Sign in to follow this  
UtkarshGaur

OpenGL Help Regarding Matrix Transforms

Recommended Posts

Hey guys, I am new to OpenGl. I wish to draw a cylinder between two 3D points. I did a lot of googling but to no avail. This question has been asked many times before but not one answer has been satisfactory. Could any experienced one from you post the algo (code snippet would b a gr8 help). This should not be a big thing for you guys. Plz help me out Thanx in advance..

Share this post


Link to post
Share on other sites
a cylinder is a circle with height

to draw a circle

glBegin( GL_POINTS );
for ( j=0;j<10;j++)
for ( i=0; i<2*PI; i+=0.1 );
glVertex3f(sin(i)*radius,j,cos(i)*radius);
glEnd();

its up to you to connect the dots

Share this post


Link to post
Share on other sites
There is actually a gluCylinder function that will do the above code snippet for you pretty easily. I forget the exact syntax, but it's easy enough to look up. I think it renders along the z-axis, but a simple extra rotation matrix will fix that.

As for the transformation part, that isn't too hard once you understand how to do it. You need to do three things:

1) move the coordinate system so that (0,0) is at point a
2) rotate the coordinate system so the y-axis is pointing at point b
3) scale the y-axis vertically so that a unit of one is the distance from a to b

Then all you have to do is render a cylinder with a height of one, and you'll get a cylinder from point a to point b.

Steps one and three are easy, but I think it's the middle part that is probably tripping you up.

There is a very easy way to make "up" in world coordinates point in any direction you want, though. Basically, you need to construct three vectors, one each for a new x, y, and z. The y vector should point from point a to point b, and x and z should be orthogonal to the new y vector and each other. Since a cylinder is symmetrical around the y-axis, x and z can point any which way so long as they are orthogonal to y.

Here is a psuedo-code example (adapted from one of my projects). The CVector class is a utiliy class that I developed, but you should be able to duplicate the functionality pretty easily.


//first, translate the matrix so that vertex a is the new world center
glTranslatef(a.x, a.y, a.z);

//apply a rotation matrix so that the cylinder renders along the y-axis
//(remember, gluCylinder renders along the z-axis)
glRotate(90, 1.0f, 0, 0);

//apply a scale matrix to scale from a to b
glScalef(cylinderWidth, a.distanceTo(b), 0);

//next, move the axes so that the y-axis points towards vertex b

//static, so we don't have to reinitialize it every time
//your world up may be different
static CVector worldUpVector(0, 1, 0);

upVector a - b;

upVector.Normalize();
CVector leftVector = upVector.CrossProduct(worlldUpVector);
leftVector.Normalize();
CVector frontVector = frontVector.CrossProduct(leftVector);
frontVector.Normalize();

GLfloat matrix[16];

//left vector is the new x-axis
matrix[0] = leftVector.x;
matrix[1] = leftVector.y;
matrix[2] = leftVector.z;
matrix[3] = 0;

//upVector is the new y-axis
matrix[4] = upVector.x;
matrix[5] = upVector.y;
matrix[6] = upVector.z;
matrix[7] = 0;

//frontVector is the new z-axis
matrix[8] = frontVector.x;
matrix[9] = frontVector.y;
matrix[10] = frontVector.z;
matrix[11] = 0;

matrix[12] = 0;
matrix[13] = 0;
matrix[14] = 0;
matrix[15] = 1;
glMultMatrixf(matrix);

//and put your code to render the cylinder here
//a cylinder of 1 unit height will stretch from a to b

GLUquadricObj *quadric = gluNewQuadric();

void gluCylinder(
quadric,
GLdouble baseRadius,
GLdouble topRadius,
GLdouble height, //should be 1
GLint slices,
GLint stacks
);







And that's it!

[Edited by - kuroioranda on October 10, 2007 6:07:08 PM]

Share this post


Link to post
Share on other sites
Thnx kuroioranda and zedz for your reply.
But kuroioranda the code u provided didnt work!!
All that i can see is probably a sheared cylinder
this is my code in C# (exactly same as yours except the starred line):






Gl.glPushMatrix();
{

Vector3 a = new Vector3(50, 0, 0);
Vector3 b = new Vector3(0, 50, 0);

//first, translate the matrix so that vertex a is the new world center
Gl.glTranslated(a.X, a.Y, a.Z);

//apply a rotation matrix so that the cylinder renders along the y-axis
//(remember, gluCylinder renders along the z-axis)
Gl.glRotatef(90, 1.0f, 0, 0);

//apply a scale matrix to scale from a to b
Gl.glScaled(10, Vector3.Distance(a, b), 0);

//next, move the axes so that the y-axis points towards vertex b

//static, so we don't have to reinitialize it every time
//your world up may be different
Vector3 worldUpVector = new Vector3(0, 1, 0);

Vector3 upVector = new Vector3();
upVector = a - b;

upVector.Normalize();
Vector3 leftVector = upVector.CrossProduct(worldUpVector);
leftVector.Normalize();
***You provided "frontVector.CrossProduct(...)" but c# throws exception as frontVector was still undefined so changed it to upVector***
Vector3 frontVector = upVector.CrossProduct(leftVector);
frontVector.Normalize();

double[] matrix = new double[16];

//left vector is the new x-axis
matrix[0] = leftVector.X;
matrix[1] = leftVector.Y;
matrix[2] = leftVector.Z;
matrix[3] = 0;

//upVector is the new y-axis
matrix[4] = upVector.X;
matrix[5] = upVector.Y;
matrix[6] = upVector.Z;
matrix[7] = 0;

//frontVector is the new z-axis
matrix[8] = frontVector.X;
matrix[9] = frontVector.Y;
matrix[10] = frontVector.Z;
matrix[11] = 0;

matrix[12] = 0;
matrix[13] = 0;
matrix[14] = 0;
matrix[15] = 1;
Gl.glMultMatrixd(matrix);

//and put your code to render the cylinder here
//a cylinder of 1 unit height will stretch from a to b
Gl.glColor3d(0.9, 0.3, 0.2);
Glu.gluCylinder(quadratic, 5, 5, 1, 20, 20);
}
Gl.glPopMatrix();


Could you plz chk what might be causing the trouble..

