If there are multiple entities walls using different kinds of ImageMaterialProperty, then the entities cannot be removed.

[emloxe]

Sandcastle example:

var base64_1 = 'data:image/png;base64,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';
var base64_2 = 'data:image/png;base64,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';

var viewer = new Cesium.Viewer('cesiumContainer');
for (var i = 0; i < 10; i++) {
    var redWall = viewer.entities.add({
        name: 'Red wall at height',
        wall: {
            positions: Cesium.Cartesian3.fromDegreesArrayHeights([-115.0, 40.0, 200000.0, -90.0, 40.0, 200000.0]),
            minimumHeights: [100000.0, 100000.0],
            material: new Cesium.ImageMaterialProperty({
                image: base64_1,
                transparent: false,
                color: new Cesium.Color(1.0, 1.0, 1.0, 1.0)
            })
        }
    });

    var greenWall = viewer.entities.add({
        name: 'Green wall from surface with outline',
        wall: {
            positions: Cesium.Cartesian3.fromDegreesArrayHeights([-115.0, 43.0, 100000.0, -90.0, 43.0, 100000.0]),
            material: new Cesium.ImageMaterialProperty({
                image: base64_2,
                transparent: false,
                color: new Cesium.Color(1.0, 1.0, 1.0, 1.0)
            }),
            outline: true
        }
    });

    var blueWall = viewer.entities.add({
        name: 'Blue wall with sawtooth heights and outline',
        wall: {
            positions: Cesium.Cartesian3.fromDegreesArrayHeights([-115.0, 50.0, 100000.0, -90.0, 50.0, 100000.0]),
            material: new Cesium.ImageMaterialProperty({
                image: base64_1,
                transparent: false,
                color: new Cesium.Color(1.0, 1.0, 1.0, 1.0)
            }),
            outline: true,
        }
    });

}
viewer.zoomTo(viewer.entities);


setTimeout(function () {
    viewer.entities.removeAll()
}, 20000)

Browser: Google Chrome 69.0.3497.100

Operating System: win 10

[hpinkos]

Thanks for including the code example @emloxe! I'll try to figure out the problem soon.

This is the code example I was testing with:

var viewer = new Cesium.Viewer('cesiumContainer');
for (var i = 0; i < 2; i++) {
    viewer.entities.add({
        wall: {
            positions: Cesium.Cartesian3.fromDegreesArray([-115.0, 43.0 + i, -90.0, 43.0]),
            maximumHeights: [100000.0, 100000.0],
            material: new Cesium.ImageMaterialProperty({
                image: '../images/bumpmap.png'
            })
        }
    });
    
    viewer.entities.add({
        wall: {
            positions: Cesium.Cartesian3.fromDegreesArray([-115.0, 40.0 + i, -90.0, 40.0]),
            maximumHeights: [100000.0, 100000.0],
            material: new Cesium.ImageMaterialProperty({
                image: '../images/Cesium_Logo_Color.jpg'
            })
        }
    });

}
viewer.zoomTo(viewer.entities);

Sandcastle.addToolbarButton('Remove', function() {
    viewer.entities.removeAll();
});

[hpinkos]

Thanks again for reporting this @emloxe! This was fixed in #7163 and will be included in the 1.51 release available on November 1st.