This tutorial will teach you about the Geometry & Appearances system available with the Primitive API. This is an advanced topic for extending Cesium with custom meshes, shapes, volumes, and appearances and is not geared towards the typical Cesium user. If you are interested in learning how to draw various shapes and volumes on the globe, check out the Visualizing Spatial Data tutorial instead.

Cesium can create a different geomtery types using entities, such as polygons and ellipsoids. For example, copy and paste the following into the Hello World Sandcastle Example to create a rectangle on the globe with a dot pattern:

var viewer = new Cesium.Viewer('cesiumContainer');

viewer.entities.add({
    rectangle : {
        coordinates : Cesium.Rectangle.fromDegrees(-100.0, 20.0, -90.0, 30.0),
        material : new Cesium.StripeMaterialProperty({
            evenColor: Cesium.Color.WHITE,
            oddColor: Cesium.Color.BLUE,
            repeat: 5
        })
    }
});

In this tutorial, we go under the hood of primitives and look at the Geometry and Appearance types that form them. A geometry defines the primitive’s structure, i.e., the triangles, lines, or points that compose the primitive. An appearance defines the primitive’s shading, including its full GLSL vertex and fragment shaders, and render state.

Cesium supports the following geometries.

A box
A circle or extruded circle
A polyline normal to the surface with a width in meters and optional extruded height
A cylinder, cone, or truncated cone
An ellipse or extruded ellipse
An ellipsoid
An rectangle or extruded rectangle
A polygon with optional holes or extruded polygon
A collection of line segments with a width in pixels
A 2D shape extruded along a polyline
A sphere
A wall perpendicular to the globe

The benefits of using geometries and appearances are:

  • Performance - When drawing a large number of static primitives (such as polygon for each zip code in the United States), using geometries directly allows us to combine them into a single geometry to reduce CPU overhead and better utilize the GPU. Combining primitives is done on a web worker to keep the UI responsive.
  • Flexibility - Primitives combine geometry and appearance. By decoupling them, we can modify each independently. We can add new geometries that are compatible with many different appearances and vice-versa.
  • Low-level access - Appearances provide close-to-the-metal access to rendering without having to worry about all the details of using the Renderer directly. Appearances make it easy to:

    • Write full GLSL vertex and fragment shaders.
    • Use custom render state.

There are also some downsides:

  • Using geometries and appearances directly requires more code and a deeper understanding of graphics. Primitives are at the level of abstraction appropriate for mapping apps; geometries and appearances have a level of abstraction closer to a traditional 3D engine.
  • Combining geometries is effective for static data, not necessarily for dynamic data.

Let’s rewrite the initial code example using geometries and appearances:

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

// original code
//viewer.entities.add({
//    rectangle : {
//        coordinates : Cesium.Rectangle.fromDegrees(-100.0, 20.0, -90.0, 30.0),
//        material : new Cesium.StripeMaterialProperty({
//            evenColor: Cesium.Color.WHITE,
//            oddColor: Cesium.Color.BLUE,
//            repeat: 5
//        })
//    }
//});

var instance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(-100.0, 20.0, -90.0, 30.0),
    vertexFormat : Cesium.EllipsoidSurfaceAppearance.VERTEX_FORMAT
  })
});

scene.primitives.add(new Cesium.Primitive({
  geometryInstances : instance,
  appearance : new Cesium.EllipsoidSurfaceAppearance({
    material : Cesium.Material.fromType('Stripe')
  })
}));

Instead of using a rectangle entity, we used the generic Primitive, which combines the geometry instance and appearance. For now, we will not differentiate between a Geometry and a GeometryInstance other than an instance is a container for a geometry.

To create the geometry for the rectangle, i.e., the triangles covering the rectangular region and that fit the curvature of the globe, we create an RectangleGeometry.

Since we know it is on the surface, we can use the EllipsoidSurfaceAppearance. This is able to save memory and support all materials given that the geometry is on the surface or at a constant height above the ellipsoid.

Combining Geometries

We see a performance benefit when we use one primitive to draw multiple static geometries. For example, let’s draw two rectangles.

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var instance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(-100.0, 20.0, -90.0, 30.0),
    vertexFormat : Cesium.EllipsoidSurfaceAppearance.VERTEX_FORMAT
  })
});

var anotherInstance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(-85.0, 20.0, -75.0, 30.0),
    vertexFormat : Cesium.EllipsoidSurfaceAppearance.VERTEX_FORMAT
  })
});

scene.primitives.add(new Cesium.Primitive({
  geometryInstances : [instance, anotherInstance],
  appearance : new Cesium.EllipsoidSurfaceAppearance({
    material : Cesium.Material.fromType('Stripe')
  })
}));

We created another instance with a different rectangle, and then provided both instances to the primitive. This draws both both instances with the same appearance.

Some appearances allow each instance to provide unique attributes. For example, we can use PerInstanceColorAppearance to shade each instance with a different color.

