Canvas取色法的实现

作者：图形开发学院
canvas
时间：2024-06-20 22:25:54
99次访问

取色法是一种高效的图形拾取技术，其核心思想在于利用Canvas的像素操作API实现图形的快速识别与提取。在绘制图像的过程中，取色法会在另一个Canvas上同步生成一个具有相同坐标位置和大小的缓存图形，但每个图形对象都采用独特的颜色进行绘制，并且会保存一份图形颜色与图形对象的对照表。

在实际操作中，通过Canvas渲染上下文对象提供的ctx.getImageData(x, y, 1, 1)方法，可以精确地获取指定位置像素的颜色信息。由于缓存图形中的颜色都是唯一的，因此可以迅速从颜色对照表中找到对应的图形对象，实现精准拾取。需要注意的是，ctx.getImageData()方法返回的数据中包含了data属性，该属性为Uint8ClampedArray类型，存储了像素数据。每个像素由四个字节的值构成，分别代表红、绿、蓝和透明度（r,g,b,a）。

对于常见的几何图形，取色法可以直接在缓存图形上进行绘制。然而，对于文本和图像这类较为复杂的图形对象，则需要将其转换为矩形，并以矩形的方式绘制在缓存图形中。这样做可以确保取色法的有效性和准确性，使其能够广泛应用于各种场景。

通过取色法，我们可以从一张包含多种图形对象的图像中快速、准确地拾取目标对象。这种方法不仅提高了图形拾取的效率，还增强了图形处理的灵活性和可扩展性。因此，取色法在图形处理领域具有广泛的应用前景。

示例效果与源代码：

运行效果

<!DOCTYPE html>
<html>

<head>
    <title>点图查询-取色法</title>
    <meta charset="UTF-8">
    <!-- frame所需脚本和样式 -->
    <script type="text/javascript" src="/examples/script/lib/jquery-1.11.2.min.js"></script>
    <link type="text/css" rel="stylesheet" href="/examples/script/bootstrap-3.3.5/css/bootstrap.min.css">
    <script src="/examples/graph/js/helper.js"></script>
</head>

<body style="overflow: hidden; margin:10px;">
    <div id="graphWrapper" style="width:500px; height:500px; border:solid 1px #CCC;"></div>
    <div id="graphColor" style="position:absolute; left:520px; top:10px; width:500px; height:500px; border:solid 1px #CCC; background-image: url('/examples/graph/images/transparent.png');">
    </div>
    <div style="margin:10px; text-align: left;">
        <div class="checkbox" style="font-size: 16px;">
            <label style="margin-right:20px"><input id="chkPoint" type="checkbox">点</label>
            <label style="margin-right:20px"><input id="chkRect" type="checkbox">矩形</label>
            <label style="margin-right:20px"><input id="chkPolyline" type="checkbox">线</label>
            <label style="margin-right:20px"><input id="chkPolygon" type="checkbox">多边形</label>
            <label style="margin-right:20px"><input id="chkCircle" type="checkbox">圆形</label>
            <label style="margin-right:20px"><input id="chkEllipse" type="checkbox">椭圆</label>
            <label style="margin-right:20px"><input id="chkText" type="checkbox">文字</label>
        </div>
    </div>
</body>
<script type="module">
    import {
        Graph, Layer, VectorSource, debug, Color, MathUtil, Collide, circle2LineRing, getStarLineRing,
        GGeometryType, Rect, Point, Circle, Ellipse, Text, Polyline, Polygon, EventType
    } from "/examples/src/index.js";

    // graph对象
    let graph = new Graph({
        "target": "graphWrapper",
        "hitGetColor": true
    });

    // 网格水印层
    let debugLayer = debug.generateGrid(Object.assign({ "interval": 10, "graph": graph }, graph.getSize()));
    debugLayer.getSource().add(new Text({
        "text": "按点拾取（取色法）Demo",
        "x": graph.getSize().width / 2,
        "y": graph.getSize().height / 2,
        "vectorSize": false,
        "style": { "fillColor": "#D0D0D0", "fontSize": 30, "fontName": "黑体", "textAlign": "center", "textBaseline": "middle" }
    }));

