Projection control

The buttons for the projection control are only visible if the 3D−Button (s. B−6) is depressed.

The projection shows how the points of the graphics objects are displayed on the output medium (screen, printer, clipboard, metafile etc.).

For plotting 3D graphics objects a parallel perspective, a point perspective or an eccentric perspective can be chosen. One can switch from one to the other using the buttons shown on the left.

In the following the terms vertical and horizontal are often used. By vertical directions parallel to the global zdirection are meant and by horizontal directions in planes parallel to the global xyplane.

Parallel perspective projection direction X Y Z X Y Z plane of picture frame of picture

The structure is depicted by parallel rays in the freely selectable projection direction projected onto the plane of the ’picture’ that lies normal to it. The rectangular frame of the picture (=border of the output medium) is placed automatically such that a vector points vertically upwards in the global zdirection in the picture. If the projection direction coincides with the global zdirection, then the picture frame can be arbitrarily rotated.

In this way we force vertical lines of the graphics objects also to be represented vertically on the output medium, and the ability of an object to rotate arbitrarily, as is usual for instance in CAD systems in mechanical engineering, is deliberately restricted.

Part B The Graphics Editor B 3 Screen Elements

Point perspective

The following figure shows the representation of the point perspective and defines the terms used for this purpose:

S P v S: object centre v: projection direction P: projection point = location of observer B: plane of figure R: figure frame = screen border R P S P v d: distance P−S d fictitious camera v B R É É picture object camera axis

Picture of an object with the fictitious camera

To get a better understanding of the control parameters for perspective drawing we imagine a fictitious camera, as sketched in the above figure. The points of intersection of the rays fromall object points P with the plane of the picture produce the picture. If one moves the camera another picture is obtained.

Both the picture plane and the picture area of the camera are rectangular. The freedom of movement of the camera is restricted to the extent that the bottom edge of this rectangle is always horizontal (indeterminate in the case of a vertical projection direction). Thus this is of importance because the picture frame, which is represented by the border of the screen, is always parallel to the camera rectangle.

S P v ÉÉ ÉÉ Variation of the projection direction S P É v É

Variation of the distance of the observer to the object

Eccentric perspective

In general in the case of perspective architectural drawings it is usual that vertical lines of a structure are also shown vertically on the drawing. Keeping to this rule is also strongly recommended here. It is fulfilled if the projection direction is left as horizontal. By means of a corresponding camera eccentricity (see below) the viewing point on the structure can be changed in the height.

An eccentric camera position results if the camera axis does not pass through the centre of the object. A vertical eccentricity is above all of importance in obtaining a higher or lower viewing point, keeping a horizontal projection direction. Horizontal eccentricities are hardly of significance. Instead of them one would normally perform a horizontal rotation of the projection direction.

The buttons for projection control

The projection direction is changed in such a way that the structure rotates itself in the figure about an axis in the global z−direction in the sense of the right hand rule. The steepness of the projection direction with respect to a horizontal plane remains unchanged.

As above, but with a rotation in the opposite direction.

The steepness of the projection direction , i.e. its angle with respect to the horizontal, is changed. The observer views from further up on the structure. For the eccentric perspective the vertical camera eccentricity is changed.

As above, but with a reversed sign.

If you keep the above four buttons depressed for a longer time, the projection direction changes until you release the button. The rotation angle of an individual step can be influenced using the input field shown.

The projection direction can also be rotated using the key combination Ctrl+Alt by moving the mouse with a depressed left mouse button. By using the key combination

Ctrl+Alt the mouse pointer changes to the curved arrow shown on the left. A horizontal movement of the mouse pointer causes a rotation about a vertical axis, and a vertical movement of the mouse pointer causes a rotation about a horizontal axis.

To reduce the distance between the observer and the object. The shorter this distance the stronger are the effects of the point perspective, while an increasing distance gradually leads to a parallel perspective. This button is not active for the parallel perspective.

To increase the distance between the observer and the object. This button is not active for the parallel perspective.

Projektion in the x, y and zdirections, respectively, (with reversed sign, if simultaneously the <Shift>key is pressed).

After changing the projection direction with the above three buttons, with this button you can go back to the last general projection direction.

With this button you can open the management of views. There you can save your own views under any name and use them again later. Views contained in brackets cannot be deleted or modified. By double−clicking on an entry in the list, you can import the settings for this view into the Graphics Editor.

Saves the current projection settings in the marked view

Creates a new view The marked view is deleted

The marked view can be modified (e.g. change of title)

Button to switch the rendering on and off (photo realistic representation) of the graphics objects. Is not available in all applications.

Part B The Graphics Editor B 3 Screen Elements