I'm a fresh in cocos3d, now I have a problem.
In cocos3d, I want to rotate a node. I got the angles in x axis, y axis, z axis, then I used the property:rotation to rotate, like this:
theNodeToBeRotated.rotation = cc3v(x,y,z);
But I found out it didn't rotate as I expected, because the document said the rotate order is y-x-z.
I want to change the order to x-y-z. Can anyone let me know how?
You might need to clarify further regarding the following: "it didn't rotate as I expected"
OpenGL ES (and ergo, cocos3D) uses the y-axis as up so the rotation order is still x-y-z. If you are importing a model, you then need to take into account the 3D editor's co-ordinate system and adapt accordingly.
If you are not used to working with three-dimensional representations, the leap from 2D to 3D can be a significant hurdle. Within Cocos3D:
the x-axis is positive on the right and negative on the left
the y-axis is positive upwards and negative downwards
the z-axis is positive moving towards you and negative moving away from you
Envisage those three lines of axis, or even better, a piece of string.
If you are rotating around the x-axis, hold the string horizontally from left to right: the object would rotating towards you or away from you.
If you are rotating around the y-axis, hold the string vertically from feet to head: the object would rotate as if like a revolving door.
If you are rotating around the z-axis, hold one end close to your chest and the other end as far away as possible: the object would rotate similar to a clock face.
-- Update
I heavily wouldn't recommend changing the rotation order as it is the OpenGL standard to use Y-X-Z. If you wish to modify it, take a look at CC3GLMatrixMath and look for kmMat4RotationYXZ - there is also kmMat4RotationZYX. If you want to have X-Y-Z, you would need to construct your own rotation matrix and update accordingly in CC3GLMatrix and CC3GLMatrixMath.
As a reference, you also have the OpenGL Red book - it should have some suggestions for you.
Related
When setting the OpenGLES draw distance using Matrix.frustumM I notice that you can’t set the near draw distance to zero and any value less than 1 gives really weird distortion. Setting the near distance to 1 works fine most of the time but when the camera moves closer to objects than this distance it looks horrible because they are not drawn (or a portion of them is not drawn). Is there anything that can be done about this?
Many thanks for your time.
Not much can be done actually. The near and far clipping planes clip the pixels closer to near or further then far. Beside this the near is a bit special as it defines your field of view with the combination of the border parameters (left, right, up and down). So if you had a quad with same coordinates as those border if would be full-screen when exactly near away. Because of this the near plane can not be zero or even negative as for instance an object that would be at zero units away using a frustum would appear to be scaled infinitely.
Still you can use values smaller then 1 without having some strange artifacts. What you should do is look at some examples on how to define the frustum by setting a field of view. Generally you define your angle (a field of view) for one of the dimensions like 45 degrees in width, then you define your near and far as you please but both should be positive. Now use the trigonometry to compute the left and right using the angle and near and use the same values for up and down but scaled by screen (view) ratio. By doing so you will have no difference as in distortion when changing the near parameter.
When several objects overlap on the same plane, they start to flicker. How do I tell the renderer to put one of the objects in front?
I tried to use .renderDepth, but it only works partly -
see example here: http://liveweave.com/ahTdFQ
Both boxes have the same size and it works as intended. I can change which of the boxes is visible by setting .renderDepth. But if one of the boxes is a bit smaller (say 40,50,50) the contacting layers are flickering and the render depth doesn't work anymore.
How to fix that issue?
When .renderDepth() doesn't work, you have to set the depths yourself.
Moving whole meshes around is indeed not really efficient.
What you are looking for are offsets bound to materials:
material.polygonOffset = true;
material.polygonOffsetFactor = -0.1;
should solve your issue. See update here: http://liveweave.com/syC0L4
Use negative factors to display and positive factors to hide.
Try for starters to reduce the far range on your camera. Try with 1000. Generally speaking, you shouldn't be having overlapping faces in your 3d scene, unless they are treated in a VERY specific way (look up the term 'decal textures'/'decals'). So basically, you have to create depth offsets, and perhaps even pre sort the objects when doing this, which all requires pretty low-level tinkering.
