Issue loading heightmap from image about three.terrain HOT 3 CLOSED

        var heightmapImage = new Image();
            heightmapImage.src = 'heightmap.png';
            heightmapImage.onload = function () { 

                THREE.ImageUtils.loadTexture('sand1.jpg', undefined, function (t1) {  
                    THREE.ImageUtils.loadTexture('grass1.jpg', undefined, function (t2) { 
                        THREE.ImageUtils.loadTexture('stone1.jpg', undefined, function (t3) { 
                            THREE.ImageUtils.loadTexture('snow1.jpg', undefined, function (t4) { 
                                var material = THREE.Terrain.generateBlendedMaterial([
                                    {texture: t1},
                                    {texture: t2, levels: [-80, -35, 20, 50]},
                                    {texture: t3, levels: [20, 50, 60, 85]},
                                    {texture: t4, glsl: '1.0 - smoothstep(65.0 + smoothstep(-256.0, 256.0, vPosition.x) * 10.0, 80.0, vPosition.z)'},
                                    {texture: t3, glsl: 'slope > 0.7853981633974483 ? 0.2 : 1.0 - smoothstep(0.47123889803846897, 0.7853981633974483, slope) + 0.2'}, 
                                ]);

                                var terrainScene = THREE.Terrain({
                                    heightmap: heightmapImage,
                                    material: material,
                                    maxHeight: 100,
                                    minHeight: -100,
                                    xSegments: 63,
                                    xSize: 1024,
                                    ySegments: 63,
                                    ySize: 1024
                                });
                                scene.add(terrainScene);
                                terrainScene.children[0].castShadow = true;
                                terrainScene.children[0].receiveShadow = true;
                            });
                        });
                    });
                });
            };