first thing I want to do is translating to the center of the screen and draw all of the objects from there.
then I would like to apply tranlsate for panning and scale for zoom. I want to zoom relative to a center point ! so how should be the order of them so that it works ?
glTranslatef(width/2, height/2, 0);
gltranslate(centerX,centerY); // go to center point
glscale(zoom);
glTranslatef(offset.x/zoom, offset.y/zoom, offset.z/zoom); // pan
I tried the above order but it doesn't go to the center point and it always zoom relative to (0,0).
I suppose you are drawing a square with both x and y between 0,1.
first you have to translate to the point the scaled object should be:
glTranslate3f(centerX,centerY,0);
glScale(zoom);
glTranslatef(-0.5f, -0.5f,0); // to the middle
draw stuff
opengl executes the transformations in reverse order since it's a pipeline.
reading the above sequence in the bottom-up direction will give the key.
Related
My goal is to create an intuitive 3D manipulator to handle rotations of meshes displayed in my 3D editor, made with Qt / QML.
To do that, when the user clicks on an entity, 3 tori are spawned around the mesh, representing the euler angles the user can act on. If the user then clicks on one torus, I want him to be able to rotate the mesh by dragging the mouse. The natural way users seem to do that is by dragging the mouse around the torus in the direction they want the mesh to rotate.
I therefore need a way to know how the user is rotating his mouse. I thought of a way: when the user clicks on the torus, I retrieve the position of the center of the torus. Then, I translate this world position to its screen position. Then, I monitor the angle between the cursor of the mouse and the center of the torus. The evolution of this angle should tell me everything I need: if the angle increases clockwise, the mesh should rotate clockwise and vice versa. This solution should yield a result good enough for my application, since it won't depend on the angle of the camera, or only very minimally.
However, I can't find a way to translate a world position to its screen position with Qt. I found the function QVector3D::project(const QMatrix4x4 &modelView, const QMatrix4x4 &projection, const QRect &viewport), but its documentation is very scarce and I couldn't find anyone using it... I might have found what to feed in for the projection argument (the projectionMatrix property from QCamera, here https://doc.qt.io/qt-5/qml-qt3d-render-camera.html), but that's it. What is the modelView ? And viewport ? Is it simply QRect(0, 0, 1920, 1080) ?
If anyone have any kind of lead, it would be amazing, I can't find anything anywhere and I'm kind of losing hope now. Or maybe another, simpler, solution to my problem ? Please note that the user can also freely move the camera around the mesh, which adds in complexity.
Thanks a lot for your time, and have a nice day !
Yes, you should be able to translate from world position to screen position using the mentioned function. You are correct about the projection argument. As for the modelView argument, you should use viewMatrix property from QCamera, which is missing from the official documentation, but it works for me. The viewport parameter represents the dimensions of the part of the screen, you are projecting on. You could use QRect(0, 0, 1920, 1080) if you use full screen Full HD projection, otherwise use something like QRect(QPoint(0, 0), view->size()), where view is the wigdet or window with your 3D image. Be careful, that the resulting screen position will have y = 0 being down and positive values being above, which the opposite to usual screen coordinates.
just wondering how someone would go about dragging 4 different
objects in openGL. I have very simple code to draw these objects:
glPushMatrix();
glTranslatef(mouse_x, mouse_y, 0);
glutSolidIcosahedron();
glPopMatrix();
glPushMatrix();
glTranslatef(mouse_x2, mouse_y2, 0);
glutSolidIcosahedron();
glPopMatrix();
glPushMatrix();
glTranslatef(mouse_x3, mouse_y3, 0);
glutSolidIcosahedron();
glPopMatrix();
glPushMatrix();
glTranslatef(mouse_x4, mouse_y4, 0);
glutSolidIcosahedron();
glPopMatrix();
I understand how to move an object, but I want to learn how to drag and drop any one of these objects.
I've been researching about the Name Stack and Selection Mode, but it just confused the hell out of me.
And I also know about having to have some sort of glutMouseFunc.
It's just the selection of each shape I'm puzzled on.
First thing that you need to do is capturing the position of mouse on the screen when the button is clicked. There are plenty of ways to do it but I believe it's outside of the scope of this question. When you have screen X,Y coords you must detect if any object is selected and which one it is. There are two possible approaches. You can either keep track of a bounding rectangle positions of each object (in screen space) and the test if the cursor is inside any of those rectangles will be quite simple. Or you can cast a ray from eye through cursor position in world space and check intersection of this ray with each object.
The second approach is more versatile for 3D graphics but you seem to be using only X and Y coords so you don't need to worry about Z order of objects.
