I draw glutSolidSphere and then I cut it using glClipPlane.
The cut sphere looks empty inside, i.e. reflection comes from inner spherical surface.
I want a planar cross section of my sphere look like a solid disc, not a hole to an empty sphere.
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Simply put - I want to draw a ray/line from the near clipping plane out to the far clipping plane using a perspective projection. I have what I believe are correctly normalized world coordinates generated from a mouse click using methods describe in various OpenGL/graphics programming guides.
The problem I am having is that it seems my ray is being drawn from outside the near clipping plane.
Background: This is for a simple model viewer I am building in Qt that requires a picking capability. I need to draw the ray in order to calculate intersections with objects in the scene. However, my basic problem is that I can seem to draw the ray correctly.
My perspective projection is defined:
gluPerspective(_fov, aspect, 0.1, 100.0);
where _fov is 45.0 degress, and aspect is the ratio of the window width/height.
Using my picking code, I've generated what I believe to be correctly normalized world coordinates based off of mouse clicks. An example of these coordinates:
-0.385753,-0.019608,-0.100000
However, when I try to draw a ray starting at that point, it looks like it is being drawn from outside of the clipping plane:
Maybe I am expecting something different, but in the example above I clicked on the nose of the airplane, generated the world coordinates above, and I am drawing the ray incorrectly (or so I believe). I was hoping to see the line being drawn from the location of the mouse click into the airplane model.
When I draw the ray I first load the identity matrix, and then draw a line from the near clipping plane coordinates to the far plane. Then I draw a sphere at the end of the ray (in this screenshot it is behind the plane).
glPushMatrix();
glLoadIdentity();
glColor3f(0,0,1);
glBegin(GL_LINES);
glVertex3f(_near_ray.x(), _near_ray.y(), _near_ray.z());
glVertex3f(_far_ray.x(), _far_ray.y(), _far_ray.z());
glEnd();
glTranslatef(_far_ray.x(), _far_ray.y(), _far_ray.z());
glColor3f(1,0,0);
glutWireSphere(1, 10, 10);
glPopMatrix();
Any hints as to what I am doing wrong? The _far_ray coordinates are the same as the _near_ray except for the Z field. I want the ray to be drawn straight into the scene.
In The End... I'd just like to know how the draw the ray itself. I understand that there might be errors in my code that generates the coordinates, but what if I just wanted to draw an arbitrary ray from the near clipping plane straight into the scene. That is that I'd like answered.
With perspective projection, a line looks like a point on the screen if and only if it passes through the eye position.
Since you revert modelview matrix to identity, the eye is located at the origin (according to this question). Pass (0, 0, 0) as one of the vertices, and hopefully you'll see that line degenerates into a point.
Generally, the two 3d vectors used as vertices must be collinear.
If you do not revert modelview matrix, then you can draw a line from (eye) to (eye + dir), where eye is the first vector passed to gluLookAt, and dir is the any sufficiently large vector looking into proper direction.
I have been wondering about how to implement this with openGL:
I have a map, with a flat floor and walls. Every thing here is 2d, there is no 3d geometry, only 2d poligons that compose the map.
Using the vertex of the polygons I cast shadows, to define the viewable area.
The shadows define the field of view, but since the cells with walls obstructi view, they are also darkened. I can draw the walls on top of the shadows, but doing so would show even walls outside the field of view.
I have been suggested to approach this problem with shadow mapping. I should render the 2D scene into 4 different 1D textures that hold the depth of the distance to the first colliding surface.
The problem is that I have no idea how to render the projection of the 2d scene into the 1D texture. If I use, for example:
gluLookAt (x, y, 0.0, 0.0, x , y+1, 0.0, 0.0, 1.0);
To render the top view, the result is still 2D. Also, nothing would be rendered since all the vertex will be at the same plane, so all surfaces will be ortogonal to the camera.
Do you have any tip or idea of how to do these 2D-to-1D projections? I have been googling for scenarios like this one, but all of them are in 3D enviroments.
