I want to implement collision of 6 torus which are randomly disturbed in the game area. It is a simple 3D space game using the perspective view and in first person. I saw some stack overflow answer suggesting to compute distance of whatever (player) to torus cell and if bigger than half or whole cell size you are colliding +/- your coordinate system and map topology tweaks. But if we take the distance that means we're only considering the z co-ordinates so if the camera moved to that distance (without considering x,y coordinates) it's always taking as a collision which is wrong right?
I'm hoping to do this using AABB algorithm. Is it ok to consider camera position and torus position as 2 boxes and check the collision (box to box collision) or camera as a point and torus as a box (point to box)? Or can somebody suggest best way to do that?
Below is the code that I've tried so far.
float im[16], m[16], znear = 0.1, zfar = 100.0, fovx = 45.0 * M_PI / 180.0;
glm::vec3 p0, p1, p2, p3, o, u, v;
//p0, p1, p2, p3 holds your znear camera screen corners in world coordinates
void ChangeSize(int w, int h)
{
GLfloat fAspect;
// Prevent a divide by zero
if(h == 0)
h = 1;
// Set Viewport to window dimensions
glViewport(0, 0, w, h);
// Calculate aspect ratio of the window
fAspect = (GLfloat)w*1.0/(GLfloat)h;
// Set the perspective coordinate system
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
// field of view of 45 degrees, near and far planes 1.0 and 1000
//that znear and zfar should typically have a ratio of 1000:1 to make sorting out z depth easier for the GPU
gluPerspective(45.0f, fAspect, 0.1f, 300.0f); //may need to make larger depending on project
// Modelview matrix reset
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// get camera matrix (must be in right place in code before model transformations)
glGetFloatv(GL_MODELVIEW_MATRIX, im); // get camera inverse matrix
matrix_inv(m, im); // m = inverse(im)
u = glm::vec3(m[0], m[1], m[2]); // x axis
v = glm::vec3(m[4], m[5], m[6]); // y axis
o = glm::vec3(m[12], m[13], m[14]); // origin
o -= glm::vec3(m[8], m[9], m[10]) * znear; // z axis offset
// scale by FOV
u *= znear * tan(0.5 * fovx);
v *= znear * tan(0.5 * fovx / fAspect);
// get rectangle coorners
p0 = o - u - v;
p1 = o + u - v;
p2 = o + u + v;
p3 = o - u + v;
}
void matrix_inv(float* a, float* b) // a[16] = Inverse(b[16])
{
float x, y, z;
// transpose of rotation matrix
a[0] = b[0];
a[5] = b[5];
a[10] = b[10];
x = b[1]; a[1] = b[4]; a[4] = x;
x = b[2]; a[2] = b[8]; a[8] = x;
x = b[6]; a[6] = b[9]; a[9] = x;
// copy projection part
a[3] = b[3];
a[7] = b[7];
a[11] = b[11];
a[15] = b[15];
// convert origin: new_pos = - new_rotation_matrix * old_pos
x = (a[0] * b[12]) + (a[4] * b[13]) + (a[8] * b[14]);
y = (a[1] * b[12]) + (a[5] * b[13]) + (a[9] * b[14]);
z = (a[2] * b[12]) + (a[6] * b[13]) + (a[10] * b[14]);
a[12] = -x;
a[13] = -y;
a[14] = -z;
}
//Store torus coordinates
std::vector<std::vector<GLfloat>> translateTorus = { { 0.0, 1.0, -10.0, 1 }, { 0.0, 4.0, -6.0, 1 } , { -1.0, 0.0, -4.0, 1 },
{ 3.0, 1.0, -6.0, 1 }, { 1.0, -1.0, -9.0, 1 } , { 4.0, 1.0, -4.0, 1 } };
GLfloat xpos, ypos, zpos, flagToDisplayCrystal;
//Looping through 6 Torus
for (int i = 0; i < translateTorus.size(); i++) {
//Get the torus coordinates
xpos = translateTorus[i][0];
ypos = translateTorus[i][1];
zpos = translateTorus[i][2];
//This variable will work as a variable to display crystal after collision
flagToDisplayCrystal = translateTorus[i][3];
//p0 min, p2 max
//Creating a square using Torus index coordinates and radius
double halfside = 1.0 / 2;
//This (xpos+halfside), (xpos-halfside), (ypos+halfside), (ypos-halfside) are //created using Torus index and radius
float d1x = p0[0] - (xpos + halfside);
float d1y = p0[1] - (ypos + halfside);
float d2x = (xpos - halfside) - p2[0];
float d2y = (ypos - halfside) - p2[1];
//Collision is checking here
//For square's min z and max z is checking whether equal to camera's min //z and max z
if ((d1x > 0.0f || d1y > 0.0f || d2x > 0.0f || d2y > 0.0f) && p2[2] == zpos && p0[2] == zpos) {
//If there is collision update the variable as 0
translateTorus[i][3] = 0;
}
else {
if (flagToDisplayCrystal == 1) {
glPushMatrix();
glEnable(GL_TEXTURE_2D);
glTranslatef(xpos, ypos, zpos);
glRotatef(fPlanetRot, 0.0f, -1.0f, 0.0f);
glColor3f(0.0, 0.0, 0.0);
// Select the texture object
glBindTexture(GL_TEXTURE_2D, textures[3]);
glutSolidTorus(0.1, 1.0, 30, 30);
glDisable(GL_TEXTURE_2D);
glPopMatrix();
}
}
}
as I mentioned in the comments you got 2 options either use OpenGL rendering or compute entirely on CPU side without it. Let start with rendering first:
render your scene
but instead of color of torus and stuff use integer indexes (for example 0 empty space, 1 obstacle, 2 torus ...) you can even have separate indexes for each object in the world so you know exactly which one is hit etc ...
