Perspective projection for a polygon Using OpenGL - c++

I'm trying to implement a perspective projection using open-GL
But when I apply gluPerspective(0,0.5,0.5,5) method, the polygon is not shown in a perspective view and shown in orthogonal view instead
here is the output
Anyone can help
my code:
#include<GL/glut.h>
float angle = 2;
void myinit(void)
{
glClearColor(1.0, 1.0, 1.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 5);
//glFrustum(-0.5, 2.4, -0.5, 0.5, -0.5, 0.5);
//glFrustum(-5.0, 5.0, -5.0, 5.0, 5, 100);
gluPerspective(0,0.5,0.5,5);
}
void polygon(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(0.0, 0.0, 1.0);
//glLineWidth(2);
//glRotatef(angle, 0.0, 0.0, 1.0);
glBegin(GL_POLYGON);
glVertex3f(0.25, 0.25, 0.0);
glVertex3f(0.75, 0.25, 0.0);
glVertex3f(0.75, 0.75, 0.0);
glVertex3f(0.25, 0.75, 0.0);
//glVertex3f(0, 0.5, 0.0);
glEnd();
glFlush();
}
int main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowPosition(50, 100);
glutInitWindowSize(1000, 1000);
glutCreateWindow("Open Gl 2D Geometric Transformation");
myinit();
glutDisplayFunc(polygon);
glutMainLoop();
return 0;
}

the first argument to to gluPerspective is wrong:
gluPerspective(0,0.5,0.5,5);
The first argument to gluPerspective is the vertical field of view angle in degrees. The value has to be greater than 0.0 and less than 180. 0.0 is an invalid argument and causes undefined behavior.
Probably the instruction doesn't set the matrix at all.
Anyway, if you set a correct angle, then the geometry will be clipped. The perspective projection matrix defines a Viewing frustum. All the geometry which is not in between the near and fare plane is clipped. In your case the near plane is 0.5 and the far plane 5.0.
Set a view matrix and translate the geometry in between the near and far plane, by shifting the geometry along the negative z axis. For instance (0, 0, -2.5).
The 2nd argument of gluPerspective is the aspect ration. Since the size of the window is 1000x1000, the aspect ratio has to be 1.0:
void myinit(void)
{
glClearColor(1.0, 1.0, 1.0, 0.0);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
GLdouble fov = 90.0; // 90 degrees
GLdouble aspect = 1.0;
GLdouble near_dist = 0.5;
GLdouble far_dist = 5.0;
gluPerspective(fov, aspect, near_dist, far_dist);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0f, 0.0f, -2.5f); // near_dist < 2.5 < far_dist
}

Related

Why does my wired sphere turn into ellipsoid when translating and changing camera angle?

I need to translate my wired sphere along z-axis back and forth while also changing camera angle. Whenever my sphere gets translated, it slowly turns into ellipsoid. I really don't understand why. Here you can see pieces of code where I believe is a mistake. Also, shapes shouldn't be changed when resizing the window, only their size.
void init() {
glClearColor(0.0, 0.0, 0.0, 0.0);
glEnable(GL_DEPTH_TEST);
}
void display(void) {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glPushMatrix();
glLoadIdentity();
gluLookAt(ex, ey, ez, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glTranslatef(0.0,0.0,tra);
glScalef(0.65, 0.65, 0.65);
glColor3f(1.0f, 0.8f, 1.0f);
glutWireSphere(0.65, 10, 15);
glPopMatrix();
glPushMatrix();
glLoadIdentity();
gluLookAt(ex, ey, ez, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glColor3f(0.1f, 0.8f, 1.0f);
glutWireTorus(0.25, 1.0, 15, 15);
glPopMatrix();
glFlush();
glFlush();
}
void reshape(int w, int h) {
glViewport(0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(70.0, (GLfloat)w / (GLfloat)h, 1.0, 80.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
[...] while also changing camera angle. Whenever my sphere gets translated, it slowly turns into ellipsoid.
That's caused by the Perspective distortion or wide-angle distortion and increases towards the edge of the view. The effect can be decreased by reducing the field of view angle, but the effect will never be canceled completely (except parallel projection).
See als How to fix perspective projection distortion?.
Also, shapes shouldn't be changed when resizing the window, only their size."
At perspective projection the size of the objects is always relative to the size of the viewport rather than the size of your screen.
If you don't want perspective distortion and if you want that the size of the objects has to be relative to the size of the screen (measured in pixel), then you have to use an orthographic projection, relative to the size of the viewport.
For instance:
void reshape(int w, int h) {
glViewport(0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//gluPerspective(70.0, (GLfloat)w / (GLfloat)h, 1.0, 80.0);
float sx = w / 100.0f;
float sy = h / 100.0f;
glOrtho(-sx/2.0f, sx/2.0f, -sy/2.0f, sy/2.0f, 1.0f, 80.0f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}

How do I correctly draw 3D axis in OpenGL?