Shadow mapping assumes either a directional light, or a spotlight, and you have a point light. But since you only need shadow on the floor, you could model it as a spotlight that hovers e.g. 2 m above the floor and points downwards. All the walls would have to be at least 2 m high. In the first shadow mapping pass, you could render the floor and all the walls into the shadow buffer.
However, I would not go with shadow mapping, but use volumetric shadows instead. If you go from 3D to 2D, a 3D volume becomes a 2D polygon.
Assuming that all the walls are on a regular grid, we can compute view rays from the player's position P to all the corners of walls. For each corner, store the adjacent walls, and ignore all the walls that face away from the player. Then cast rays from P to each corner, convert the rays to polar coordinates, and sort them by their angle, say counter-clockwise. Now go through this sorted list in a sweeping motion, and build the shadow polygon.
The shadow polygon consists either of corner points in this list, or of intersections between a) a line that is parallel to a wall and b) a line that goes through P and a corner. The only thing that makes this a bit complicated is that you have to find the wall that receives the shadow. Since the input is so small, I would probably start with brute force (check the corner against each wall), and see how slow it is. Note that only player-facing walls can cast shadows. Also note that the point closest to the player doesn't need to be visible.
It's probably going to look really cool with a moving character.
I'm working with OpenGL, I need to draw a plane in front of a triangle in the three dimensional space. So if one of the triangle points changes, the plane also changes
I have the 3 points, and using cross product, I can get the normal vector, so, to draw the plane, I only need to translate the triangle to the origin of the world in reference of one of the triangle points, translate a distance over the normal, rotate the normal angles in X, Y and Z, and draw the plane.
I need to know how to translate over the normal, and how to rotate the new plane, so, when one of the vertex changes, the normal changes, and the plane also changes.
As I understand, I can use the normal vector in glRotatef(angle, normal[x, y, z]), with angle =0. But the plane doesn't change when I change one of the triangle vertex.
OpenGl is not a scene graph. It will not deal with transforming objects for you. All OpenGL does is render what you tell it to render.
If you tell it to render a vertex (which YOU changed), and do not tell it to change the way it draws the plane, then of course the plane will not change.
Look into scene graphs, and how to do matrix and vector math. A simple scene graph is relatively easy to create.
I am trying to draw a very simple, black and white stick figure whose head is a sphere. The background is black and I am drawing with white color. The orthographic projection of the sphere head should be a white circle with line width of 3 pixels. Everything behind the head should be hidden (DEPTH_TEST enabled). I would like to avoid using glut but glu is OK.
What is the simplest way to achive this?
Not using a sphere. A sphere will be composed of several triangles, and if drawn using a line mode you'll end up with a wireframe sphere. Silouhette Shaders are overkill here. The simplemost solution is drawing a line circle, that's oriented towards the camera (=origin), i.e. a billboard effect.
If you really want to use a sphere and orthographic projection I recommend looking at gluSphere.
An alternative would be gluDisk (which already is 2D).
However, for pure 2D graphics (as it sounds) I recommend that you think about using point-sprites for the circles: One Quad with the circle as texture
http://nehe.gamedev.net/data/articles/article.asp?article=19
I need to draw a circle on some arbitrary non plane surface, but this circle should lay on surface and follow surface's irregular form. In other words ( that is actuially can be one of possible solutions) want to have a "shadow" like projection on non plane surface near the mouse pointer. Do I need to create in memory a sphere and project it on the surface ? Are there some other techniques to achieve the same goal?
Thank you in advance.
There are two ways to do this. First would be, to create a cylinder and intersect it with the surface, get the intersection segments, and draw them. If you already have a math library which you can leverage, and if you don't have to do intersections every frame, then this might be a good idea. You will get accurately what you want.
The other option, as you already suggested would be to use a projection. I am not sure though that, you will be able to see clearly the shadow of a circle on a surface. If however, you have parametric texture coordinates for your surface, you can create a texture with the circle imprinted on it, and apply this texture to the surface.