If the far range reduction helps, then you're experiencing a lack of precision (depending on the device). Also look up 'z fighting'
UPDATE
Don't overlap planes.
How do I tell the renderer to put one of the objects in front?
You put one object in front of the other :)
For example if you have a camera at 0,0,0 looking at an object at 0,0,10, if you want another object to be behind the first object put it at 0,0,11 it should work.
UPDATE2
What is z-buffering:
http://en.wikipedia.org/wiki/Z-buffering
http://msdn.microsoft.com/en-us/library/bb976071.aspx
Take note of "floating point in range of 0.0 - 1.0".
What is z-fighting:
http://en.wikipedia.org/wiki/Z-fighting
...have similar values in the z-buffer. It is particularly prevalent with
coplanar polygons, where two faces occupy essentially the same space,
with neither in front. Affected pixels are rendered with fragments
from one polygon or the other arbitrarily, in a manner determined by
the precision of the z-buffer.
"The renderer cannot reposition anything."
I think that this is completely untrue. The renderer can reposition everything, and probably does if it's not shadertoy, or some video filter or something. Every time you move your camera the renderer repositions everything (the camera is actually the only thing that DOES NOT MOVE).
It seems that you are missing some crucial concepts here, i'd start with this:
http://www.opengl-tutorial.org/beginners-tutorials/tutorial-3-matrices/
About the depth offset mentioned:
How this would work, say you want to draw a decal on a surface. You can 'draw' another mesh on this surface - by say, projecting a quad onto it. You want to draw a bullet hole over a concrete wall and end up with two coplanar surfaces - the wall, the bullet hole. You can figure out the depth buffer precision, find the smallest value, and then move the bullet hole mesh by that value towards the camera. The object does not get scaled (you're doing this in NDC which you can visualize as a cube and moving planes back and forth in the smallest possible increment), but does translate in depth direction, ending up in front of the other.
I don't see any flicker. The cube movement in 3D seems to be super-smooth. Can you try in a different computer (may be faster one)? I used Chrome on Macbook Pro.
Currently, I'm taking each corner of my object's bounding box and converting it to Normalized Device Coordinates (NDC) and I keep track of the maximum and minimum NDC. I then calculate the middle of the NDC, find it in the world and have my camera look at it.
<Determine max and minimum NDCs>
centerX = (maxX + minX) / 2;
centerY = (maxY + minY) / 2;
point.set(centerX, centerY, 0);
projector.unprojectVector(point, camera);
direction = point.sub(camera.position).normalize();
point = camera.position.clone().add(direction.multiplyScalar(distance));
camera.lookAt(point);
camera.updateMatrixWorld();
This is an approximate method correct? I have seen it suggested in a few places. I ask because every time I center my object the min and max NDCs should be equal when their are calculated again (before any other change is made) but they are not. I get close but not equal numbers (ignoring the negative sign) and as I step closer and closer the 'error' between the numbers grows bigger and bigger. IE the error for the first few centers are: 0.0022566539084770687, 0.00541687811360958, 0.011035676399427596, 0.025670088917273515, 0.06396864345885889, and so on.
Is there a step I'm missing that would cause this?
I'm using this code as part of a while loop to maximize and center the object on screen. (I'm programing it so that the user can enter a heading an elevation and the camera will be positioned so that it's viewing the object at that heading and elevation. After a few weeks I've determined that (for now) it's easier to do it this way.)
However, this seems to start falling apart the closer I move the camera to my object. For example, after a few iterations my max X NDC is 0.9989318709122867 and my min X NDC is -0.9552042384799428. When I look at the calculated point though, I look too far right and on my next iteration my max X NDC is 0.9420058636660581 and my min X NDC is 1.0128126740876888.
Your approach to this problem is incorrect. Rather than thinking about this in terms of screen coordinates, think about it terms of the scene.