I am trying to draw the 3D axis. I have also a cone which is rotating around its vertex and I would want the axis starting from there.
Here I have my function to draw the cone and I wrote the functions to draw the axis after gluLookAt:
GLfloat xRotated, yRotated, zRotated;
// Cone
GLdouble base=0.5;
GLdouble height=1.3;
GLint slices =20;
GLint stacks =20;
std::vector<std::array<GLfloat, 3>> data;
void displayCone(void)
{
// set matrix mode
glMatrixMode(GL_MODELVIEW);
// clear model view matrix
glLoadIdentity();
// multiply view matrix to current matrix
gluLookAt(0,2.,0.,0.,0.,-4.5,0,1,0); // <----------------------- add
// ******
glPushMatrix();
glLoadIdentity();
glTranslatef(0.0, 0.0, -4.5);
glBegin(GL_LINES);
glColor3f (1.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(2.0, 0.0, 0.0);
glColor3f (1.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 2.0, 0.0);
glColor3f (1.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 0.0, 2.0);
glEnd();
glPopMatrix();
// ******
// clear the drawing buffer.
glClear(GL_COLOR_BUFFER_BIT);
// traslate the draw by z = -4.0
// Note this when you decrease z like -8.0 the drawing will looks far , or smaller.
glTranslatef(0.0,0.0,-4.5);
// Red color used to draw.
glColor3f(0.8, 0.2, 0.1);
// changing in transformation matrix.
// rotation about X axis
glRotatef(xRotated,1.0,0.0,0.0);
// rotation about Y axis
glRotatef(yRotated,0.0,1.0,0.0);
// rotation about Z axis
glRotatef(zRotated,0.0,0.0,1.0);
// scaling transfomation
glScalef(1.0,1.0,1.0);
// built-in (glut library) function , draw you a Cone.
// move the peak of the cone to the origin
glTranslatef(0.0, 0.0, -height);
glutSolidCone(base,height,slices,stacks);
// Flush buffers to screen
// gluLookAt(3,3,3,0,0,-4.5,0,1,0); <----------------------- delete
glFlush();
// sawp buffers called because we are using double buffering
// glutSwapBuffers();
}
I thought to draw the 3D axis in this way but I am making some mistakes.
What am I missing?
The lies are cleared immediately after they've been drawn by glClear. Do glClear(GL_COLOR_BUFFER_BIT); at the begin of displayCone:
If you want to draw the lines in view space, then the view matrix has to be set. Remove glLoadIdentity before drawing the lines.
e.g.
void displayCone(void)
{
// clear the drawing buffer.
glClear(GL_COLOR_BUFFER_BIT); // <---- add
// set matrix mode
glMatrixMode(GL_MODELVIEW);
// clear model view matrix
glLoadIdentity();
// multiply view matrix to current matrix
gluLookAt(3.0, 3.0, 3.0-4.5, 0.0, 0.0,-4.5,0,1,0);
// ******
glPushMatrix();
// glLoadIdentity(); <---- delete
glTranslatef(0.0, 0.0, -4.5);
glBegin(GL_LINES);
glColor3f (1.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(2.0, 0.0, 0.0);
glColor3f (1.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 2.0, 0.0);
glColor3f (1.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 0.0, 2.0);
glEnd();
glPopMatrix();
// clear the drawing buffer.
// glClear(GL_COLOR_BUFFER_BIT); // <---- delete
// traslate the draw by z = -4.0
// Note this when you decrease z like -8.0 the drawing will looks far , or smaller.
glTranslatef(0.0,0.0,-4.5);
// Red color used to draw.
glColor3f(0.8, 0.2, 0.1);
// changing in transformation matrix.
// rotation about X axis
glRotatef(xRotated,1.0,0.0,0.0);
// rotation about Y axis
glRotatef(yRotated,0.0,1.0,0.0);
// rotation about Z axis
glRotatef(zRotated,0.0,0.0,1.0);
// scaling transfomation
glScalef(1.0,1.0,1.0);
// built-in (glut library) function , draw you a Cone.
// move the peak of the cone to the origin
glTranslatef(0.0, 0.0, -height);
glutSolidCone(base,height,slices,stacks);
// Flush buffers to screen
// gluLookAt(3,3,3,0,0,-4.5,0,1,0); <----------------------- delete
glFlush();
// sawp buffers called because we are using double buffering
// glutSwapBuffers();
}

