Personal Computer News

22nd December 1984
3D Graphics

 
Published in Personal Computer News #092

With Ian Wood's Three Dimensional Graphics program you can explore and design your own 3D line drawings on the Beeb.

3D Drawing

With Ian Wood's Three Dimensional Graphics program you can explore and design your own 3D line drawings on the Beeb

3D Line Drawings On The BBC Micro

The BBC's graphics capabilities are excellent, but to take full advantage of them you need the right software. Three Dimensional Graphics for the BBC lets you design a three dimensional line drawing which can then be rotated in any direction. You can also save and load your own programs.

Shape Menu

  1. Use Demonstration Shape
    Clears the current shape and loads the built-in demonstration shape.
  2. Load New Shape From Tape/Disk
    Clears the current shape and asks for the filename of the program to be loaded. Pressing RETURN without a filename loads the next program on tape.
  3. Create New Shape
    Clears the current shape and enters the editor.
  4. Edit Shape
    Enters the editor.
  5. Change Title
    Is the only way to change the title, as clearing the shape does not. A null string may be entered. When the shape is saved, the title is saved with it.
  6. Exit Menu
    Redraws the shape.

Rotation And Movement Menu

Both these give a choice of six directions plus the 'Exit Menu' option. When a direction has been chosen, the program asks how far you want to rotate or move the shape.

The movement menu also has a 'Change Camera Position' option. This is included in the movement menu because the 'Move' options actually change the camera position.

Editor

The editor is entered from the shape menu simply by selecting either the 'Create New Shape' option or the 'Edit Shape' option.

Two cursors are displayed, together with their x,y,z coordinates, and can be moved around the screen. If the space bar is pressed a line will be drawn between the cursors and if the Delete key is pressed, the line will be deleted. Note that it's possible to draw up to one hundred lines.

To move the square cursor, use the arrow keys, and 'I' and 'O' for 'in' and 'out' (away from and towards you). To move the cross cursor, shift the keys.

When the shape is finished, press Return.

Procedure And Functions

The procedures and functions appear to be rather disorganised, but in fact they are in chronological order. When I thought of something new to put in the program, I just added another procedure to the end.

FNmovex Works out the coordinate for drawing the shape on the screen from the x and z coordinates, taking into account the camera position.
FNmovey Does the same for the y coordinate.
PROCplot Draws a line on the screen using the x,y,z coordinates of the start and end of the line.
PROCdraw Draws the whole shape and prints the title.
PROCroll Works out the clockwise/anticlockwise movement of the shape.
PROCpitch Works out up and down movement.
PROCyaw Works out left and right movement.
PROCdemo Initialises the demonstration shape.
PROCsave Saves the shape and title.
PROCload Loads the shape.
PROCenter This is the main editor procedure. It initialises variables, reads the keyboard and then counts the number of lines.
PROCpoint1 Moves square cursor.
PROCpoint2 Moves cross cursor.
PROCline Draws a line. First checks to see if line already exists.
PROCedit Deletes a line. First checks to see if line already exists.
PROCdelete Called by PROCedit. Deletes a line and compacts the list of coordinates (removes empty spaces in list).
PROCcompact Called by PROCdelete. Compacts the list of coordinates.
PROCmain_menu Prints main menu, reads keyboard and calls a procedure depending on choice made.
PROCrotate_menu As above, for rotation.
PROCinput Inputs the number of degrees that the shape is to be rotated.
PROCshape_menu As PROCmain_menu for shape menu.
PROCerror Puts colours and cursor back to normal, empties the keyboard buffer and prints an error message.
PROClear Called by PROCshape menu. Clears the shape.
PROCno_room Called by PROCenter, if the arrays holding the list of coordinates are full. Prints an error message on the screen.
FNcheck Called by PROCline. Checks to see if line to be drawn already exists.
PROCinfo Prints the instructions.
PROCtitle Inputs a new title.
PROCmove_menu As PROCmain_menu but for movement menu.
PROCpoints Inputs the number of points that the shape is to be moved.
PROCout Moves shape out (towards screen). With a negative value, moves shape in (away).
PROCup Moves shape up or down.
PROCleft Moves shape left or right.
PROCc_pos Inputs a new camera position.

Main Variables

x%,y%,z% These three arrays make up the list of coordinates that form the shape.
lines% The number of lines in the shape minus one.
line% Used throughout the program. Current line being worked on.
key$ Used in menu procedures. Contains the last key pressed.
cx%,cy%,cx% The coordinates of the camera position.
title$ The title of the shape.

Ian Wood