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var instance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(-100.0, 20.0, -90.0, 30.0),
    vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
  }),
  attributes : {
    color : new Cesium.ColorGeometryInstanceAttribute(0.0, 0.0, 1.0, 0.8)
  }
});

var anotherInstance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(-85.0, 20.0, -75.0, 30.0),
    vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
  }),
  attributes : {
    color : new Cesium.ColorGeometryInstanceAttribute(1.0, 0.0, 0.0, 0.8)
  }
});

scene.primitives.add(new Cesium.Primitive({
  geometryInstances : [instance, anotherInstance],
  appearance : new Cesium.PerInstanceColorAppearance()
}));

Each instance has a Color attribute. The primitive is then constructed with a PerInstanceColorAppearance, which knows to use each instance’s color attribute to determine shading.

Combining geometries allows Cesium to efficiently draw A LOT of geometries. The following example draws 2,592 uniquely colored rectangles. It will optimize the geometry, then drawing is very fast.

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var instances = [];

for (var lon = -180.0; lon < 180.0; lon += 5.0) {
  for (var lat = -85.0; lat < 85.0; lat += 5.0) {
    instances.push(new Cesium.GeometryInstance({
      geometry : new Cesium.RectangleGeometry({
        rectangle : Cesium.Rectangle.fromDegrees(lon, lat, lon + 5.0, lat + 5.0),
        vertexFormat: Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
      }),
      attributes : {
        color : Cesium.ColorGeometryInstanceAttribute.fromColor(Cesium.Color.fromRandom({alpha : 0.5}))
      }
    }));
  }
}

scene.primitives.add(new Cesium.Primitive({
  geometryInstances : instances,
  appearance : new Cesium.PerInstanceColorAppearance()
}));

Picking

After instances are combined, they are still independently accessible. In particular, we can assign an id to an instance and use it to determine if the instance is picked with Scene.pick. id can be any JavaScript type: a string, a number, an object with its own properties, etc.

The following example creates an instance with a id, and writes a message to the console when it is clicked.

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var instance = new Cesium.GeometryInstance({
  geometry : new Cesium.RectangleGeometry({
    rectangle : Cesium.Rectangle.fromDegrees(-100.0, 30.0, -90.0, 40.0),
    vertexFormat: Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
  }),
  id : 'my rectangle',
  attributes : {
    color : Cesium.ColorGeometryInstanceAttribute.fromColor(Cesium.Color.RED)
  }
});

scene.primitives.add(new Cesium.Primitive({
  geometryInstances : instance,
  appearance : new Cesium.PerInstanceColorAppearance()
}));

var handler = new Cesium.ScreenSpaceEventHandler(scene.canvas);
handler.setInputAction(function (movement) {
    var pick = scene.pick(movement.position);
    if (Cesium.defined(pick) && (pick.id === 'my rectangle')) {
      console.log('Mouse clicked rectangle.');
    }
  }, Cesium.ScreenSpaceEventType.LEFT_CLICK);

Using id, instead of the reference to the instance itself, allows the primitive - and our application - to avoid keeping a reference to the full instance, including its reference to the geometry, in memory after the primitive is constructed. Since a geometry can contain several large typed arrays, this allows us to save a significant amount of memory.

Geometry Instances

Thus far, we have defined a geometry instance only as a container for a geometry. In addition, instances are used to position, scale, and rotate the same geometry in different parts of the scene. This is possible because multiple instances can reference the same Geometry, and each instance can have a different modelMatrix. This allows us to only compute the geometry once and reuse it many times.

The following example creates one EllipsoidGeometry and two instances. Each instance references the same ellipsoid geometry, but places it using a different modelMatrix resulting in one ellipsoid being on top of the other.

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var ellipsoidGeometry = new Cesium.EllipsoidGeometry({
    vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT,
    radii : new Cesium.Cartesian3(300000.0, 200000.0, 150000.0)
});

var cyanEllipsoidInstance = new Cesium.GeometryInstance({
    geometry : ellipsoidGeometry,
    modelMatrix : Cesium.Matrix4.multiplyByTranslation(
        Cesium.Transforms.eastNorthUpToFixedFrame(Cesium.Cartesian3.fromDegrees(-100.0, 40.0)),
        new Cesium.Cartesian3(0.0, 0.0, 150000.0)
    ),
    attributes : {
        color : Cesium.ColorGeometryInstanceAttribute.fromColor(Cesium.Color.CYAN)
    }
});

var orangeEllipsoidInstance = new Cesium.GeometryInstance({
    geometry : ellipsoidGeometry,
    modelMatrix : Cesium.Matrix4.multiplyByTranslation(
        Cesium.Transforms.eastNorthUpToFixedFrame(Cesium.Cartesian3.fromDegrees(-100.0, 40.0)),
        new Cesium.Cartesian3(0.0, 0.0, 450000.0)
    ),
    attributes : {
        color : Cesium.ColorGeometryInstanceAttribute.fromColor(Cesium.Color.ORANGE)
    }
});

scene.primitives.add(new Cesium.Primitive({
    geometryInstances : [cyanEllipsoidInstance, orangeEllipsoidInstance],
    appearance : new Cesium.PerInstanceColorAppearance({
        translucent : false,
        closed : true
    })
}));

