Author: Eber Irigoyen
In this article I'll show you how to create 3D shapes, which is just an extension
of the last article, also you'll see how to rotate around the same shape axis or
rotate around other object (like the solar system)
Answer:
Ok, this is the 3rd of a series of articles on OpenGL, so far we have seen:
OpenGL I: Hello World, Setup a Delphi OpenGL Application
OpenGL II: Moving and rotating 2D shapes
Now we're gonna expand on the last article to create 3D shapes, rotate them around
their own axis and rotating objects around some other object (like a solar system)
To accomplish such thing we're still going to use the same basic OpenGL
instructions but because this is graphics, one line in the wrong place makes a huge
difference As I stated in my last article, OpenGL just follows instructions like
"move forward", "turn left X degrees", etc, so just with that you know is not the
same:
"move forward 10 units", "turn left 90 degrees"
than:
"turn left 90 degrees", "move forward 10 units"
simple, right??
well, just those simple rules will make a difference when you want to rotate an
object around it's own axis or around other object axis
Rotating in the object own axis would be something like:
"move to 0, 0, 0", "move forward 10 units", "turn left X degrees", "draw a shape",
"inc(X, 1.0)"
very simple, we move to a fixed position (10 units forward) turn X degrees, X gets
incremented later, so that makes the shape rotate around its own axis and then we
draw the shape, and increment the angle
In Delphi would be something like:
1 glTranslatef(-0.5, 0.0, -15.0); // Move 0.5 Units Left And 15.0 units Into
2 The Screen
3 glRotatef(rquad, 0.0, 1.0, 1.0); // Rotate The Quad On The Y and Z axis ( NEW )
4 glBegin(GL_QUADS); // We are going to draw a cube, so let's use QUADS
5 glColor3f(0.0, 1.0, 0.0);
6 glVertex3f(0.5, 0.5, -0.5); // Top Left
7 glVertex3f(-0.5, 0.5, -0.5); // Top Right
8 {.
9 .
10 .}
11 glEnd();
12
13 rquad := rquad + 1.0;
Now to rotate around that object we just drew, we would have to do the following:
"cancel previous rotation", "turn left X degrees", "move 3 units into screen",
"turn right Y degrees", "draw shape"
The first important thing to notice here is that we didn't use "move 0, 0, 0",
which means we're going to keep drawing from the position where we drew the last
shape
The second thing is cancelling the previous rotation, why is that? Because if you
don't, when you move forward X units, is going to move into whatever direction the
last angle was, that might be what you want, but if you want your shape to rotate
in a different axis, then you have to cancel the rotation, and then rotate to where
you want and then move... confusing? luckily you'll get to play with the source
code and see what happens when you cancel and when you don't.
The third thing is rotating to the angle that you want (this actually makes the
rotation around the first object posible)
Move 3 units into screen, This to separate the new shape from the last, if we don't
move to any direction, we will draw this shape in the same place!, maybe rotated to
a different angle but in the same coordinates
Finally we do a last rotation around a new variable angle and draw the shape. This
last rotation will make the new object rotate around it's own axis just like the
solar system, the earth rotates around the sun, and then also rotates around it's
own axis, makes sense? I hope so
let's look at the Delphi code:
14 //Notice we don't use glLoadIdentity here, which means we are at whatever
15 position we drew the quad
16 glRotatef(-rquad, 0.0, 1.0, 1.0); // Cancel the previous rotation!!!
17 glRotatef(rtri, 0.0, 1.0, 0.0); // Rotate The Triangle On The Y axis (around the
18 quad)
19 glTranslatef(0.0, 0.0, 3.0); // Move 3.0 Units Into The screen
20 glRotatef(rtri, 0.0, 0.0, 1.0);
21 // Let's rotate the pyramid, while we rotate around the quad
22 glBegin(GL_TRIANGLES); // We're gonna draw a pyramid, so let's use TRIANGLES
23 glColor3f(1.0, 0.0, 0.0); // Red
24 glVertex3f(0.0, 0.5, 0.0); // Top Of Triangle (Front)
25 {.
26 .
