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So I draw an 'I' and use gluLookAt(0.f,0.f,3.f,0.f,0.f,0.f,0.f,1.f,0.f), and the I is moderate size. Then I add a drawScene() function which draw the background with gradient color, and then the 'I' becomes super big. I guess it is because I change matrix mode to GL_PROJECTION and GL_MODELVIEW in drawScene(), and those change the perspective maybe? I guess glPushMatrix() and glPopMatrix() are needed to reserve matrix status, but I have hard time finding where to put them. So how can I make the 'I' look normal size? Here are my drawI() and drawScene():
void drawI(int format)
{
glBegin(format);
glColor3f(0, 0, 1);
glVertex2f(point[3][0], point[3][1]);
glVertex2f(point[2][0], point[2][1]);
glVertex2f(point[1][0], point[1][1]);
glVertex2f(point[12][0], point[12][1]);
glVertex2f(point[10][0], point[10][1]);
glEnd();
glBegin(format);
glVertex2f(point[10][0], point[10][1]);
glVertex2f(point[11][0], point[11][1]);
glVertex2f(point[12][0], point[12][1]);
glEnd();
glBegin(format);
glVertex2f(point[9][0], point[9][1]);
glVertex2f(point[10][0], point[10][1]);
glVertex2f(point[3][0], point[3][1]);
glVertex2f(point[4][0], point[4][1]);
glVertex2f(point[6][0], point[6][1]);
glColor3f(1, 0.5, 0);
glVertex2f(point[7][0], point[7][1]);
glVertex2f(point[8][0], point[8][1]);
glEnd();
glBegin(format);
glColor3f(0, 0, 1);
glVertex2f(point[5][0], point[5][1]);
glVertex2f(point[6][0], point[6][1]);
glVertex2f(point[4][0], point[4][1]);
glEnd();
}
void drawScene()
{
glBegin(GL_QUADS);
//red color
glColor3f(1.0,0.0,0.0);
glVertex2f(-1.0,-1.0);
glVertex2f(1.0,-1.0);
//blue color
glColor3f(0.0,0.0,1.0);
glVertex2f(1.0, 1.0);
glVertex2f(-1.0, 1.0);
glEnd();
}
Thanks a lot!
So I take glMatrixMode() and glLoadIdentity() out of drawScene() and drawI() and put them in display(). I changed drawScene() and drawI() above, and here is my display()
void display()
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(70.f,1.f,0.001f,30.f);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
drawScene();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0.f,0.f,3.f,0.f,0.f,0.f,0.f,1.f,0.f);
drawI(GL_TRIANGLE_FAN);
glutSwapBuffers();
}
The normal way to do this (in a 3D mode) is in your code, before you call drawI or drawScene would be:
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(fov, aspect, near, far); // fov is camera angle in degrees, aspect is width/height of your viewing area, near and far are your near and far clipping planes.
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0.0,0.0,3.0,0.0,0.0,0.0,0.0,1.0,0.0)
In your 2D rendering, you probably don't need the call to gluPerspective, but these calls should be done in your code before you call drawI or drawScene. Do this and delete the glMatrixMode() and glLoadIdentity() calls from drawI and drawScene.
Edit:
If your "I" is still too big, there are a number of things you could do, but you should probably be operating in 3D (giving a Z coordinate also).
You could scale the object:
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0.0,0.0,3.0,0.0,0.0,0.0,0.0,1.0,0.0)
glScalef(0.5, 0.5, 0.5);
You could move the camera further back (you'll need to include the gluPerspective() call as well):
gluLookAt(0.0,0.0,50.0,0.0,0.0,0.0,0.0,1.0,0.0);
Perhaps the easiest way to control the rendered image size in a 3D mode is to both move the camera (eye) back a way and then control the image size by changing the camera aperture angle (fov in the gluPerspective() call). A wider fov will shrink the rendered image; a smaller fov will enlarge it.
I don't know what the values for your coordinates are in drawI since they're variables, but a camera position of 3.0, an fov of 70.0 and an aspect of 1 should give you left, right, top and bottom clipping planes of about +/- 2.1 at Z = 0.
If you kept everything else the same and moved the camera to 50.0, the clipping planes would be at about +/- 35.0, so your "I" would occupy a much smaller portion of the viewing area.
