# Layer Modify

- Projection
- Position
- Visibility
- View

Change here the space region of layer (or list of layers, previously selected with Layer->Select, when Global is pressed), to where the user is looking. Parameters for empty entries or Local choices remain unchanged.

### Eye

Change the user position, in the layer space. Any position in 3D space is valid. Initially, the user is at (0, 0, +z), looking down to (0, 0, 0), and the up direction is (0, 1, 0), so the x axis points to the right, the y axis points to the top, and the z axis points to the user.

### Center

Change the position to where the user is looking. The distance from Eye to Center must be within the limits of visibility (from Near to Far).

### Up

Change the direction in layer space that points up, to the top of the screen. This direction is converted internally to a unit vector normal to the direction Eye -> Center, so any non-zero vector that is not parallel to that direction is valid.

The user position and orientation can be defined by a (X,Y,Z) layer referential, pointing from the middle of the screen to: the right (X), the top (Y), the user (Z). Eye is the origin, Eye <- Center defines the Z axis, and Up the Y axis, permiting to get the X axis by the cross product of the other two axes. Initially, the axes are orientated along (1,0,0) for X, (0,1,0) for Y and (0,0,1) for Z, but this is no longer the case when the layer referential is rotated.

To move the user around the objects, Eye, Center and Up must change. A simple, interactive way to do this is to press Axes in the window toolbar and then either drag the mouse or use the ruler, as described in Introduction Mouse and Introduction Tools.

Each layer has its own position parameters, so in a window with multiple visible layers, the image rendered in the graphic area is the superposition of the various, totally independent, layer views.
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