OpenGL 3D Graphics in Liberty BASIC

Lesson Two: Moving objects around in 3D

by Robert McAllister

There are two ways to move an object on the screen in 3D, change the location of the object or change your relative view of the object. Both methods require the scene to be redrawn for each frame.

Moving an object can be done by setting up a for/next loop and changing one or more of the X Y or Z values with each pass. You’ll notice there is a pause added to each step so we can see what happens.

 'Moving an object 
 FOR a = 1 TO 20 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 CALL glBegin GL.TRIANGLES 
 CALL glColor4fv 1 , 0 , 0 , 1 
 CALL glVertex (-2 + xAdjustment) , -1 , 0 
 CALL glColor4fv 0 , 1 , 0 , 1 
 CALL glVertex (-1 + xAdjustment) , 1 , 0 
 CALL glColor4fv 0 , 0 , 1 , 1 
 CALL glVertex (0 + xAdjustment) , -1 , 0 
 CALL glEnd 
 CALL RefreshView 
 CALL Pause 150 
 xAdjustment = xAdjustment + .1 
 NEXT a 
 WAIT 

Moving the camera can accomplish the same effect. The following is a line you will see used often in these programs.

 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 

The values eyeX , eyeY , eyeZ can be thought of as the location of a camera taking a snapshot of the scene. And centerX , centerY , centerZ represents the location the camera is aimed at in the scene.

Setting up a for/next loop that changes the value of eyeX and centerX gives the same result as the program above.

 'Moving the camera 
 FOR a = 1 TO 20 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 CALL glBegin GL.TRIANGLES 
 CALL glColor4fv 1 , 0 , 0 , 1 
 CALL glVertex -2 , -1 , 0 
 CALL glColor4fv 0 , 1 , 0 , 1 
 CALL glVertex -1 , 1 , 0 
 CALL glColor4fv 0 , 0 , 1 , 1 
 CALL glVertex 0 , -1 , 0 
 CALL glEnd 
 CALL RefreshView 
 CALL Pause 150 
 eyeX = eyeX - .1 
 centerX = centerX - .1 
 NEXT a 
 WAIT 

Here's a little program that moves a triangle around in a circle. What would happen if you put xAdjustment or yAdjustment in the Z value for the vertex calls?

 'Moving an object in a circle 
 FOR a = 1 TO 360 
 angle=(a/57.2957) 
 xAdjustment=0+(sin(angle)) 
 yAdjustment=0-(cos(angle)) 
 CALL ClearView eyeX , eyeY , eyeZ , centerX , centerY , centerZ , upX , upY , upZ 
 CALL glBegin GL.TRIANGLES 
 CALL glColor4fv 1 , 0 , 0 , 1 
 CALL glVertex (-1 + xAdjustment) , (-1 + yAdjustment) , 0 
 CALL glColor4fv 0 , 1 , 0 , 1 
 CALL glVertex (0 + xAdjustment) , (1 + yAdjustment) , 0 
 CALL glColor4fv 0 , 0 , 1 , 1 
 CALL glVertex (1 + xAdjustment) , (-1 + yAdjustment) , 0 
 CALL glEnd 
 CALL RefreshView 
 CALL Pause 10 
 NEXT a 
 WAIT 

In the next lesson we will learn about "Rotation and scaling"