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| 6.
TRANSFORMATION IN OPENGL |
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1. |
From the previous tutorial, if you run the program using
GL_POLYGON in
step number 8, you will get this result:
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2. |
Now, replace the program with this program.
When you run it, it will be like this:
Now we will go through the program
explanation.
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TRANSLATION
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3. |
At line 18 in the program, glTranslatef is used to
move the object along the X, Y and Z axis. The parameters involved are:
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void
glTranslatef
(float X, float Y, float Z); |
In this example, the
function glTranslatef() is used to move the polygon by 1
unit along the X-axis in the right (positive) direction.
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glTranslatef(1.0, 0.0, 0.0); |
See this diagram for clearer view:
Simply said, the vertices
at coordinate X, will be added with 1.0. For vertices at
coordinate Y and Z, they will be added with 0.0.
V0
( 0 + 1.0,
0 + 0.0,
0 + 0.0)
V1 (
1 + 1.0,
0 + 0.0,
0 + 0.0)
V2 (
1 + 1.0,
1 + 0.0,
0 + 0.0)
V3 (0.5 + 1.0,
1.5 + 0.0,
0 + 0.0)
V4 (
0 + 1.0,
1 + 0.0,
0 + 0.0) |
Now, the latest vertices coordinates
will be like this:
V0 ( 0,
0, 0)
V1 (
2, 0, 0)
V2 (
2, 1, 0)
V3
(1.5, 1.5, 0)
V4 (
1, 1, 0) |
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SCALING
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| 4. |
Now, replace line 18
(glTranslatef) with this:
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5. |
This time, the output will be like this:
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6. |
glScalef is used to resize the
object. The parameters involved are:
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void
glScalef
(float X, float Y, float Z); |
In the example above, all the vertices
at coordinate X, will be multiplied by 2.0. For vertices at
coordinate Y and Z, they will be multiplied by 1.0.
V0 ( 0 x
2.0, 0 x
1.0, 0 x
1.0)
V1 (
1 x
2.0, 0 x
1.0, 0 x
1.0)
V2 (
1 x
2.0, 1 x
1.0, 0 x
1.0)
V3 (0.5 x
2.0, 1.5 x
1.0, 0 x
1.0)
V4 (
0 x
2.0, 1 x
1.0, 0 x
1.0) |
Now, the latest vertices coordinates
will be like this:
V0 ( 0, 0, 0)
V1 ( 2, 0, 0)
V2 ( 2, 1, 0)
V3 (1.0, 1.5, 0)
V4 ( 0, 1, 0) |
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ROTATION
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| 7. |
Now, replace line 18
(glScalef) with this:
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glRotatef(-90.0, 0.0, 0.0, 1.0); |
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8. |
This time, the output will be like this:
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9. |
glRotatef is used to rotate the object
around the axis. The parameters involved are:
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void
glRotatef
(float Angle, float X, float
Y, float Z); |
In the example, the object is
rotated around Z-axis, with -90 degrees rotation.
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10.
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Now, you can try the combination of the transformations in your
program such as this, and see the result:
glPushMatrix();
glTranslatef(1.0, 2.0, 0.0);
glRotatef(180.0, 0.0, 0.0, 1.0);
glScalef(1.0,
2.0, 1.0);
glBegin(GL_POLYGON);
glVertex3f( 0.0, 0.0, 0.0);
// V0 ( 0, 0, 0)
glVertex3f( 1.0f, 0.0, 0.0);
// V1 ( 1, 0, 0)
glVertex3f( 1.0f, 1.0f, 0.0);
// V2 ( 1, 1, 0)
glVertex3f( 0.5f, 1.5f, 0.0);
// V3 (0.5, 1.5, 0)
glVertex3f( 0.0, 1.0f, 0.0);
// V4 ( 0, 1, 0)
glEnd();
glPopMatrix();
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