If you then left the camera position at 50.0, but changed the fov to 40.0, the clipping planes would be at about +/- 18.2. Your "I" would fill a larger area than it did at cameraZ = 50.0, fov = 70.0, but a smaller area than cameraZ = 3.0, fov = 70.0.
You can play with camera position and fov to get the image size you want, or you could just scale the image. I like to keep camera position constant and change the fov. If I provide a function that changes the fov based on user input (maybe a mouse scroll), it's a good way to provide a zoom in/out effect.
BTW, in your original code, if you called:
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(0.0,0.0,3.0,0.0,0.0,0.0,0.0,1.0,0.0)
Then later in DrawI or drawScene call:
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
You've trashed the matrix loaded by your earlier call to gluLookAt().
I was trying to understand OpenGL a bit more deep and I got stuck with below issue.
This segment describes my understanding, and the outputs are as assumed.
glViewport(0, 0 ,800, 480);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-400.0, 400.0, -240.0, 240.0, 1.0, 100.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0, 0, -1);
glRotatef(0, 0, 0, 1);
glBegin(GL_QUADS);
glVertex3f(-128, -128, 0.0f);
glVertex3f(128, -128, 0.0f);
glVertex3f(128, 128, 0.0f);
glVertex3f(-128, 128, 0.0f);
glEnd();
The window coordinates (Wx, Wy, Wz) for the above snippet are
(272.00000286102295, 111.99999332427979, 5.9604644775390625e-008)
(527.99999713897705, 111.99999332427979, 5.9604644775390625e-008)
(527.99999713897705, 368.00000667572021, 5.9604644775390625e-008)
(272.00000286102295, 368.00000667572021, 5.9604644775390625e-008)
I did a glReadPixels() and dumped to a bmp file. In the image I get a quad as expected with the (Wx, Wy) mentioned above ( since incase of images, the origin is at the top left, while verifying the bmp image I took care of subtracting the the window height i.e 480). This output was as per my understanding - (Wx, Wy) will be used as a 2D coordinate and Wz will be used for depth purpose.
Now comes the issue. I tried the below code snippet.
glViewport(0, 0 ,800, 480);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-400.0, 400.0, -240.0, 240.0, 1.0, 100.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(100, 0, -1);
glRotatef(30, 0, 1, 0);
glBegin(GL_QUADS);
glVertex3f(-128, -128, 0.0f);
glVertex3f(128, -128, 0.0f);
glVertex3f(128, 128, 0.0f);
glVertex3f(-128, 128, 0.0f);
glEnd()
The window coordinates for the above snippet are
(400.17224205479812, 242.03174613770986, 1.0261343689191909)
(403.24386530741430, 238.03076912806583, 0.99456100555566640)
(403.24386530741430, 241.96923087193414, 0.99456100555566640)
(400.17224205479812, 237.96825386229017, 1.0261343689191909)
When I dumped output to a bmp file, I expected to have a very small parallelogram(approx like a 4 x 4 square transformed to a parallelogram) based on the above (Wx, Wy). But this was not the case. The image had a different set of coordinates as below
(403, 238)
(499, 113)
(499, 366)
(403, 241)
I have mentioned the coordinates in CW direction as seen on the image.
I got lost here. Can anyone please help in understanding what and why it is happening in the 2nd case??
How come I got a point (499, 113) on the screen when it was no where in the calculated window coordinates?
I used gluProject() to the window coordinates.
Note : I'm using OpenGL 2.0. I'm just trying to understand the concepts here, so please don't suggest to use versions > OpenGL 3.0.
edit
This is an update for the answer posted by derhass
The homogenous coordinates after the projection matrix for the 2nd case is as follows
(-0.027128123630699719, -0.53333336114883423, -66.292930483818054, -63.000000000000000)
(0.52712811245482882, -0.53333336114883423, 64.292930722236633, 65.00000000000000)
(0.52712811245482882, 0.53333336114883423, 64.292930722236633, 65.000000000000000)
(-0.027128123630699719, 0.53333336114883423, -66.292930483818054, 63.000000000000000)
So here for the vertices where z > -1, the vertices will get clipped at the near plane. When this is the case, shouldn't GL use the projected point at z = -1 plane?
The thing you are missing here is clipping.
After this
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-400.0, 400.0, -240.0, 240.0, 1.0, 100.0);
you basically have a camera at origin, looking along the -z direction, and the near plane at z=-1, the far plane at z=-100. Now you draw a 128x128 square rotated at 30 degrees aliong the y (up) axis, and shifted by -1 along z (and 100 along x, but that is not the crucial point here). Since You rotated the square around its center point, the z value for two of the points will be way before the near plane, while the other two should fall into the frustum. (And you can also see that as those two points match your expectations).
