Low-Level Camera Properties

Camera Properties You Can Set

Camera graphics is based on a group of axes properties that control the position and orientation of the camera. In general, the camera commands, such as campos, camtarget, and camup, make it unnecessary to access these properties directly.

Property	Description
`CameraPosition`	Specifies the location of the viewpoint in axes units.
`CameraPositionMode`	In `automatic` mode, the scene determines the position. In `manual` mode, you specify the viewpoint location.
`CameraTarget`	Specifies the location in the axes pointed to by the camera. Together with the `CameraPosition`, it defines the viewing axis.
`CameraTargetMode`	In `automatic` mode, MATLAB^® specifies the `CameraTarget` as the center of the axes plot box. In `manual` mode, you specify the location.
`CameraUpVector`	The rotation of the camera around the viewing axis is defined by a vector indicating the direction taken as up.
`CameraUpVectorMode`	In `automatic` mode, MATLAB orients the up vector along the positive y-axis for 2-D views and along the positive z-axis for 3-D views. In `manual` mode, you specify the direction.
`CameraViewAngle`	Specifies the field of view of the "lens." If you specify a value for CameraViewAngle, MATLAB does not stretch-the axes to fit the figure.
`CameraViewAngleMode`	In `automatic` mode, MATLAB adjusts the view angle to the smallest angle that captures the entire scene. In `manual` mode, you specify the angle. Setting `CameraViewAngleMode` to `manual` overrides stretch-to-fill behavior.
`Projection`	Selects either an orthographic or perspective projection.

Default Viewpoint Selection

When all the camera mode properties are set to auto (the default), MATLAB automatically controls the view, selecting appropriate values based on the assumption that you want the scene to fill the position rectangle (which is defined by the width and height components of the axes Position property).

By default, MATLAB

Sets the CameraPosition so the orientation of the scene is the standard MATLAB 2-D or 3-D view (see the view command)
Sets the CameraTarget to the center of the plot box
Sets the CameraUpVector so the y-direction is up for 2-D views and the z-direction is up for 3-D views
Sets the CameraViewAngle to the minimum angle that makes the scene fill the position rectangle (the rectangle defined by the axes Position property)
Uses orthographic projection

This default behavior generally produces desirable results. However, you can change these properties to produce useful effects.

Moving In and Out on the Scene

You can move the camera anywhere in the 3-D space defined by the axes. The camera continues to point towards the target regardless of its position. When the camera moves, MATLAB varies the camera view angle to ensure the scene fills the position rectangle.

Moving Through a Scene

You can create a fly-by effect by moving the camera through the scene. To do this, continually change CameraPosition property, moving it toward the target. Because the camera is moving through space, it turns as it moves past the camera target. Override the MATLAB automatic resizing of the scene each time you move the camera by setting the CameraViewAngleMode to manual.

If you update the CameraPosition and the CameraTarget, the effect is to pass through the scene while continually facing the direction of movement.

If the Projection is set to perspective, the amount of perspective distortion increases as the camera gets closer to the target and decreases as it gets farther away.

Example — Moving Toward or Away from the Target

To move the camera along the viewing axis, you need to calculate new coordinates for the CameraPosition property. This is accomplished by subtracting (to move closer to the target) or adding (to move away from the target) some fraction of the total distance between the camera position and the camera target.

The function movecamera calculates a new CameraPosition that moves in on the scene if the argument dist is positive and moves out if dist is negative.

function movecamera(dist) %dist in the range [-1 1]
set(gca,'CameraViewAngleMode','manual')
newcp = cpos - dist * (cpos - ctarg);
set(gca,'CameraPosition',newcp)
function out = cpos
out = get(gca,'CameraPosition');
function out = ctarg
out = get(gca,'CameraTarget');

Setting the CameraViewAngleMode to manual can cause an abrupt change in the aspect ratio.

Making the Scene Larger or Smaller

Adjusting the CameraViewAngle property makes the view of the scene larger or smaller. Larger angles cause the view to encompass a larger area, thereby making the objects in the scene appear smaller. Similarly, smaller angles make the objects appear larger.

Changing CameraViewAngle makes the scene larger or smaller without affecting the position of the camera. This is desirable if you want to zoom in without moving the viewpoint past objects that will then no longer be in the scene (as could happen if you changed the camera position). Also, changing the CameraViewAngle does not affect the amount of perspective applied to the scene, as changing CameraPosition does when the figure Projection property is set to perspective.

Revolving Around the Scene

You can use the view command to revolve the viewpoint about the z-axis by varying the azimuth, and about the azimuth by varying the elevation. This has the effect of moving the camera around the scene along the surface of a sphere whose radius is the length of the viewing axis. You could create the same effect by changing the CameraPosition, but doing so requires you to perform calculations that MATLAB performs for you when you call view.

For example, the function orbit moves the camera around the scene.

function orbit(deg)
[az, el] = view;
rotvec = 0:deg/10:deg;
for i = 1:length(rotvec)
    view([az+rotvec(i) el])
    drawnow
end

Rotation Without Resizing

When CameraViewAngleMode is auto, MATLAB calculates the CameraViewAngle so that the scene is as large as can fit in the axes position rectangle. This causes an apparent size change during rotation of the scene. To prevent resizing during rotation, you need to set the CameraViewAngleMode to manual (which happens automatically when you specify a value for the CameraViewAngle property). To do this in the orbit function, add the statement

set(gca,'CameraViewAngleMode','manual')

Rotation About the Viewing Axis

You can change the orientation of the scene by specifying the direction defined as up. By default, MATLAB defines up as the y-axis in 2-D views (the CameraUpVector is [0 1 0]) and the z-axis for 3-D views (the CameraUpVector is [0 0 1]). However, you can specify up as any arbitrary direction.

The vector defined by the CameraUpVector property forms one axis of the camera's coordinate system. Internally, MATLAB determines the actual orientation of the camera up vector by projecting the specified vector onto the plane that is normal to the camera direction (i.e., the viewing axis). This simplifies the specification of the CameraUpVector property, because it need not lie in this plane.

In many cases, you might find it convenient to visualize the desired up vector in terms of angles with respect to the axes x-, y-, and z-axis. You can then use direction cosines to convert from angles to vector components. For a unit vector, the expression simplifies to

where the angles α, β, and γ are specified in degrees.

XComponent = cos(α*(pi/180));

YComponent = cos(β*(pi/180));

ZComponent = cos(γ*(pi/180));

Consult a mathematics book on vector analysis for a more detailed explanation of direction cosines.

Calculating a Camera Up Vector

To specify an up vector that makes an angle of 30° with the z-axis and lies in the y-z plane, use the expression

upvec = [cos(90*(pi/180)),cos(60*(pi/180)),cos(30*(pi/180))];

and then set the CameraUpVector property.

set(gca,'CameraUpVector',upvec)

Drawing a sphere with this orientation produces

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