    // 新建绘图图层
    let layer = graph.addLayer();
    // 浮动层
    let overLayer = graph.addOverLayer();
    
    let lastPosition = [0, 0];

    // pageload
    $(document).ready(function () {

        let colorCanvas = graph.getRenderer().getHitImage();
        $("#graphColor").append(colorCanvas);
        
        let tmpCanvas = document.createElement("canvas");
        $("#graphColor").append(tmpCanvas)
        tmpCanvas.width = colorCanvas.width;
        tmpCanvas.height = colorCanvas.height;
        tmpCanvas.style.position = "absolute";
        tmpCanvas.style.top = 0;
        tmpCanvas.style.left = 0;
        let ctxColor = tmpCanvas.getContext('2d');

        // 鼠标移动事件
        graph.getRenderObject().on('mousemove', function (e) {
            let point = [e.offsetX, e.offsetY];
            let coord = graph.getCoordinateFromPixel(point, true);

            // 清空浮动层数据
            overLayer.getSource().clearData();

            // 碰撞检测
            _collideCheck(point);

            // 鼠标位置点
            overLayer.getSource().add(new Point({
                "x": coord[0],
                "y": coord[1],
                "size": -5,
                "style": { "fillColor": "blue", "fillStyle": 1, "color": "none" }
            }));
            
            lastPosition = [point[0], point[1]];
        });
        
        // 在颜色Canvas上绘制圆点
        graph.on(EventType.RenderAfter, function (args) {
			ctxColor.clearRect(0, 0, ctxColor.canvas.width, ctxColor.canvas.height);
            // 绘制圆点
            ctxColor.beginPath();
            ctxColor.arc(lastPosition[0], lastPosition[1], 3, 0, 2*Math.PI);
            ctxColor.fillStyle = "blue";
            ctxColor.fill();
        });

        // 取色法进行碰撞检测
        function _collideCheck(coord) {
            // 显示颜色值
            let hitColor = graph.getRenderer().getColor(coord);
            if (hitColor) {
                if (graph.viewGeomList.has(hitColor)) {
                    let geom = graph.viewGeomList.get(hitColor).clone();
                    geom.setStyle({ "fillColor": "#FF2020", "color": "#FF2020", "lineWidth":4 });
                    overLayer.getSource().add(geom);
                    return true;
                }
            }
            return false;
        }

        // 复选框状态事件：清空或随机生成对象
        $("#chkPoint").on("change", function () {
            if ($(this).prop("checked")) {
                for (let i = 0; i < MathUtil.getRandomNum(6, 10); i++) {
                    layer.getSource().add(new Point({
                        "x": MathUtil.getRandomNum(50, 450),
                        "y": MathUtil.getRandomNum(50, 450),
                        "size": MathUtil.getRandomNum(10, 20),
                        "style": { "fillColor": "#FFDFDF", "fillStyle": 1, "lineWidth": 1, "color": "#FFA5E8" }
                    }));
                }
            } else {
                layer.getSource().clearTypeData(GGeometryType.POINT);
            }
            graph.render();
        });

        // 复选框状态事件：清空或随机生成对象
        $("#chkRect").on("change", function () {
            if ($(this).prop("checked")) {
                for (let i = 0; i < MathUtil.getRandomNum(3, 5); i++) {
                    layer.getSource().add(new Rect({
                        "x": MathUtil.getRandomNum(50, 450),
                        "y": MathUtil.getRandomNum(50, 450),
                        "width": MathUtil.getRandomNum(40, 70),
                        "height": MathUtil.getRandomNum(20, 40),
                        "style": { "fillColor": "#FFDFDF", "fillStyle": 1, "lineWidth": 1, "color": "#FFA5E8" }
                    }));
                }
            } else {
                layer.getSource().clearTypeData(GGeometryType.RECT);
            }
            graph.render();
        });

