Translucent appearances break transparent backgrounds.

[mramato]

As reported on the mailing list, https://groups.google.com/d/msg/cesium-dev/dh3O12530hA/7IqwI3x71lgJ, having geometry with a translucent appearance will cause the alpha value of scene.backgroundColor to be ignored. Below is a complete sandcastle example which shows the problem.

If the appearance or scene.backgroundColor is opaque, everything works as expected, it's only when the appearance and background are both translucent does the background alpha get thrown out.

require(['Cesium'], function(Cesium) {
    "use strict";

    document.body.style.background = '#0F0';

    var viewer = new Cesium.Viewer('cesiumContainer', {
      contextOptions : {
          webgl : {
              alpha : true
          }
      }
    });

    viewer.scene.skyBox.destroy();
    viewer.scene.skyBox = undefined;
    viewer.scene.sun.destroy();
    viewer.scene.sun = undefined;
    viewer.scene.moon.destroy();
    viewer.scene.moon = undefined;
    viewer.scene.skyAtmosphere.destroy();
    viewer.scene.skyAtmosphere = undefined;

    viewer.scene.backgroundColor = new Cesium.Color(0,0,0,0);

    Sandcastle.addToolbarButton('Draw Box', function() {
        var primitives = viewer.scene.primitives;

        // Create box and position with a model matrix
        var dimensions = new Cesium.Cartesian3(400000.0, 300000.0, 500000.0);
        var positionOnEllipsoid = Cesium.Cartesian3.fromDegrees(-105.0, 45.0);
        var boxModelMatrix = Cesium.Matrix4.multiplyByTranslation(
            Cesium.Transforms.eastNorthUpToFixedFrame(positionOnEllipsoid),
            new Cesium.Cartesian3(0.0, 0.0, dimensions.z * 0.5));

        var boxGeometry = Cesium.BoxGeometry.fromDimensions({
            vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT,
            dimensions : dimensions
        });

        var boxGeometryInstance = new Cesium.GeometryInstance({
            geometry : boxGeometry,
            modelMatrix : boxModelMatrix,
            attributes : {
                color : Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(1.0, 0.0, 0.0, 0.5))
            }
        });

        primitives.add(new Cesium.Primitive({
            geometryInstances : boxGeometryInstance,
            appearance : new Cesium.PerInstanceColorAppearance({
                closed: true
            })
        }));
    });

    Sandcastle.finishedLoading();
});

[rahwang]

Came up on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/ICr8w3B-Ufs