I just want to create a perspective where the eye of the camera would be, at, say: (2, 2, -2), looking right at the origin. I'm trying to use a combination of gluLookAt() and glFrustum(), but for some reason, though it is rendering, my objects look very distorted:
glMatrixMode( GL_PROJECTION );
glLoadIdentity();
glFrustum (-1.0, 1.0, -1.0, 1.0, 1.0, 500.0);
...
gluLookAt (-2.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, -1.0, 0.0);
What am I doing wrong?
EDIT: Here is a screenshot. The left side defines a profile curve to be swept around the y-axis. It should be a cylinder in the perspective view, but it's... warped.
EDIT 2: Also, those axis in the perspective view are set up as followed, which I know isn't correct:
// draw the axis
glBegin(GL_LINES);
// x
glVertex3f(500.0, 0.0, 0.0);
glVertex3f(-500.0, 0.0, 0.0);
// y
glVertex3f(0.0, -500.0, 0.0);
glVertex3f(0.0, 500.0, 0.0);
// z
glVertex3f(0.0, 0.0, -500.0);
glVertex3f(0.0, 0.0, 500.0);
glEnd();
EDIT 3: Also, none of the vertices of that shape have an x, y, or z value greater than 1.0..
What am I doing wrong?
I don't thing you're doing wrong anything. The kind of view you want to have in your left pane is usually done using a orthographic projection. The distortion you see is just a perspective distortion; you're not looking perpendicular onto the curve (the curve lies in the XY plane, and you're looking at the XY plane from some angle).
Side note:
The projection matrix only defines the "lens" of OpenGL. It must not be used to place the "camera". Any eyepoint positioning (the view) is defined as part of the modelview transformation. Thus gluLookAt is meant to be used on the modelview matrix.
Your frustum is set up as if the screen is square. If your screen isn't square this will distort the objects badly.
Also, if the objects are close to your camera, the near and far distances can affect the image.
And one more thing, your up axis in gluLookAt is pointing down.
Posting a screenshot would help identify the problem.
Related
I dont understand how this GluLookAt works in OpenGl.
I would like to know how to transform this two lines :
gluLookAt(5.0, 15.0, 2.0, 0.0, 0.0, 0.0, 1.0, 0.0, -1.0);
gluLookAt(5.0, 0.0, 5.0, 0.0, 0.0, 0.0, 1.0, -1.0, 0.0);
using glRotatef and glTranslatef.
After some searches, it seems to exist a way for making that thing :
glRotatef();
glRotatef();
glTranslatef(5.0,15.0,2.0);
glRotatef();
glRotatef();
glTranslatef(5.0,0.0,5.0);
So just by using two rotations and one translation.
But I dont understand how can i find the angles and the axes of these rotations.
I tried to explain how the functions work below. Hope it makes you understand the concept. For rotation and translation you can check this link to see how it is handled.
struct Triple
{
float x,y,z;
}
//CameraPosition
Triple Cp(a,b,c); //initialise your camera position
//LookatPosition
Triple Lp(e,f,g); //initialise your lookat position
//Up vector
Triple Up(k,l,m); //initialise your up vector
UpdateCamera()
{
//Update Cp, Lp here
//if you move your camera use translatef to update camera position
//if you want to change looking direction use correct rotation and translation to update your lookat position
//if you need to change up vector simply change it to
Up = Triple(knew,lnew,mnew);
}
display()
{
gluLookAt(Cp.x,Cp.y,Cp.z,Lp.x,Lp.y,Lp.z,Up.x,Up.y,Up.z);
//Your object drawings Here
}
I'd like to sidestep the glRotate and glTranslate and use glLoadMatrix instead (glLoadMatrix replaces the current matrix on the stack use glMultMatrix if you want to multiply): you would then use an array of floats containing the matrix in column major order:
xaxis.x yaxis.x zaxis.x 0
xaxis.y yaxis.y zaxis.y 0
xaxis.z yaxis.z zaxis.z 0
-dot(xaxis, camP) -dot(yaxis, camP) -dot(zaxis, camP) 1
where
zaxis = normal(At - camP)
xaxis = normal(cross(Up, zaxis))
yaxis = cross(zaxis, xaxis)
and camP the position of the camera, At the point the camera is looking at and Up the up-vector.
One of the ways to view a scene is to either use gluLookAt or to create your own custom viewing routine. I came across this custom viewing routine in a flight simulator in a book.
void pilotView(GLdouble planex, GLdouble planey,
GLdouble planez, GLdouble roll,
GLdouble pitch, GLdouble heading)
{
glRotated(roll, 0.0, 0.0, 1.0);
glRotated(pitch, 0.0, 1.0, 0.0);
glRotated(heading, 1.0, 0.0, 0.0);
glTranslated(-planex, -planey, -planez);
}
Rotating the camera by x degrees clockwise is equivalent to rotating the target object by x degrees counterclockwise. Thus, when we specify the "roll" angle for example, shouldn't we be writing "glRotated(-roll, 0.0, 0.0, 1.0)" instead of "glRotated(roll, 0.0, 0.0, 1.0)"?
No, we should not, in OpenGL triangles are transformed not camera OpenGL does not have notion of camera; so when you add a rotation to the transform your object will be rotated by this updated transform.
gluLookAt just sets up transformation of the objects as if you are looking at it, but still what is transformed is the object.
