I have the following code:
glNormal3f(0, 0, 1);
glColor3f(1, 0, 0);
glBegin(GL_POINTS);
glVertex3f(-45, 75, -5);
glVertex3f(-45, 90, -5);
glVertex3f(-30, 90, -5);
glVertex3f(-30, 80, -5);
glVertex3f(-35, 80, -5);
glVertex3f(-35, 75, -5);
glVertex3f(-45, 75, -5);
glEnd();
glColor3f(1, 1, 0);
glBegin(GL_POLYGON);
glVertex3f(-45, 75, -5);
glVertex3f(-45, 90, -5);
glVertex3f(-30, 90, -5);
glVertex3f(-30, 80, -5);
glVertex3f(-35, 80, -5);
glVertex3f(-35, 75, -5);
glVertex3f(-45, 75, -5);
glEnd();
Notice how the code between glBegin and glEnd in each instance is identical.
But the vertices of the GL_POLYGON (yellow) don't match up with the GL_POINTS (red).
Here is a screenshot:
The more I use openGL the more I'm hating it. But I guess it's probably something I'm doing wrong... What is up?
That's because your polygon is not convex - the top right corner is concave. GL_POLYGON only works for convex polygons.
Try using GL_TRIANGLE_FAN instead and start from the lower-left corner: Your polygon is star-shaped, so you can draw it with a single GL_TRIANGLE_FAN if you start from a point in its kernel (of your vertices, the lower-left one is the only one that satisfies this condition).
If you expect more complicated polygons, you need to break them up into convex bits (triangles would be best) to render them.
The specification says for GL_POLYGON:
Only convex polygons are guaranteed to be drawn correctly by the GL. If a
specied polygon is nonconvex when
projected onto the window, then the
rendered polygon need only lie within
the convex hull of the projected
vertices dening its boundary.
Since you are defining a concave polygon, this is a valid behaviour.
Try using a triangle strip instead of a polygon. It would be much faster because a polygon needs to be triangulated by the GL.
BTW: I haven't used GL_POLYGON so far. But I think you do not need to specify the last vertex (which equals the first one). As far as I know, it will be connected automatically.
The GLU Tesselator is a handy way to automatically convert concave (and other complex) polygons into proper opengl friendly ones. Check it out:
http://glprogramming.com/red/chapter11.html
http://www.songho.ca/opengl/gl_tessellation.html
Related
I'm attempting to draw an eclipse-shaped object so I used a sphere and scaled it as necessary. I'm attempting to rotate this eclipse-shaped object with the following code but it won't budge (or at least it doesn't look like it's budging). I've tested by simply changing to another 3D shape and it rotates. Is there something I'm doing wrong? What's so different about a glutSolidSphere?
glPushMatrix();
glTranslatef(subx, suby + y, subz);
glScalef(9.0, 1.75, 1.75);
glRotatef(angle, 0, 1, 0);
glTranslatef(-subx, -suby, -subz);
glTranslatef(subx, suby, subz);
glutSolidSphere(1.0, 50, 50);
glPopMatrix();
Issue as pointed out above in comments was that I was rotating the sphere before I stretched it. Scaling and then rotating fixed the issue.
glPushMatrix();
glTranslatef(subx, suby + y, subz);
glRotatef(angle, 0, 1, 0);
glScalef(9.0, 1.75, 1.75);
glTranslatef(-subx, -suby, -subz);
glTranslatef(subx, suby, subz);
glutSolidSphere(1.0, 50, 50);
glPopMatrix();
I have the code for a room provided to me, I need to add a sphere in the room. It goes like this:
PushMatrix();
//draw floor,walls,ceilings
PushMatrix();
//draw some boxes on front wall
PopMatrix();
PushMatrix();
//drawing sphere;
glLoadIdentity();
glColor3f(1, 0, 0);
glTranslatef(0, ypos, 0);
glutSolidSphere(2, 20, 20);
PopMatrix();
PopMatrix();
But all the walls etc turn red (and no sphere) when I do this. Why does that happen even after pushing another matrix?
Calling glColor*() sets the current color. This color doesn't change before you call glColor*() again. In other words glPushMatrix() and glPopMatrix() has no effect on the current color.
Thus if you in //draw some boxes on front wall don't call glColor*(). Then due to you setting the color to red by calling glColor3f(1, 0, 0) then everything is going to be red from that point on.
Considering:
glMatrixMode(GL_PROJECTION);
gluPerspective(40, 1, 1, 40);
glMatrixMode(GL_MODELVIEW);
Then:
glPushMatrix();
glTranslatef(0, 0, -10);
glColor3f(1, 0, 0);
glutSolidSphere(2, 20, 20);
glPopMatrix();
By doing the above you should see a sphere.
Thus if you before didn't see the sphere at all. Then it's probably due to ypos being outside your view. If you were referring to the whole screen being red, then I'm assuming ypos to be around 0, which means that it would be filling the whole screen (Again assuming you don't translate the view in any other way).
I am rather new to programming so I may not use the correct terminology. I am trying to create a dog out of only glutwirecubes, however I cannot figure out how to define a starting position for the back legs, nor can I figure out how to close the space between my 'shoulder' and 'elbows'. I have each body part assigned to rotate by key press. I also realize that my use of glPushMatrix and glPopMatrix may not be correct as I do not fully understand how the matrix stack is saved/loaded.
glPushMatrix();
glTranslatef(-1, 0, 0);
glRotatef((GLfloat)body,1, 0, 0);//sets rotations about x,y,z axis
glTranslatef(1, 0, 0);
glPushMatrix();
glScalef(2.0, 0.4, 0.5);//sets dimensions of cube
glutWireCube(2.0);//sets scale of wire cube
glPopMatrix();
glPopMatrix();
glPushMatrix();
glTranslatef(-1, 0, 0);
glRotatef((GLfloat)shoulder, 0, 0, 1);//sets rotations about x,y,z axis
glTranslatef(1, 0, 0);
glPushMatrix();
glScalef(1.5, 0.4, 0.5);//sets dimensions of cube
glutWireCube(.75);//sets scale of wire cube
glPopMatrix();
glTranslatef(1,0,0);
glRotatef((GLfloat)elbow,0,0,1);//sets rotations about x,y,z axis
glTranslatef(1,0,0);
glPushMatrix();
glScalef(1.5,0.4,0.5);//sets dimensions of cube
glutWireCube(.75);//sets scale of wire cube
glPopMatrix();
glPopMatrix();
glutSwapBuffers();
The picture is the position I am aiming for, however my cubes always start oriented horizontally.
with glTranslate right before, as for its friends !
For instance you might translate so that the center is now at the corner, so that your rotations rotate around this new handle.
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 import .obj files but cant even get a triangle rendered. Here is what I'm trying to do just to test if it can show something.
glBegin(GL_TRIANGLES);
glColor3f(0.5, 0.5, 0.5);
glVertex3f(0, 0, 0);
glVertex3f(1, 1, 1);
glVertex3f(2, 2, 2);
glEnd();
This doesn't display anything. However when I change GL_TRIANGLES for GL_LINES it displays the line.
How can I get the triangle to display?
Language: C++
OS: Mac 10.7.3
A triangle where all three points are colinear (in the same line) has zero area. And a triangle renderer only renders the area of a triangle, not the edges of it. So it renders zero pixels.