### 27.1.1 3D geometry graphics screen

The Alt+H command brings up a display screen for 3D graphics (see Figure 27.1). This screen has its own menu and command lines. This screen also automatically appears whenever there is a 3D graphic command.

> Figure 27.1: A 3D-geometry screen in Xcas

The plane of vision for a 3D graphic screen is perpendicular to the observer’s line of vision. The plane of vision is also indicated by dotted lines showing its intersection with the parallelepiped. The axis of vision for a 3D graphic screen is

The 3D graphic screen starts with the image of a parallelepiped bounding the graphics and vectors in the x, y and z directions. At the top of the screen is the equation of the plane of vision, which is a plane perpendicular to the observer’s line of vision. The plane of vision is shown graphically with dotted lines indicating where it intersects the plane of vision.

Clicking in the graphic screen outside of the parallelepiped and dragging the mouse moves the x, y and z directions relative to the observer; these directions are also changed with the x, X, y, Y, z and Z keys. Scrolling the mouse wheel moves the plane of vision along the line of vision. The in and out buttons on the graphic screen menu zoom in and out of the picture.

The graphical features available for 2D graphics (see Section 26.3) are also available for 3D graphics, but to see the points the markers must be squares with width (point_width) at least 3.

The graphic screen menu has a cfg button which brings up a configuration screen. Among other things, this screen has

- Ortho proj button, which determines whether the drawing uses orthogonal projection or perspective projection.
- Lights button, which determines whether the objects are lit or not; the locations of eight points for lighting are set using the buttons L1, …, L7, which specify the points with homogeneous coordinates.
- Show axis button, which determines whether or not the outlining parallelepiped is visible.