Scale and Orientation in 3D
draft topic outline
Scale and orientation have always been two major variables when authoring a map.
Indicating Scale and Orientation
The Scale Bar and North Arrow are key components of a map, they allow user to interpret scale and orientation, distance and direction. With 3D scenes and models these items become problematic as scale and orientation will vary for differing locations across the view.
As the authors and developers of 3D scenes and views we may need to find alternative ways of indicating scale and orientation.
Looking through a Camera
When we are viewing a 3D scene we looking at it as though we were looking through a camera looking down at the world. The camera has a position (latitude/longitude/height), a heading, a tilt and a field of view.
When we look though a camera at the real world we see that things get smaller towards the horizon, as they disappear into the distance. We are are reasonably good a judging the magnitude of distance based on the perceived size of an object. When we have two cameras, or a pair of eyes we can see in stereo have an additional tool to judge distance.
If we think of a simple perspective drawing, with a single vanishing point, it is easy to see how the orientation varies as we move from left to right. In the real world there is an additional, slight variation as we move further up the view, due to the earths curvature.
The Orientation varies across the extent of the view
Returning to our simple perspective drawing, lines will converge towards the vanishing point. The rendered size of an object, or its scale will decrease as it moves towards the vanishing point, this is also slightly effected by the earths curvature.
The Scale varies across the extent of the view
Impact of Tilt
If we are viewing a 3D scene from above, with the camera tilted vertically towards the ground then scale and orientation will be quite similar across the view.
As we start to tilt the camera towards the horizon the scale and orientation will start to vary across the view. The more the camera is tilted towards the horizon the greater the impact on scale and orientation at differing points on the view.
Scale also becomes different depending on if we are measuring it along the baseline of the view, or perpendicular to the baseline. So at any point across the view scale there are two or more scales.
Impact of Field of View
If we look at a grid of squares from above, there will be a slight distortion, the further we are away from the centre.