        $("#chkCircle").on("change", function () {
            if ($(this).prop("checked")) {
                for (let i = 0; i < MathUtil.getRandomNum(3, 5); i++) {
                    layer.getSource().add(new Circle({
                        "x": MathUtil.getRandomNum(50, 450),
                        "y": MathUtil.getRandomNum(50, 450),
                        "radius": MathUtil.getRandomNum(20, 40),
                        "style": { "fillColor": "#DFFFBF", "fillStyle": 1, "lineWidth": 1, "color": "#FFA5E8" }
                    }));
                }
            } else {
                layer.getSource().clearTypeData(GGeometryType.CIRCLE);
            }
            graph.render();
        });

        $("#chkText").on("change", function () {
            if ($(this).prop("checked")) {
                for (let i = 0; i < MathUtil.getRandomNum(3, 5); i++) {
                    layer.getSource().add(new Text({
                        "x": MathUtil.getRandomNum(50, 450),
                        "y": MathUtil.getRandomNum(50, 450),
                        "rotation": MathUtil.getRandomNum(0, 90),
                        "text": "碰撞检测文本",
                        "style": { "fontSize": 24, "fontName": "黑体", "fillColor": "#0000FF", }
                    }));
                }
            } else {
                layer.getSource().clearTypeData(GGeometryType.TEXT);
            }
            graph.render();
        });

        $("#chkEllipse").on("change", function () {
            if ($(this).prop("checked")) {
                for (let i = 0; i < MathUtil.getRandomNum(2, 4); i++) {
                    layer.getSource().add(new Ellipse({
                        "x": MathUtil.getRandomNum(50, 450),
                        "y": MathUtil.getRandomNum(50, 450),
                        "radiusX": MathUtil.getRandomNum(20, 40),
                        "radiusY": MathUtil.getRandomNum(20, 20),
                        "style": { "fillColor": "#9FFFFF", "fillStyle": 1, "lineWidth": 1, "color": "#FFA5E8" }
                    }));
                }
            } else {
                layer.getSource().clearTypeData(GGeometryType.ELLIPSE);
            }
            graph.render();
        });

        $("#chkPolyline").on("change", function () {
            if ($(this).prop("checked")) {
                for (let i = 0; i < MathUtil.getRandomNum(3, 5); i++) {
                    let [x, y] = [MathUtil.getRandomNum(50, 400), MathUtil.getRandomNum(50, 400)];
                    layer.getSource().add(new Polyline({
                        "coords": [[x, y], [x + MathUtil.getRandomNum(10, 300), y + MathUtil.getRandomNum(-100, 100)]],
                        "style": { "lineWidth": 4, "color": "#00957D" }
                    }));
                }
            } else {
                layer.getSource().clearTypeData(GGeometryType.POLYLINE);
            }
            graph.render();
        });

        $("#chkPolygon").on("change", function () {
            if ($(this).prop("checked")) {
                for (let i = 0; i < MathUtil.getRandomNum(3, 5); i++) {
                    let center = [MathUtil.getRandomNum(150, 400), MathUtil.getRandomNum(50, 400)];
                    let sideNum = MathUtil.getRandomNum(3, 8);
                    let radius = MathUtil.getRandomNum(20, 40);
                    layer.getSource().add(new Polygon({
                        "coords": circle2LineRing(center, radius, sideNum),
                        "style": { "fillColor": "#E5FFF5", "fillStyle": 1, "lineWidth": 1, "color": "#FF9F9F" }
                    }));

                    center = [MathUtil.getRandomNum(150, 620), MathUtil.getRandomNum(50, 400)];
                    layer.getSource().add(new Polygon({
                        "coords": getStarLineRing(center, radius, null, sideNum >= 6 ? sideNum - 3 : sideNum),
                        "style": { "fillColor": "#E5FFF5", "fillStyle": 1, "lineWidth": 1, "color": "#FF9F9F" }
                    }));
                }
            } else {
                layer.getSource().clearTypeData(GGeometryType.POLYGON);
            }
            graph.render();
        });
    })

</script>

</html>

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