27 .}
28 glEnd();
29
30 rtri := rtri + 2.0;
31
32 //ok, that's it on explanations, here's the source code for the drawing procedure I
33 added a polygon to the scene just to show you how to create them Also check the
34 coloring mix on the pyramid
35
36 function DrawGLScene(): Bool; { All Rendering Done Here }
37 begin
38 glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT); //Clear Screen and Depth
39 Buffer
40 glLoadIdentity(); //Reset The View
41
42 glTranslatef(-0.5, 0.0, -15.0); // Move 0.5 Units Left And 15.0 units Into The
43 Screen
44 glRotatef(rquad, 0.0, 1.0, 1.0); // Rotate The Quad On The Y and Z axis ( NEW )
45 glBegin(GL_QUADS); // We are going to draw a cube, so let's use QUADS
46 glColor3f(0.0, 1.0, 0.0);
47 glVertex3f(0.5, 0.5, -0.5); // Top Left
48 glVertex3f(-0.5, 0.5, -0.5); // Top Right
49 glVertex3f(-0.5, 0.5, 0.5); // Bottom Right
50 glVertex3f(0.5, 0.5, 0.5); // Bottom Left
51
52 glColor3f(1.0, 0.5, 0.0);
53 glVertex3f(0.5, -0.5, -0.5); // Top Left
54 glVertex3f(-0.5, -0.5, -0.5); // Top Right
55 glVertex3f(-0.5, -0.5, 0.5); // Bottom Right
56 glVertex3f(0.5, -0.5, 0.5); // Bottom Left
57
58 glColor3f(1.0, 0.0, 0.0);
59 glVertex3f(0.5, 0.5, 0.5); // Top Left
60 glVertex3f(-0.5, 0.5, 0.5); // Top Right
61 glVertex3f(-0.5, -0.5, 0.5); // Bottom Right
62 glVertex3f(0.5, -0.5, 0.5); // Bottom Left
63
64 glColor3f(1.0, 1.0, 0.0);
65 glVertex3f(0.5, -0.5, -0.5); // Top Left
66 glVertex3f(-0.5, -0.5, -0.5); // Top Right
67 glVertex3f(-0.5, 0.5, -0.5); // Bottom Right
68 glVertex3f(0.5, 0.5, -0.5); // Bottom Left
69
70 glColor3f(0.0, 0.0, 1.0);
71 glVertex3f(-0.5, 0.5, 0.5); // Top Left
72 glVertex3f(-0.5, 0.5, -0.5); // Top Right
73 glVertex3f(-0.5, -0.5, -0.5); // Bottom Right
74 glVertex3f(-0.5, -0.5, 0.5); // Bottom Left
75
76 glColor3f(1.0, 0.0, 1.0);
77 glVertex3f(0.5, 0.5, -0.5); // Top Left
78 glVertex3f(0.5, 0.5, 0.5); // Top Right
79 glVertex3f(0.5, -0.5, 0.5); // Bottom Right
80 glVertex3f(0.5, -0.5, -0.5); // Bottom Left
81 glEnd();
82
83 //Notice we don't use glLoadIdentity here, which means we are at whatever
84 position we drew the quad
85 glRotatef(-rquad, 0.0, 1.0, 1.0); // Cancel the previous rotation!!!
86 glRotatef(rtri, 0.0, 1.0, 0.0);
87 // Rotate The Triangle On The Y axis (around the quad)
88 glTranslatef(0.0, 0.0, 3.0); // Move 3.0 Units Into The screen
89 glRotatef(rtri, 0.0, 0.0, 1.0);
90 // Let's rotate the pyramid, while we rotate around the quad
91 glBegin(GL_TRIANGLES); // We're gonna draw a pyramid, so let's use TRIANGLES
92 glColor3f(1.0, 0.0, 0.0); // Red
93 glVertex3f(0.0, 0.5, 0.0); // Top Of Triangle (Front)
94 glColor3f(0.0, 1.0, 0.0); // Green
95 glVertex3f(-0.5, -0.5, 0.5); // Left Of Triangle (Front)
96 glColor3f(0.0, 0.0, 1.0); // Blue
97 glVertex3f(0.5, -0.5, 0.5); // Right Of Triangle (Front)
98
99 glColor3f(1.0, 0.0, 0.0); // Red
100 glVertex3f(0.0, 0.5, 0.0); // Top Of Triangle (Right)
101 glColor3f(0.0, 0.0, 1.0); // Blue
102 glVertex3f(0.5, -0.5, 0.5); // Left Of Triangle (Right)
103 glColor3f(0.0, 1.0, 0.0); // Green
104 glVertex3f(0.5, -0.5, -0.5); // Right Of Triangle (Right)
105
106 glColor3f(1.0, 0.0, 0.0); // Red
107 glVertex3f(0.0, 0.5, 0.0); // Top Of Triangle (Back)
108 glColor3f(0.0, 1.0, 0.0); // Green
109 glVertex3f(0.5, -0.5, -0.5); // Left Of Triangle (Back)
110 glColor3f(0.0, 0.0, 1.0); // Blue
111 glVertex3f(-0.5, -0.5, -0.5); // Right Of Triangle (Back)
112
113 glColor3f(1.0, 0.0, 0.0); // Red
114 glVertex3f(0.0, 0.5, 0.0); // Top Of Triangle (Left)
115 glColor3f(0.0, 0.0, 1.0); // Blue
116 glVertex3f(-0.5, -0.5, -0.5); // Left Of Triangle (Left)
117 glColor3f(0.0, 1.0, 0.0); // Green
118 glVertex3f(-0.5, -0.5, 0.5); // Right Of Triangle (Left)
119 glEnd();
120
121 glLoadIdentity(); // Move to (0, 0, 0)
122 glTranslatef(1.5, 0.0, -6.0); // Move 1.5 Right and -6.0 intro screen
123 glRotatef(rpol, 0.0, 0.0, 1.0); // rotate on Z axis
124 glColor3f(0.0, 0.0, 1.0); // Add some color
125 glBegin(GL_POLYGON); // Draw A Polygon (I can put many points in here)
126 glVertex3f(-0.5, 0.5, 0.0); // Top Left
127 glVertex3f(0.0, 0.75, 0.0); // Upper point
128 glVertex3f(0.5, 0.5, 0.0); // Top Right
129 glVertex3f(0.5, -0.5, 0.0); // Bottom Right
130 glVertex3f(0.0, -0.75, 0.0); // Lower point
131 glVertex3f(-0.5, -0.5, 0.0); // Bottom Left
132 glEnd();
133
134 rtri := rtri + 2.0;
135 rquad := rquad + 1.0;
136 rpol := rpol + 1.0;
137
138 Result := True
139 end;
That's it, I hope it wasn't too confusing, play with the code, see what happens when you comment this or that line.
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