Author: Eber Irigoyen
On this article I'll show you some basic movement and rotating, I also try to
explain how openGL works
Answer:
After showing you how to setup an openGL window and draw a simple quad (OpenGL I:
Hello World) plus doing all the message handling and correctly shutting down your
openGL application on this article we will add some movement and rotating to our
shapes This article is based on the first article, so if you didn't read it you may
do so now, so it makes more sense
After reading this article you should be able to move shapes around the screen and
rotate them in all the different axis, what can you do with that? well, some basic
2D games... of course moving a quad on the screen is not so exciting, but at least
we're moving now =o) later we will see how to add textures to our shapes so they
look way better.
Drawing in openGL is relatively easy, because when you draw, say a quad and you
want to rotate it, you don't have to do any calculations, openGL works using a
turtle graphics kinda of thing, where the pen that is drawing just follows
instructions like "go forward", "turn left 90 degrees", etc... and so, as I say the
advantage on this is that that many paths are more simply described in relative
than in absolute terms.
For example, it's easy to indicate the absolute coordinates of the corners of a
square with vertical and horizontal sides, but it's not so easy to find the corners
of an inclined square.
A good way to describe this is if you hold a map (is the screen) and then walk
following the directions, then you find a square that is facing you, you see it
with vertical and horizontal sides to you (then pen), but when you first saw the
map that same square was inclined, make sense?
those instructions in OpenGL look like this:
1 glTranslatef(-1.5, 0.0, PosZ);
2 //move to position 3.0, 0.0, PosZ (coming and going to/from view)
3 glRotatef(Angle, 0.0, 0.0, 1.0); // Rotate The quad On The Z axis
4 glBegin(GL_QUADS); // Draw A Quad
5 glVertex3f(-1.0, 1.0, 0.0); // Top Left
6 glVertex3f(1.0, 1.0, 0.0); // Top Right
7 glVertex3f(1.0, -1.0, 0.0); // Bottom Right
8 glVertex3f(-1.0, -1.0, 0.0); // Bottom Left
9 glEnd(); // end of the Quad
The glTranslatef is the "go -1.5 in the X axis", "go O in the Y" and "go PosZ in
the Z axis", as you can see there's only one variable, and that's the one that is
going to allow my shape to move (only in the Z axis for now, which is going into
the screen depth or coming out)
Then the glRotatef is "rotate Angle degrees in the Z axis", if I had put 1.0 in the
second parameter is would rotate in the X axis, the third parameter in the Y axis
and the last parameter in the Z axis.
Then I tell openGL, glBegin(GL_QUAD): "I'm going to draw a square", and then you
specify the four points of the Quad, because you already told openGL that you're
going to draw a square is expecting four points if you give it five or six points
it will discard them, it will only take four by four (which make a quad) If I put 8
points it will create 2 quads, and so on the same applies if you tell openGL that
you are going to draw triangles, you have to give 3 by 3 points
Ok, with no more here's the main code of our article:
Since we are going to do some movement, we now need some variables
10 var
11 Angle: glfloat; //angle of the shapes
12 PosZ: glfloat; //Position in the Z axis
13 DForward: Boolean; //going forward? or backwards if false
14
15 //That's all we need, now we initialize our variables on the InitGL part of our
16 program:
17
18 Angle := 0;
19 PosZ := -20.0;
20 DForward := True;
21
22 //And that's all we need, we can draw now:
23
24 function DrawGLScene(): Bool; { All Rendering Done Here }
25 begin
26 glClear(GL_COLOR_BUFFER_BIT or GL_DEPTH_BUFFER_BIT); //Clear Screen and Depth
27 Buffer
28 glLoadIdentity(); //Reset The View (move to 0, 0, 0)
29 glColor3f(0.0, 0.0, 1.0); //set the color (1.0=totally blue)
30 glTranslatef(-1.5, 0.0, -15.0); //Draw triangle always at same position
31 glRotatef(Angle, 0.0, 1.0, 0.0); // Rotate The Triangle On The Y axis ( NEW )
32 glBegin(GL_TRIANGLES); // Drawing Using Triangles
33 glVertex3f(0.0, 1.0, 0.0); // Top
34 glVertex3f(-1.0, -1.0, 0.0); // Bottom Left
35 glVertex3f(1.0, -1.0, 0.0); // Bottom Right
36 glEnd(); // end of the triangle
37
38 glLoadIdentity(); //move to position 0, 0, 0
39 glColor3f(0.5, 0.0, 0.5); //set the color (0.5 red and 0.5 blue)
40 glTranslatef(-1.5, 0.0, PosZ);
41 //move to position 3.0, 0.0, PosZ (coming and going to/from view)
42 glRotatef(Angle, 0.0, 0.0, 1.0); // Rotate The quad On The Z axis
43 glBegin(GL_QUADS); // Draw A Quad
44 glVertex3f(-1.0, 1.0, 0.0); // Top Left
45 glVertex3f(1.0, 1.0, 0.0); // Top Right
46 glVertex3f(1.0, -1.0, 0.0); // Bottom Right
47 glVertex3f(-1.0, -1.0, 0.0); // Bottom Left
48 glEnd(); // end of the Quad
49
50 if (DForward) then
51 //control the position of the quad, it just goes forward or backward
52 begin
53 PosZ := PosZ + 0.05; //go Forward
54 if (PosZ > -10.5) then
55 //have I gone too far? go backwards now (towards screen depth)
56 DForward := False
57 end
58 else
59 begin //go backward
60 PosZ := PosZ - 0.05;
61 if (PosZ < -20.0) then //have I gone too far into depth? go forward (towards
62 user)
63 DForward := True
64 end;
65 Angle := Angle + 0.4; //change the angle
66 DrawGLScene := True
67 end;
Simple right? once you understand how the drawing is done "internally" it's easier
to figure out what we can do with openGL.
That's all for this article, let me know what you think about these openGL articles I try to make them as easy to understand as posible, because I know that is not that easy at the begining but let me know if you are interested in more advanced openGL articles and I will try to post some more topics soon.
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