As for this function my guess is that it sets transformation to set it so that view would be aligned with plane instead of being aligned with horizontal and hang in the center instead of behind that is why we have + sign on rotations and - sign on translations. Note that calls do not set transformation, but update it.
I'm having problems in OpenGL getting my object (a planet) to rotate relative to the current camera rotation. It seems to work at first, but then after rotating a bit, the rotations are no longer correct/relative to the camera.
I'm calculating a delta (difference) in mouseX and mouseY movements on the screen. The rotation is stored in a Vector3D called 'planetRotation'.
Here is my code to calculate the rotation relative to the planetRotation:
Vector3D rotateAmount;
rotateAmount.x = deltaY;
rotateAmount.y = deltaX;
rotateAmount.z = 0.0;
glPushMatrix();
glLoadIdentity();
glRotatef(-planetRotation.z, 0.0, 0.0, 1.0);
glRotatef(-planetRotation.y, 0.0, 1.0, 0.0);
glRotatef(-planetRotation.x, 1.0, 0.0, 0.0);
GLfloat rotMatrix[16];
glGetFloatv(GL_MODELVIEW_MATRIX, rotMatrix);
glPopMatrix();
Vector3D transformedRot = vectorMultiplyWithMatrix(rotateAmount, rotMatrix);
planetRotation = vectorAdd(planetRotation, transformedRot);
In theory - what this does is, sets up a rotation in the 'rotateAmount' variable. It then gets this into model space, by multiplying this vector with the inverse model transform matrix (rotMatrix).
This transformed rotation is then added to the current rotation.
To render this is the transform being setup:
glPushMatrix();
glRotatef(planetRotation.x, 1.0, 0.0, 0.0);
glRotatef(planetRotation.y, 0.0, 1.0, 0.0);
glRotatef(planetRotation.z, 0.0, 0.0, 1.0);
//render stuff here
glPopMatrix();
The camera sort of wobbles around, the rotation I'm trying to perform, doesn't seem relative to the current transform.
What am I doing wrong?
GAH! Don't do that:
glPushMatrix();
glLoadIdentity();
glRotatef(-planetRotation.z, 0.0, 0.0, 1.0);
glRotatef(-planetRotation.y, 0.0, 1.0, 0.0);
glRotatef(-planetRotation.x, 1.0, 0.0, 0.0);
GLfloat rotMatrix[16];
glGetFloatv(GL_MODELVIEW_MATRIX, rotMatrix);
glPopMatrix();
OpenGL is not a math library. There are proper linear algebra libraries for that kind of job.
As for your problems. A vector is not fit to store a rotation. You need at least a Vector (axis of rotation) and the angle itself, or better yet a Quaternion.
Also rotations don't add. They're no commutative, however addition is a commutative operation. Rotations in fact multiply.
How to fix your code: Rewrite it from scratch using the proper mathematical methods. For this please read up the topics of "Rotation matrices" and "Quaternions" (Wikipedia has them).
Suppose I have a point at (250,125,-20).
After the following transformation,
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(100.0, 50.0, 0.0);
glRotatef(-25.0, 0.0, 1.0, 0.0);
How can I get the value of current coordinates of that point?
Need I write a subroutine to multiply a matrix to a vector?
Are there any built-in solutions?
You can't get the coordinates for a specific vertex (point) after a transformation, however for this particular case you can get the ModelViewMatrix after the translate/rotate is applied.
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(100.0, 50.0, 0.0);
glRotatef(-25.0, 0.0, 1.0, 0.0);
glGetFloatv(GL_MODELVIEW_MATRIX , *your_matrix*);
//print your matrix to check if that is the desired transformation coordinates
There is no magic tape in OpenGL, you will have to write your own framework e.g: for every objects in your world a class where you hold the vertices and what data you find relevant.
My instructor says they can be used interchangeably to affect the projection matrix. He argues that gluLookAt() is preferably used because of its 'relative simplicity due to its ability to define the viewing angle'. Is this true? I've seen code examples using both gluLookAt() and glFrustum() and I'm wondering why would the programmmer mix them.
Like in the cube.c example of the redbook appears:
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glColor3f(1.0, 1.0, 1.0);
glLoadIdentity();
gluLookAt(0.0, 0.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
**//why isn't this a call to glFrustum?**
glScalef(1.0, 2.0, 1.0);
glutWireCube(1.0);
glFlush();
}
void reshape(int w, int h)
{
glViewport(0, 0, (GLsizei) w, (GLsizei) h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-1.0, 1.0, -1.0, 1.0, 1.5, 20.0);
//why isn't this a call a call to gluLookAt()?
glMatrixMode(GL_MODELVIEW);
}
Both glFrustum and gluLookAt perform simple matrix multiplication. Check the man pages for equations for those matrices:
gluLookAt
glFrustum
Both of those can be replaced by a glMultMatrix* call.
The most important difference is that glFrustum is used most of the time to establish a perspective projection matrix (used internally by gluPerspective), and gluLookAt is a convenience method for specifying model-view matrices (usually: implementing a camera).
gluLookAt affects the ModelView matrix whereas, glFrustum/gluPerspective affect the Projection matrix. These are entirely two different matrices in the pipeline. gluLookAt is a utility method which is a combination of glRotate and glTranslate calls to move the "camera" to the appropriate point in space and point it to the desired location. Whereas, glFrustum/gluPerspective are methods which define a perspective viewing volume. Check out for more details in the OpenGL Red Book.