3d letter with axes rotation doesn't work - opengl

Question

I need to draw a 3d letter A and axes using opengl. In order to draw letter A I draw 2d letter A with z == 50 and then draw letter A with z = 50 - offset.
Here my opengl and letter a coordinates initialization
glutInit(&argc,argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowSize(WIDTH, HEIGHT);
glutCreateWindow("PROJECT");
glClearColor(1.0, 1.0, 1.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0, WIDTH, 0, HEIGHT, -100, 100);
glMatrixMode(GL_MODELVIEW);
p1.x = 0; p1.y = 400 ; p1.z = 50;
p2.x = -300; p2.y = -500; p2.z = 50;
p3.x = -200; p3.y = -500; p3.z = 50;
p4.x = -100; p4.y = -200; p4.z = 50;
p5.x = 100; p5.y = -200; p5.z = 50;
p6.x = 200; p6.y = -500; p6.z = 50;
p7.x = 300; p7.y = -500; p7.z = 50;
p8.x = -130; p8.y = -100; p8.z = 50;
p9.x = 130; p9.y = -100; p9.z = 50;
p10.x = 0; p10.y = 100; p10.z = 50;
glutDisplayFunc(display);
glutMainLoop();
In function display I draw axes and two letters A.
void display() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
axes();
drawLetter(0);
drawLetter(100);
glFlush();
glutSwapBuffers();
}
Here initial values for axes
float angle = 17;
float AXIS_X[3] = { 250, 0, 0 },
AXIS_Y[3] = { 0, 250, 0 },
AXIS_Z[3] = { 0, 0, 150 };
I draw axes like this
void axes() {
glPushMatrix();
glTranslatef(WIDTH/2, HEIGHT/2, 0);
glBegin(GL_LINES);
glColor3f(1, 0, 0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3fv(AXIS_X);
glEnd();
glBegin(GL_LINES);
glColor3f(0, 0, 1);
glVertex3f(0.0, 0.0, 0.0);
glVertex3fv(AXIS_Y);
glEnd();
glBegin(GL_LINES);
glColor3f(0, 1, 0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3fv(AXIS_Z);
glEnd();
glPopMatrix();
}
Here is code for the letter
void drawLetter(float offset) {
glPushMatrix();
glTranslatef(WIDTH/2, HEIGHT/2, 0);
glScalef(0.3, 0.3, 0.3);
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(p1.x, p1.y, p1.z - offset);
glVertex3f(p2.x, p2.y, p2.z - offset);
glVertex3f(p3.x, p3.y, p3.z - offset);
glVertex3f(p10.x, p10.y, p10.z - offset);
glEnd();
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(p1.x, p1.y, p1.z - offset);
glVertex3f(p10.x, p10.y, p10.z - offset);
glVertex3f(p6.x, p6.y, p6.z - offset);
glVertex3f(p7.x, p7.y, p7.z - offset);
glEnd();
glBegin(GL_POLYGON);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(p8.x, p8.y, p8.z - offset);
glVertex3f(p4.x, p4.y, p4.z - offset);
glVertex3f(p5.x, p5.y, p5.z - offset);
glVertex3f(p9.x, p9.y, p9.z - offset);
glEnd();
glPopMatrix();
}
By default it looks like this
but when I add glRotatef(angle, 0.0, 1.0, 0.0); before axes() it looks like this(simple green line yes)
What's the problem?
Also a question - how to create orthogonal and perspective projections on the different planes(xy, yz, xz) for my letter? I need to use glFrustrum and glOrtho?

Answer 1

Normally you want to do the operations in this order:
Scale
Rotate
Translate
Since OpenGL uses column-major matrices, you want:
Translate
Rotate
Scale
NOTE: Conceptually the order is the same, you just have to do matrix multiplication in a different order when you use column-major.
I would start by addressing this.
The other important thing to note is that if you rotate outside of either drawLetter (...) or axes (...) in this code, the rotation is going to accumulate over multiple frames (since you do not load an identity matrix). You should either load an identity matrix for modelview each frame, or do the rotation inside your PushMatrix / PopMatrix blocks (you need to do this for proper rotation anyway).