In case of the first solution the main problem is: how to know how big is your object on the screen. glutSolidIcosahedron() renders an object of radius 1. To calculate it's screen radius you can either use some matrix math or in that case a simple trigonometry. You will need to know the distance from camera to the drawing plane (I believe you're using some glTranslatef(0,0,X) before you render. X is your distance) You also need to know the view angle of the camera. You set it in projection matrix. Now take a piece of paper, draw a cone of angle alpha, intersecting a plane at distance X and knowing that an object has radius 1 you can easily calculate how large area of the screen it occupies. (I'll leave this calculation for you)
Now if you know the radius on screen, simply test the distance from your click position to each object. if the distance is below radius it's selected. If more than one object passes this test just select first one of them.
I am working on my first real OpenGL Project. It is a 3x3x3 Rubiks Cube. Here is a link to a simple screenshot of what i have so far(my rubiks cube)
Rotating the cube is done with dragging the mouse while holding the right mouse button. This works using the example of a arcball from NeHe Tutorials(NeHe Arcball)
I have the class singleCubes which represents one cube via 6 actual quads, stored in a display list that can be used in it´s draw method.
Class ComplexCube has an array of 3x3x3 singleCubes and is used as interface when interacting with the complete rubiks cube.
Now i want to rotate each specific face according to the mousedragging with left mouse button down. I use picking to get the id of the corresponding side of the single cube the user clicked on. This works also. So i click on a side of one cube on a face and depending on the direction of the dragging i set a rotation and offset factor of the cubes that get affected. (i also want to implement that u actually see the face rotate instead of just changing the color)
Now my Problem is that when i rotate the rubiks cube in any direction with right mouse dragging, it becomes upside down for example. So when i click on a side and want to rotate the face to the right, it´s going the wrong direction because i can´t keep track if the cube is upside down or whatever. Due to the use of the arcball rotation i dont have a x- or y-rotation angle which i could use to determine this.
Question 1: How can i keep track or later on get the information if the cube is upside down, tilted etc in order to translate the mouse dragging information(when rotating one face) when using the arcball example linked above?
// In render function
glPushMatrix();
{
glMultMatrixf(Transform.M); // Rotation applied by arcball object
complCube.draw(); // Draw all the cubes using display lists
}
glPopMatrix();
Setup: C++ with Microsoft Visual Studio 2008, GLEW, freeglut
You could use gluUnProject to convert mouse coordinates to 3d space and get a vector (difference between two points). This vector could then be used to apply a "force" to the selected face. Since gluUnProject uses the projection matrix, it would automatically deal with the orientation of the camera.
Basically, once you get your "force" vector, you project it onto the three axes (so onto (1,0,0), (0,1,0), (0,0,1)). Then choose the one with the largest magnitude. Then you have to convert this direction into a rotation axis as in the diagram below (sorry for the bad paint skills):
So what we have is the "force" vector in black and the selected rubiks face in grey. To get the rotation axis, just take the cross product the "force" vector with the normal of the selected face. This gives the red arrow. From that, you should be able to rotate your cubes in the right direction.
Edit to answer the question in more detail
So continuing from my explanation, I will give an example of how this will help you. Let's first assume your screen is 800x800 pixels and your rubiks cube is always centred. Now lets also assume that, as per your drawings in the comments, that we are in the case on the left.
We drag the mouse and get two positions which using gluUnProject are transformed into world coordinates (the numbers were chosen to show my point, not by any calculation):
p1 : (600, 600) -> (1, -0.5, 0)
p2 : (630, 605) -> (1.3, -0.505, 0)
Now we get the difference vector: p2 - p1 = v = (0.3, -0.05, 0). The reason that I was saying to "project onto the three axes" is so that you extract your major movement (which in this case is 0.3 in the x axis) (since the rubiks cube can't rotate along diagonals). To do the "projection" you just have to take the x, y, z axes individually and create vectors from them so you wind up with:
v1 = (0.3, 0, 0)
v2 = (0, -0.05, 0)
v3 = (0, 0, 0)
Now take the magnitudes and discard the smallest vectors, so we are left with the vector v1 = (0.3, 0, 0). This is your movement vector in world space. Now you take the cross product of that vector, with the normal vector of the selected face (which in this case would be (0, 0, 1)). This gives you a vector which points down (0, 1, 0) (after normalization) (in this step you will probably also have to extract the largest component only (0.02, 1.2, 0.8) -> (0, 1, 0) otherwise you would get bizarre rotations if your camera was not pointing directly along the main axes). You can now use that vector as the rotation axis and use 0.3 as your rotation amount (if it rotates in the opposite direciton to that expected, just put a -).
