let [dx, dy] = ol.extent.getCenter(extent);
    let [sx, sy] = [canvas.width / ol.extent.getWidth(extent), canvas.height / ol.extent.getHeight(extent)];
    line= translate(line, [-dx, -dy]);
    line= scale(line, [Math.min(sx, sy), -Math.min(sx, sy)]);
    line= translate(line, [canvas.width / 2, canvas.height / 2]);

    let feature = new ol.Feature({
        geometry: new ol.geom.Polygon([line]),
        style: style
    });

    let vtx = ol.render.toContext(canvas.getContext("2d"));
    vtx.drawFeature(feature, style);
}

这是我的 TRS 逻辑：

function translate(points: number[][], vector: number[]) {
    return points.map(p => vector.map((v, i) => v + p[i]));
}

function rotate(points: number[][], a: number) {
    return points.map(p => {
        let [x, y, cos, sin] = [p[0], p[1], Math.cos(a), Math.sin(a)];
        return [
            x * cos - y * sin,
            x * sin + y * cos
        ];
    });
}

function scale(points: number[][], vector: number[]) {
    return points.map(p => vector.map((v, i) => v * p[i]));
}

我们如何使用样式而不使用地图将 OpenLayers 3 功能渲染为 canvas？

How can we render an OpenLayers 3 feature to a canvas using a style but not using a map?

openlayers-3