You need to work out how much the camera needs to move so that a ray from it hits the centre of the object. Imagine you are standing in a field and opposite you are two people Alex and Burt, Burt is standing 2 meters to the right of Alex. You are currently looking directly at Alex but want to look at Burt without turning. If you know the distance and direction between them, 2 meters and to the right. You merely need to move that distance and direction, i.e. right and 2 meters.
In a mathematical context you need to do the following:
Get the centre of the object you are focusing on in 3d space, and then project a plane parallel to your camera, i.e. a tangent to the direction the camera is facing, which sits on that point.
Next from your camera raycast to the plane in the direction the camera is facing, the resultant difference between the centre point of the object and the point you hit the plane from the camera is the amount you need to move the camera. This should work irrespective of the direction or position of the camera and object.
You are playing the what came first problem. The chicken or the egg. Every time you change the camera attributes you are effectively changing where your object is projected in NDC space. So even though you think you are getting close, you will never get there.
Look at the problem from a different angle. Place your camera somewhere and try to make it as canonical as possible (ie give it a 1 aspect ratio) and place your object around the cameras z-axis. Is this not possible?
I have a situation that I'm not realy sure how I can handle. I have an openGl object of about 20k vertices, and I need to offer the user the possibility to select any one of these vertices (let's say with a smallest margin of error possible). Now here is what I want to do in order to do this:
Next to the 3D canvas of the object, I also offer the user 3 'slices' done by the planes x=0; y=0 and z=0. Say for the simplest example for a sphere these would be 3 circles, correponding to 'cutting' out one of the dimensions. Now let's take the z=0 one for the purpose of the example. When the user clicks on a point say (x_circle, y_circle) i would like to get the actual point in the 3d representation where he clicked. The z would be 0 of course but I can't figure out a way to get the x and y. I can easily translate that (x_circle, y_circle) -> (x_screen, y_screen) which would have the same result as a click on the canvas at those coordinates, but I need to find a way to translate that into the (x, y, 0) coordinate in 3D view.
The same thing would need to be done with x=0, y=0 but I think if I can understand/implement a way for z=0 I can just apply more or less the same solution with an added rotation over something. If anyone can help with any examples/code or even math behind this it would help a lot because at the moment I'm not really sure how to proceed.
When the user clicks, you can render the vertices using GL.POINTS (with a certain size, if you like) to an off-screen buffer using a shader that renders each vertex' index into RGBA. Then you read back the pixel on the mouse position and see what index it is.
Given the 3D vector of the direction that the camera is facing and the orientation/direction vector of a 3D object in the 3D space, how can I calculate the 2-dimensional slope that the mouse pointer must follow on the screen in order to visually be moving along the direction of said object?
Basically I'd like to be able to click on an arrow and make it move back and forth by dragging it, but only if the mouse pointer drags (roughly) along the length of the arrow, i.e. in the direction that it's pointing to.
thank you
I'm not sure I 100% understand your question. Would you mind posting a diagram?
You might find these of interest. I answered previous questions to calculate a local X Y Z axis given a camera direction (look at) vector, and also a question to translate an object in a plane parallel to the camera.
Both of these examples use Vector dot product, Vector cross product to compute the required vectors. In your example the vector dot product can be also used to output the angle between two vectors once you have found them.
It depends to an extent on the transformation that you are using to convert your 3d real world coordinates to 2d screen coordinates, e.g. perspective, isometric, etc... You will typically have a forward (3d -> 2d) and backward (2d -> 3d) transformation in play, where the backward transformation loses information. (i.e. going forward each 3d point will map to a unique 2d point, but going back from the point may not yield the same 3d point). You can often project the mouse point onto the object to get the missing dimension.
For mouse dragging, you typically get the user to specify an operation (translation on the plane of projection, zooming in or out, or rotating about an anchor point). Your input is the mouse coordinate at the start and end of the drag, which you transform into your 3d coordinate system to get two 3d coordinates, which will give you dx, dy, dz for dragging / translation etc...