Now directly projecting all 4 points to window space is not what GL does. It transforms the points to clip space, intersects the primitives with all 6 sides of the viewing frustum and finally projects the clipped primitives into window space for rasterization.
The projection you did is actually only meaningful for points which lie inside the frustum. Two of your points lie behind the camrea, and projecting points behind the camera will create an mirrored image of these points in front of the camera.
I am trying to test gluLookAt using this code. But I can see only a black screen. What is wrong with this code ? Is there any basic concept about glulookAt (or opengl camera) that I need to understand.
glViewport(0,0,640,480);
glEnable(GL_DEPTH_TEST);
glClearColor(0,0,0,1);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluLookAt(0,0,5,0,0,0,0,0,1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBegin(GL_POLYGON);
glColor3f(1, 0, 0);
glVertex2d(0.25, 0.25);
glVertex2d(-0.25, 0.25);
glVertex2d(-0.25, -0.25);
glVertex2d(0.25, -0.25);
glEnd();
One issue is the up vector of gluLookAt is in the same direction as the look direction.
All you need to do is set +Y up and it should work...
gluLookAt(0, 0, 0.5, //position is +0.5 along Z (NOTE: 0.5, not 5. see below),
0, 0, 0, //looking at a 0.5x0.5 X/Y quad at the origin
0, 1, 0 //rotated such that +Y is up
);
The other issue is that gluLookAt shouldn't be applied to the projection matrix. It'll work for now but will break lighting later. Move it to the modelview matrix:
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(... as before ...);
Assuming the projection matrix hasn't been set, changing the from position of gluLookAt back to your 5 will make the quad disappear. This is because the default projection gives a viewing volume of an orthographic -1 to 1 cube. With the "camera" now too far away it won't see anything. This is where you'll want to investigate changing the projection matrix. Maybe increase the size of the orthographic projection with glOrtho(), or look into the more complex but natural gluPerspective().
That's problem: I'm currently in OpenGL eye-space, now I want make a black rectangle to cover all window's area. How can I determine a exactly X, Y, Z position to do this?
Update
Or can someone tell me how can I determine the X, Y (top-left) of window when we have a Z value?
You can try to calculate a rectangle that fits the camera exactly and go from there. To do that, you would need to take into account the projection matrix and calculate an inverse.
However, there is a simpler method. All you have to do is to change the camera temporarily, so that you know exactly how to draw the rectangle. A simple enough camera is the default orthogonal camera, with simple limits(from -1 to 1). The following does that:
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
glLoadIdentity();
glBegin(GL_QUAD);
glVertex2f(-1.f, -1.f);
glVertex2f(1.f, -1.f);
glVertex2f(1.f, 1.f);
glVertex2f(-1.f, 1.f);
glEnd();
glPopMatrix();
glMatrixMode(GL_PROJECTION);
glPopMatrix();
glMatrixMode(GL_MODELVIEW);
I've just started playing with OpenGl to render a number of structure each comprising a number of polygon.
Basically I want to perform the equivalent of setting a camera at (0,0,z) in the world (structure) coordinates and rotate it about the x,y and z-axes of the world axes (in that order!) to render a view of each structure (as I understand it it common practice to do use the inverse camera matrix). Thus as I understand it I need to translate (to world origin i.e. (0,0,-z)) * rotateZrotateYrotateX * translate (re-define world origin see below)
So I think I need something like:
//Called when the window is resized
void handleResize(int w, int h) {
glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(9.148, (double)w / (double)h, 800.0, 1500.0);
}
float _Zangle = 10.0f;
float _cameraAngle = 90.0f;
//Draws the 3D scene
void drawScene() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW); //Switch to the drawing perspective
glLoadIdentity(); //Reset the drawing perspective
glTranslatef(0.0f, 0.0f, -z); //Move forward Z (mm) units
glRotatef(-_Zangle, 0.0f, 0.0f, 1.0f); //Rotate "camera" about the z-axis
glRotatef(-_cameraAngle, 0.0f, 1.0f, 0.0f); //Rotate the "camera" by camera_angle about y-axis
glRotatef (90.0f,1.0f,0.0f,0.0f); // rotate "camera" by 90 degrees about x-axis
glTranslatef(-11.0f,189.0f,51.0f); //re-define origin of world coordinates to be (11,-189,-51) - applied to all polygon vertices
glPushMatrix(); //Save the transformations performed thus far
glBegin(GL_POLYGON);
glVertex3f(4.91892,-225.978,-50.0009);
glVertex3f(5.73534,-225.978,-50.0009);
glVertex3f(6.55174,-225.978,-50.0009);
glVertex3f(7.36816,-225.978,-50.0009);
.......// etc
glEnd();
glPopMatrix();
However when I compile and run this the _angle and _cameraAngle seem to be reversed i.e. _angle seems to rotate about y-axis (Vertical) of Viewport and _cameraAngle about z-axis (into plane of Viewport)? What am I doing wrong?