Now how does this help if your cube is upside down? Suppose we click on the screen in the same way. We now get:
p1 : (600, 600) -> (-1, 0.5, 0)
p2 : (630, 605) -> (-1.3, 0.505, 0)
See the difference in the world coordinates? They are inverted! So when you take the difference vector p2 - p1 = v = (-0.3, 0.05, 0). Extracting the largest component vector gives (-0.3, 0, 0). Doing the cross product once again gives you the rotation axis, but now the rotation is in the opposite direction, which is what you want.
Another reason for the cross product with the normal of the face is that if you were to select the faces on the top (in our drawings), then it would either give a rotation axis along the x or z axes (to the left, or into the screen) which is what you want for the top faces.
Like most of us, you will encounter the famous problem called Gimbal Lock.
see: http://www.opengl.org/discussion_boards/ubbthreads.php?ubb=showflat&Number=208925
This problem is extremely well documented so there is not much point for me to go into details here. I am sure you will find a ton of information about it.
I have an OpenGL scene with a top left coordinate system. When I glScale it zooms in from (0,0) the top left. I want it to zoom in from the mouse's coordinate (relative to the OGL frame). How is this done?
Thanks
I believe this can be done in four steps:
Find the mouse's x and y coordinates using whatever function your windowing system (i.e. GLUT or SDL) has for that, and use gluUnProject to get the object coordinates that correspond to those window coordinates
Translate by (x,y,0) to put the origin at those coordinates
Scale by your desired vector (i,j,k)
Translate by (-x,-y,0) to put the origin back at the top left
I did a smooth zoom in using glortho . The skeleton of my solution is
glortho(initial viewport x,y & size)
glcalllist(my display list)
render
.
.
loop to gradually go to final viewrport coordinates/size . Implement your timing and FPS requirements
.
.
glortho(final viewport x,y & size)
glcalllist(my display list)
render
I hope you get the general idea. There are few other methods to acheive this, but I find glortho the method the easiest to comprehend.
Using OpenGL I'm attempting to draw a primitive map of my campus.
Can anyone explain to me how panning, zooming and rotating is usually implemented?
For example, with panning and zooming, is that simply me adjusting my viewport? So I plot and draw all my lines that compose my map, and then as the user clicks and drags it adjusts my viewport?
For panning, does it shift the x/y values of my viewport and for zooming does it increase/decrease my viewport by some amount? What about for rotation?
For rotation, do I have to do affine transforms for each polyline that represents my campus map? Won't this be expensive to do on the fly on a decent sized map?
Or, is the viewport left the same and panning/zooming/rotation is done in some otherway?
For example, if you go to this link you'll see him describe panning and zooming exactly how I have above, by modifying the viewport.
Is this not correct?
They're achieved by applying a series of glTranslate, glRotate commands (that represent camera position and orientation) before drawing the scene. (technically, you're rotating the whole scene!)
There are utility functions like gluLookAt which sorta abstract some details about this.
To simplyify things, assume you have two vectors representing your camera: position and direction.
gluLookAt takes the position, destination, and up vector.
If you implement a vector class, destinaion = position + direction should give you a destination point.
Again to make things simple, you can assume the up vector to always be (0,1,0)
Then, before rendering anything in your scene, load the identity matrix and call gluLookAt
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt( source.x, source.y, source.z, destination.x, destination.y, destination.z, 0, 1, 0 );
Then start drawing your objects
You can let the user span by changing the position slightly to the right or to the left. Rotation is a bit more complicated as you have to rotate the direction vector. Assuming that what you're rotating is the camera, not some object in the scene.
One problem is, if you only have a direction vector "forward" how do you move it? where is the right and left?
My approach in this case is to just take the cross product of "direction" and (0,1,0).
Now you can move the camera to the left and to the right using something like:
position = position + right * amount; //amount < 0 moves to the left
You can move forward using the "direction vector", but IMO it's better to restrict movement to a horizontal plane, so get the forward vector the same way we got the right vector:
forward = cross( up, right )
To be honest, this is somewhat of a hackish approach.
The proper approach is to use a more "sophisticated" data structure to represent the "orientation" of the camera, not just the forward direction. However, since you're just starting out, it's good to take things one step at a time.
All of these "actions" can be achieved using model-view matrix transformation functions. You should read about glTranslatef (panning), glScalef (zoom), glRotatef (rotation). You also should need to read some basic tutorial about OpenGL, you might find this link useful.
Generally there are three steps that are applied whenever you reference any point in 3d space within opengl.
Given a Local point
Local -> World Transform
World -> Camera Transform
Camera -> Screen Transform (usually a projection. depends on if you're using perspective or orthogonal)
Each of these transforms is taking your 3d point, and multiplying by a matrix.
When you are rotating the camera, it is generally changing the world -> camera transform by multiplying the transform matrix by your rotation/pan/zoom affine transformation. Since all of your points are re-rendered each frame, the new matrix gets applied to your points, and it gives the appearance of a rotation.