OpenGL Can't draw z axis

Im very new to OpenGL and can't understand why my code doesn't work.
It is supposed to draw 3 axis: x y and z. But only x and y are displayed.
int main()
int main(int argc, char * argv[])
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowSize(800, 600);
glutCreateWindow("OpenGL lesson");
glutReshapeFunc(reshape);
glutDisplayFunc(display);
glutMainLoop();
return 0;
}
void reshape ()
void reshape(int w, int h)
{
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-15, 15, -15, 15, -15, 15);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClearColor(1, 1, 1, 0);
}
void display()
void display()
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 0.0, 0.0);
glBegin(GL_LINES);
// draw line for x axis
glColor3f(1.0, 0.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(10.0, 0.0, 0.0);
// draw line for y axis
glColor3f(0.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 10.0, 0.0);
// draw line for Z axis
glColor3f(0.0, 0.0, 1.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 0.0, 10.0);
glEnd();
glutSwapBuffers();
}
Could someone help, please?
Axiz Z is drawn, but due to the camera's default position, there will be just a single pixel drawn.
The camera's default position is looking in the Z direction, perpendicular to the XY plane. You might want to look into gluLookAt to place your camera elsewhere or gluPerspective for a different viewing volume (more commonly used for 3D rendering than glOrtho).

Opengl: how to make this triangle centred on the window

#include <GL/glut.h>
GLint winWidth = 600, winHeight = 600;
GLfloat x0 = 100.0, y0 = 100.0, z0 = 50.0;
GLfloat xref = 50, yref = 50.0, zref = 0.0;
GLfloat Vx = 0.0, Vy = 1.0, Vz = 0.0;
GLfloat xwMin = -40.0, ywMin = -60.0, xwMax = 40.0, ywMax = 60.0;
GLfloat dnear = 25.0, dfar = 125.0;
void init (void)
{
glClearColor (1.0, 1.0, 1.0, 0.0);
//glMatrixMode(GL_MODELVIEW);
//gluLookAt(x0, y0, z0, xref, yref, zref, Vx, Vy, Vz);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//glOrtho(0,1,0,1, 0,0.1);
//gluOrtho2D(0, 1,0,1);
//gluPerspective(45, 1.2, 1, 10);
glFrustum(0, 1, 0, 1, 0, 1);
//gluPerspective(45.0, 1, 1, 15);
}
void displayFcn (void)
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(0.0, 1.0, 0.0);
//glPolygonMode(GL_FRONT, GL_FILL);
//glPolygonMode(GL_BACK, GL_FILL);
glBegin(GL_TRIANGLES);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(1.0, 0.0, 0.0);
glVertex3f(0.5, 1.0, 0.0);
glEnd();
glFlush();
}
void reshapeFcn(GLint newWidth, GLint newHeight)
{
glViewport(0,0,newWidth, newHeight);
winWidth = newWidth;
winHeight = newHeight;
}
void main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE|GLUT_RGB);
glutInitWindowPosition(400,200);
glutInitWindowSize(winWidth, winHeight);
glutCreateWindow("Test");
init();
glutDisplayFunc(displayFcn);
glutReshapeFunc(reshapeFcn);
glutMainLoop();
}
This is the full source code, you can copy and paste to your VS solution and compile.
You'll need to have glut installed.
The result comes up like this:
The center of the window is 0,0 in opengl. So, when you calculate the vertices, you have to calculate them such that the center of the triangle is 0,0.
glBegin(GL_TRIANGLES);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(1.0, 0.0, 0.0);
glVertex3f(0.5, 1.0, 0.0);
glEnd();
those coords will need to be updated, you can find a discussion on finding the centers of triangles at this question: finding center of 2D triangle which sounds like it's from a similar homework assignment.
glTranslatef(-0.5f, -0.5f, 0.0f);
The default perspective for OpenGL is the origin centered and the window x and y ranging from -1 to 1. You can either change this by changing the default viewing volume or changing the coordinates of your triangle.
Either
glTranslatef(-.5f, -.5f, .0f);
or
glBegin(GL_TRIANGLES);
glVertex3f(-.5, -.5, 0.0);
glVertex3f(.50, -.5, 0.0);
glVertex3f(0, .5, 0.0);
glEnd();
will work.
It looks like you're trying to use glFrustum where you want to use glOrtho
glFrustum is supposed to be used for generating a perspective matrix. zNear is never 0. The way you call it right now generates an error GL_INVALID_VALUE, so you get the default projection matrix, the identity.

