BabylonJS - 絲帶

絲帶將路徑陣列作為輸入，並在這些路徑上繪製線條。它使用複雜的邏輯來獲取座標。在以下給出的示例中，我們使用貝塞爾曲線方程來繪製絲帶。貝塞爾曲線主要用於 3D 遊戲中，用於對平滑曲線建模。曲線需要控制點，且曲線沿著控制點繪製。

演示

<!doctype html>
<html>
   <head>
      <meta charset = "utf-8">
      <title>BabylonJs - Basic Element-Creating Scene</title>
      <script src = "babylon.js"></script>
      <style>
         canvas {width: 100%; height: 100%;}
      </style>
   </head>

   <body>
      <canvas id = "renderCanvas"></canvas>
      <script type = "text/javascript">
         var canvas = document.getElementById("renderCanvas");
         var engine = new BABYLON.Engine(canvas, true);
         
         var createScene  = function() {

            var scene = new BABYLON.Scene(engine);
            scene.clearColor = new BABYLON.Color3(0.8, 0.8, 0.8);
            
            var camera = new BABYLON.ArcRotateCamera("Camera", 3 *Math.PI / 2, Math.PI / 2, 50, BABYLON.Vector3.Zero(), scene);
            camera.attachControl(canvas, false);

            // lights
            var light = new BABYLON.HemisphericLight("hemi", new BABYLON.Vector3(0, 1, 0), scene);
            light.groundColor = new BABYLON.Color3(0.2, 0.2, 0.5);
            light.intensity = 0.6;


            var light2 = new BABYLON.PointLight("light2", new BABYLON.Vector3(-20, 0, -20), scene);
            light2.diffuse = BABYLON.Color3.White();
            light2.specular = BABYLON.Color3.Green();
            light2.intensity = 0.6;

            // material
            var mat = new BABYLON.StandardMaterial("mat1", scene);
            mat.alpha = 1.0;
            mat.diffuseColor = new BABYLON.Color3(0.5, 0.5, 1.0);
            //mat.backFaceCulling = false;
            mat.wireframe = true;

            // cubic Bézier function
            // cubicBezier(vector3Origin, vector3Control1, vector3Control2, vector3Destination, segmentNumber)
            var cubicBezier = function(v0, v1, v2, v3, nb) {
               var bez = [];
               var step = 1 / nb;
               var equation = function(t, val0, val1, val2, val3) {
                  var res = (1 -t)*(1-t)*(1-t) * val0 + 3 * t * (1-t)*(1-t) * val1 + 3 * t*t *(1-t) * val2 + t*t*t * val3;
                  return res;
               };

               for(var i = 0; i <= 1; i += step) {
                  bez.push( new BABYLON.Vector3(equation(i, v0.x, v1.x, v2.x, v3.x), equation(i, v0.y, v1.y, v2.y, v3.y), equation(i, v0.z, v1.z, v2.z, v3.z)) );
               }
               bez.push(v3);
               return bez;
            };

            var populatePath = function(p) {
               var pi2 = Math.PI * 2;
               var i = p / 25 ;
               var path = cubicBezier( new BABYLON.Vector3(3 * Math.cos(pi2 * i),-10, 3 * Math.sin(pi2 *i)),
               new BABYLON.Vector3(12 * Math.cos(pi2 * i), p/5 ,10 * Math.sin(pi2 * i)),
               new BABYLON.Vector3(15 * Math.cos(pi2 * i), p/2 ,8 * Math.sin(pi2 * i)),
               new BABYLON.Vector3(3 * Math.cos(pi2 * i), 10, 3 * Math.sin(pi2 * i)), 50);
               return path;
            };

            // path visualizer helper
            var showPath = function(path, scene) {
               var line = BABYLON.Mesh.CreateLines("line", path, scene )
            };

            var paths = [];
            for (var p = 0; p < 20; p++) {
               paths[p] = populatePath(p); 
               showPath(paths[p], scene);
            }

            var ribbon = new BABYLON.Mesh.CreateRibbon("ribbon", paths, false, false, null, scene);
            ribbon.material = mat;

            scene.registerBeforeRender(function() {
               light2.position = camera.position;
            });
            return scene;
         };
         var scene = createScene();
         engine.runRenderLoop(function() {
            scene.render();
         });
      </script>
   </body>
</html>

輸出

以上程式碼行生成以下輸出 -

語法

var ribbon = new BABYLON.Mesh.CreateRibbon("ribbon", paths, false, false, null, scene);

絲帶使用 x、y 和 z 方向上的路徑陣列。上面使用的 Bezeir 曲線方程在此處計算 -

var cubicBezier = function(v0, v1, v2, v3, nb) {
   var bez = [];
   var step = 1 / nb;
   var equation = function(t, val0, val1, val2, val3) {
      var res = (1 -t)*(1-t)*(1-t) * val0 + 3 * t * (1-t)*(1-t) * val1 + 3 * t*t *(1-t) * val2 + t*t*t * val3;
      return res;
   };
   
   for(var i = 0; i <= 1; i += step) {
      bez.push( new BABYLON.Vector3(equation(i, v0.x, v1.x, v2.x, v3.x), equation(i, v0.y, v1.y, v2.y, v3.y), equation(i, v0.z, v1.z, v2.z, v3.z)) );
   }
   
   bez.push(v3);
   return bez;
};

貝塞爾曲線方程

var res = (1 -t)*(1-t)*(1-t) * val0 + 3 * t * (1-t)*(1-t) * val1 + 3 * t*t *(1-t) * val2 + t*t*t * val3;

babylonjs_parametric_shapes.htm

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