Thnx

Share this post


Link to post
Share on other sites
Move this line

//apply a scale matrix to scale from a to b
Gl.glScaled(10, Vector3.Distance(a, b), 0);

to after all the rotation. And correct the z-scale to 10.

Therefore,

//...
matrix[13] = 0;
matrix[14] = 0;
matrix[15] = 1;
Gl.glMultMatrixd(matrix);

Gl.glScaled(10, Vector3.Distance(a, b), 10); // move here

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

Sign in to follow this  

  • Forum Statistics

    • Total Topics
      627737
    • Total Posts
      2978873
  • Similar Content

    • By DelicateTreeFrog
      Hello! As an exercise for delving into modern OpenGL, I'm creating a simple .obj renderer. I want to support things like varying degrees of specularity, geometry opacity, things like that, on a per-material basis. Different materials can also have different textures. Basic .obj necessities. I've done this in old school OpenGL, but modern OpenGL has its own thing going on, and I'd like to conform as closely to the standards as possible so as to keep the program running correctly, and I'm hoping to avoid picking up bad habits this early on.
      Reading around on the OpenGL Wiki, one tip in particular really stands out to me on this page:
      For something like a renderer for .obj files, this sort of thing seems almost ideal, but according to the wiki, it's a bad idea. Interesting to note!
      So, here's what the plan is so far as far as loading goes:
      Set up a type for materials so that materials can be created and destroyed. They will contain things like diffuse color, diffuse texture, geometry opacity, and so on, for each material in the .mtl file. Since .obj files are conveniently split up by material, I can load different groups of vertices/normals/UVs and triangles into different blocks of data for different models. When it comes to the rendering, I get a bit lost. I can either:
      Between drawing triangle groups, call glUseProgram to use a different shader for that particular geometry (so a unique shader just for the material that is shared by this triangle group). or
      Between drawing triangle groups, call glUniform a few times to adjust different parameters within the "master shader", such as specularity, diffuse color, and geometry opacity. In both cases, I still have to call glBindTexture between drawing triangle groups in order to bind the diffuse texture used by the material, so there doesn't seem to be a way around having the CPU do *something* during the rendering process instead of letting the GPU do everything all at once.
      The second option here seems less cluttered, however. There are less shaders to keep up with while one "master shader" handles it all. I don't have to duplicate any code or compile multiple shaders. Arguably, I could always have the shader program for each material be embedded in the material itself, and be auto-generated upon loading the material from the .mtl file. But this still leads to constantly calling glUseProgram, much more than is probably necessary in order to properly render the .obj. There seem to be a number of differing opinions on if it's okay to use hundreds of shaders or if it's best to just use tens of shaders.
      So, ultimately, what is the "right" way to do this? Does using a "master shader" (or a few variants of one) bog down the system compared to using hundreds of shader programs each dedicated to their own corresponding materials? Keeping in mind that the "master shaders" would have to track these additional uniforms and potentially have numerous branches of ifs, it may be possible that the ifs will lead to additional and unnecessary processing. But would that more expensive than constantly calling glUseProgram to switch shaders, or storing the shaders to begin with?
      With all these angles to consider, it's difficult to come to a conclusion. Both possible methods work, and both seem rather convenient for their own reasons, but which is the most performant? Please help this beginner/dummy understand. Thank you!
    • By JJCDeveloper
      I want to make professional java 3d game with server program and database,packet handling for multiplayer and client-server communicating,maps rendering,models,and stuffs Which aspect of java can I learn and where can I learn java Lwjgl OpenGL rendering Like minecraft and world of tanks
    • By AyeRonTarpas
      A friend of mine and I are making a 2D game engine as a learning experience and to hopefully build upon the experience in the long run.

      -What I'm using:
          C++;. Since im learning this language while in college and its one of the popular language to make games with why not.     Visual Studios; Im using a windows so yea.     SDL or GLFW; was thinking about SDL since i do some research on it where it is catching my interest but i hear SDL is a huge package compared to GLFW, so i may do GLFW to start with as learning since i may get overwhelmed with SDL.  
      -Questions
      Knowing what we want in the engine what should our main focus be in terms of learning. File managements, with headers, functions ect. How can i properly manage files with out confusing myself and my friend when sharing code. Alternative to Visual studios: My friend has a mac and cant properly use Vis studios, is there another alternative to it?  
    • By ferreiradaselva
      Both functions are available since 3.0, and I'm currently using `glMapBuffer()`, which works fine.
      But, I was wondering if anyone has experienced advantage in using `glMapBufferRange()`, which allows to specify the range of the mapped buffer. Could this be only a safety measure or does it improve performance?
      Note: I'm not asking about glBufferSubData()/glBufferData. Those two are irrelevant in this case.
    • By xhcao
      Before using void glBindImageTexture(    GLuint unit, GLuint texture, GLint level, GLboolean layered, GLint layer, GLenum access, GLenum format), does need to make sure that texture is completeness. 
  • Popular Now