true isometric projection with opengl

Is there a simple way to have isometric projection?
I mean the true isometric projection, not the general orthogonal projection.
(Isometric projection happens only when projections of unit X, Y and Z vectors are equally long and angles between them are exactly 120 degrees.)
Try using gluLookAt
glClearColor(0.0, 0.0, 0.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
/* use this length so that camera is 1 unit away from origin */
double dist = sqrt(1 / 3.0);
gluLookAt(dist, dist, dist, /* position of camera */
0.0, 0.0, 0.0, /* where camera is pointing at */
0.0, 1.0, 0.0); /* which direction is up */
glMatrixMode(GL_MODELVIEW);
glBegin(GL_LINES);
glColor3d(1.0, 0.0, 0.0);
glVertex3d(0.0, 0.0, 0.0);
glVertex3d(1.0, 0.0, 0.0);
glColor3d(0.0, 1.0, 0.0);
glVertex3d(0.0, 0.0, 0.0);
glVertex3d(0.0, 1.0, 0.0);
glColor3d(0.0, 0.0, 1.0);
glVertex3d(0.0, 0.0, 0.0);
glVertex3d(0.0, 0.0, 1.0);
glEnd();
glFlush();
Results in
We can draw a cube to check that parallel lines are indeed parallel
glPushMatrix();
glTranslated(0.5, 0.5, 0.5);
glColor3d(0.5, 0.5, 0.5);
glutWireCube(1);
glPopMatrix();
An isometric projection is just a matter of using an orthographic projection with a specific rotation angle.
You should be able to choose any of the 8 potential orientations, with a orthographic projection, and get a perfect isometric view of your model. Just follow the math in your referenced Wiki article for setting up the view matrix, and do an orthographic projection for your projection matrix, and you're all set.
Maybe I'm not quite grokking the math correctly, but couldn't you just position your camera as it explains in that Wikipedia link and use a standard orthogonal projection?
Even if it's not the same, the projection stack is entirely up to you.
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
// your isometric matrix here (see math on Wikipedia)
glMatrixMode(GL_MODELVIEW);
If you do not want to use GLU, here is bare bones using glOrtho
void gl_enter_2_5d_mode (void)
{
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
double scale = 50;
glOrtho(-scale,
scale,
-scale * 0.7,
scale * 0.7,
-scale,
scale);
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glRotatef(35.264f, 1.0f, 0.0f, 0.0f);
glRotatef(-45.0f, 0.0f, 1.0f, 0.0f);
}
void gl_leave_2_5d_mode (void)
{
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
glMatrixMode(GL_PROJECTION);
glPopMatrix();
}
then draw a cube in it
void cube (double size)
{
glBegin(GL_QUADS);
glVertex3f(size,size,size);
glVertex3f(-size,size,size);
glVertex3f(-size,-size,size);
glVertex3f(size,-size,size);
glVertex3f(size,size,-size);
glVertex3f(-size,size,-size);
glVertex3f(-size,-size,-size);
glVertex3f(size,-size,-size);
glVertex3f(size,size,size);
glVertex3f(size,-size,size);
glVertex3f(size,-size,-size);
glVertex3f(size,size,-size);
glVertex3f(-size,size,size);
glVertex3f(-size,-size,size);
glVertex3f(-size,-size,-size);
glVertex3f(-size,size,-size);
glVertex3f(size,size,size);
glVertex3f(-size,size,size);
glVertex3f(-size,size,-size);
glVertex3f(size,size,-size);
glVertex3f(size,-size,size);
glVertex3f(-size,-size,size);
glVertex3f(-size,-size,-size);
glVertex3f(size,-size,-size);
glEnd();
}
void test (void)
{
glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
glBindTexture(GL_TEXTURE_2D, 0);
cube(1.0);
glPolygonMode(GL_FRONT_AND_BACK,GL_FILL);
}
and call something like this
gl_enter_2_5d_mode()
test()
gl_leave_2_5d_mode()
should you want to toggle between 2d and 2.5d (so you can draw your UI) then I have similar functions to enter and leave 2d mode e.g.
void gl_init_2d_mode (void)
{
/*
* Enable Texture Mapping
*/
glEnable(GL_TEXTURE_2D);
/*
* Enable alpha blending for sprites
*/
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
/*
* Setup our viewport
*/
glViewport(0, 0, game.video_pix_width,
game.video_pix_height);
/*
* Make sure we're changing the model view and not the projection
*/
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
/*
* Reset the view
*/
glLoadIdentity();
gl_init_fbo();
}
void gl_enter_2d_mode (void)
{
/*
* Change to the projection matrix and set our viewing volume.
*/
glMatrixMode(GL_PROJECTION);
glPushMatrix();
/*
* Reset the view
*/
glLoadIdentity();
/*
* 2D projection
*/
glOrtho(0,
game.video_gl_width, game.video_gl_height,
0, -1200.0, 1200.0);
/*
* Make sure we're changing the model view and not the projection
*/
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
/*
* Reset the view
*/
glLoadIdentity();
}