yeah, it should maybe ure using a very high level of ambient in the directional light i usually use this one
var light:DirectionalLight3D = new DirectionalLight3D({color:0xFFFFFF,
ambient:0.2, diffuse:0.75, specular:0.1});
light.x = 10000;
light.z = 50000;
light.y = 50000;
scene.addChild(light);
put -50000 in z to check for changes... let me know.