Thanks for taking the time to read this
The short answer is: Use gluLookAt(). This utility function creates the proper viewing matrix.
The longer answer is that each OpenGL transformation call takes the current matrix and multiplies it by a matrix built to accomplish the transformation. By calling a series of OpenGL transformation function you build one transformation matrix that will apply the combination of transformations. Effectively, the matrix will be M = M1 * M2 * M3 . . . Mathematically, the transformations are applied from right to left in the above equation.
Your code doesn't move the camera. It stays at the origin, and looks down the negative z-axis. Your transformations move everything in model space to (11,-189,-51), rotates everything 90 degrees about the x-axis, rotates everything 90 degrees about the y-axis, rotates everything 10 degrees about the z-axis, then translates everything -z along the z-axis.
EDIT: More information
I'm a little confused about what you want to accomplish, but I think you want to have elements at the origin, and have the camera look at those elements. The eye coordinates would be where you want the camera, and the center coordinates would be where you want the objects to be. I'd use a little trigonometry to calculate the position of the camera, and point it at the origin.
In this type of situation I usually keep track of camera position using longitude, latitude, and elevation centered on the origin. Calculating x,y,z for the eye coordinates is simplyx = elv * cos(lat) * sin(lon), y = elv * sin(lat), z = elv * cos(lat) * cos(lat).
My gluLookAt call would be gluLookAt(x, y, z, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
You could rotate the up on the camera by changing the last three coordinates for gluLookAt.
The z axis is coming from the center of the monitor into you. So, rotating around the z-axis should make the camera spin in place (like a 2D rotation on just the xy plane). I can't tell, but is that what's happening here?
It's possible that you are encountering Gimbal Lock. Try removing one of the rotations and see if things work the way they should.
While it's true that you can't actually move the camera in OpenGL, you can simulate camera motion by moving everything else. This is why you hear about the inverse camera matrix. Instead of moving the camera by (0, 0, 10), we can move everything in the world by (0, 0, -10). If you expand those out into matrices, you will find that they are inverses of each other.
I also noticed that, given the code presented, you don't need the glPushMatrix()/glPopMatrix() calls. Perhaps there is code that you haven't shown that requires them.
Finally, can you provide an idea of what it is you are trying to render? Debugging rotations can be hard without some context.
Short answer :Good tip
Longer answer: Yes the order of matrix multiplication is clear... that's what I meant by inverse camera matrix to indicate moving all the world coordinates of structures into the camera coordinates (hence the use of "camera" in my comments ;-)) instead of actually translating and rotating camera into the world coordinates.
So if I read between the lines correctly you suggest something like:
void drawScene() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glMatrixMode(GL_MODELVIEW); //Switch to the drawing perspective
glLoadIdentity(); //Reset the drawing perspective
gluLookAt(0.0,0.0,z,11.0,-189.0,-51.0,0.0,1.0,0.0); //eye(0,0,z) look at re-defined world origin(11,-189,-51) and up(0.0,1.0,0.0)
glRotatef(-_Zangle, 0.0f, 0.0f, 1.0f); //Rotate "camera" (actually structures) about the z-axis
glRotatef(-_cameraAngle, 0.0f, 1.0f, 0.0f); //Rotate the "camera" (actually structures!) by camera_angle about y-axis
glRotatef (90.0f,1.0f,0.0f,0.0f); // rotate "camera" (actually structures) by 90 degrees about x-axis
glPushMatrix();
Or am I still missing something?
I think you are mixing axes of your world with axes of the camera,
GLRotatef only uses axes of the camera, they are not the same as your the world axes once the camera is rotated.