Updating Per-Instance Attributes

It is also possible to update the per-instance attributes of the geometries after they are added to the primitive. Per-instance attributes include:

This example shows how to change the color of the geometry instance:

var viewer = new Cesium.Viewer('cesiumContainer');
var scene = viewer.scene;

var circleInstance = new Cesium.GeometryInstance({
    geometry : new Cesium.CircleGeometry({
        center : Cesium.Cartesian3.fromDegrees(-95.0, 43.0),
        radius : 250000.0,
        vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
    }),
    attributes : {
        color : Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(1.0, 0.0, 0.0, 0.5))
    },
    id: 'circle'
});
var primitive = new Cesium.Primitive({
    geometryInstances : circleInstance,
    appearance : new Cesium.PerInstanceColorAppearance({
        translucent : false,
        closed : true
    })
});
scene.primitives.add(primitive);

setInterval(function() {
    var attributes = primitive.getGeometryInstanceAttributes('circle');
    attributes.color = Cesium.ColorGeometryInstanceAttribute.toValue(Cesium.Color.fromRandom({alpha : 1.0}));
},2000);

The attributes of the geometry instance can be retrieved from the primitive using primitive.getGeometryInstanceAttributes. The properties of the attributes can be changed directly. In this case, we are changing attributes.color to be equal to a new randomly generated color every 2000 milliseconds.

Appearances

Geometry defines structure. The other key property of a primitive, appearance, defines the primitive’s shading, i.e., how individual pixels are colored. A primitive can have many geometry instances, but it can only have one appearance. Depending on the type of appearance, an appearance will have a material that defines the bulk of the shading.

Cesium has the following appearances.

An appearance that works with all geometry types and supports materials to describe shading.
A version of `MaterialAppearance` that assumes geometry is parallel to the surface of the globe, like a polygon, and uses this assumption to save memory by procedurally computing many vertex attributes.
Uses per-instance color to shade each instance.
Supports materials to shade a Polyline.
Uses either per-vertex or per-segment coloring to shade a Polyline.

Appearances define the full GLSL vertex and fragment shaders that execute on the GPU when the primitive is drawn. We rarely touch these unless we are writing a custom appearance. Appearances also define the full render state, which controls the GPU’s state when the primitive is drawn. We can define the render state directly or use higher-level properties like closed and translucent, which the appearance will convert into render state. For example:

// Perhaps for an opaque box that the viewer will not enter.
//  - Backface culled and depth tested.  No blending.

var appearance  = new Cesium.PerInstanceColorAppearance({
  translucent : false,
  closed : true
});

// This appearance is the same as above
var anotherAppearance  = new Cesium.PerInstanceColorAppearance({
  renderState : {
    depthTest : {
      enabled : true
    },
    cull : {
      enabled : true,
      face : Cesium.CullFace.BACK
    }
  }
});

Once an appearance is created, we can’t change its renderState property, but we can change its material. Likewise, we can also change a primitive’s appearance property.

Most appearances also have flat and faceForward properties, which indirectly control the GLSL shaders.

  • flat - Flat shading. Do not take lighting into account.
  • faceForward - When lighting, flip the normal so it is always facing the viewer. The avoids black areas on back-faces, e.g., the inside of a wall.
flat : true faceForward : false faceForward : true

Geometry and Appearance Compatibility

We’ve already seen that not all appearances work with all geometries. For example, EllipsoidSurfaceAppearance is not appropriate for WallGeometry since a wall is perpendicular to the globe, not parallel.

Beyond semantics like this, for an appearance to be compatible with a geometry, they must have matching vertex formats, which means the geometry must have the data that the appearance expects as input. A VertexFormat can be provided when creating a geometry.

We can keep things simple, but inefficient and wasteful, by requesting a geometry compute all vertex attributes, which will make the geometry compatible with all appearances (ignoring per-instance attributes; see below).

var geometry = new Cesium.RectangleGeometry({
  vertexFormat : Cesium.VertexFormat.ALL
  // ...
});


If we are using EllipsoidSurfaceAppearance, for example, we can get away with just requesting positions.

var geometry = new Ceisum.RectangleGeometry({
  vertexFormat : Ceisum.VertexFormat.POSITION_ONLY
  // ...
});

In general, how do we know what vertex format to use for a given appearance? Most appearances have a vertexFormat property or even a VERTEX_FORMAT static constant.

var geometry = new Ceisum.RectangleGeometry({
  vertexFormat : Ceisum.EllipsoidSurfaceAppearance.VERTEX_FORMAT
  // ...
});

var geometry2 = new Ceisum.RectangleGeometry({
  vertexFormat : Ceisum.PerInstanceColorAppearance.VERTEX_FORMAT
  // ...
});

var appearance = new Ceisum.MaterialAppearance(/* ... */);
var geometry3 = new Ceisum.RectangleGeometry({
  vertexFormat : appearance.vertexFormat
  // ...
});

Also, a geometry’s vertexFormat determines if it can be combined with another geometry. Two geometries do not have to be the same type; they just need matching vertex formats.

Resources

In the reference documentation, see:

For more on materials, see Fabric.

For future plans, see the Geometry and Appearances Roadmap.