so: clear screen with empty color, render your scene (using indexes instead of color with glColor??(???)) without lighting or shading or whatever. But Do not swap buffers !!! as that would show the stuff on screen and cause flickering.
read rendered screen and depth buffers
you simply use glReadPixels to copy your screen and depth buffers into CPU side memory (1D arrays) lets call them scr[],zed[].
scan the scr[] for color matching torus indexes
simply loop through all pixels and if torus pixel found check its depth. If it is close enough to camera you found your collision.
render normally
now clear screen again and render your screen with colors and lighting... now you can swap buffers too.
Beware depth buffer will be non linear which requires linearization to obtain original depth in world units. For more about it and example of reading both scr,zed see:
depth buffer got by glReadPixels is always 1
OpenGL 3D-raypicking with high poly meshes
The other approach is is much faster in case you have not too many torus'es. You simply compute intersection between camera znear plane and torus. Which boils down to either AABB vs rectangle intersection or cylinder vs. rectangle intersection.
However if you not familiar with 3D vector math you might get lost quickly.
let assume the torus is described by AABB. Then intersection between that and rectangle boils down to checking intersection between line (each edge of AABB) and rectangle. So simply finding instersection between plane and line and checking if the point is inside rectangle.
if our rectangle is defined by its vertexes in CW or CCW order (p0,p1,p2,p3) and line by endpoints q0,q1 then:
n = normalize(cross(p1-p0,p2-p1)) // is rectangle normal
dq = normalize(q1-q0) // is line direction
q = q0 + dq*dot(dq,p1-p0) // is plane/line intersection
So now just check if q is inside rectangle. There are 2 ways either test if all crosses between q-edge_start and edge_end-edge_start have the same direction or all dots between all edge_normal and q-edge_point has the same sign or zero.
The problem is that both AABB and rectangle must be in the same coordinate system so either transform AABB into camera coordinates by using modelview matrix or transform the rectangle into world coordinates using inverse of modelview. The latter is better as you do it just once instead of transforming each torus'es AABB ...
For more info about math side see:
Cone to box collision
Understanding 4x4 homogenous transform matrices
The rectangle itself is just extracted from your camera matrix (part of modelviev) position, and x,y basis vectors gives you the "center" and axises of your rectangle... The size must be derived from the perspective matrix (or parameters you passed to it especially aspect ratio, FOV and znear)
Well first you need to obtain camera (view) matrix. The GL_MODELVIEW usually holds:
GL_MODELVIEW = Inverse(Camera)*Rendered_Object
so you need to find the place in your code where your GL_MODELVIEW holds just the Inverse(Camera) transformation and there place:
float aspect=float(xs)/float(ys); // aspect from OpenGL window resolution
float im[16],m[16],znear=0.1,zfar=100.0,fovx=60.0*M_PI/180.0;
vec3 p0,p1,p2,p3,o,u,v; // 3D vectors
// this is how my perspective is set
// glMatrixMode(GL_PROJECTION);
// glLoadIdentity();
// gluPerspective(fovx*180.0/(M_PI*aspect),aspect,znear,zfar);
// get camera matrix (must be in right place in code before model transformations)
glGetFloatv(GL_MODELVIEW_MATRIX,im); // get camera inverse matrix
matrix_inv(m,im); // m = inverse(im)
u =vec3(m[ 0],m[ 1],m[ 2]); // x axis
v =vec3(m[ 4],m[ 5],m[ 6]); // y axis
o =vec3(m[12],m[13],m[14]); // origin
o-=vec3(m[ 8],m[ 9],m[10])*znear; // z axis offset
// scale by FOV
u*=znear*tan(0.5*fovx);
v*=znear*tan(0.5*fovx/aspect);
// get rectangle coorners
p0=o-u-v;
p1=o+u-v;
p2=o+u+v;
p3=o-u+v;
// render it for debug
glColor3f(1.0,1.0,0.0);
glBegin(GL_QUADS);
glColor3f(1.0,0.0,0.0); glVertex3fv(p0.dat);
glColor3f(0.0,0.0,0.0); glVertex3fv(p1.dat);
glColor3f(0.0,0.0,1.0); glVertex3fv(p2.dat);
glColor3f(1.0,1.0,1.0); glVertex3fv(p3.dat);
glEnd();
Which basicaly loads the matrix into CPU side variables inverse it like this:
void matrix_inv(float *a,float *b) // a[16] = Inverse(b[16])
{
float x,y,z;
// transpose of rotation matrix
a[ 0]=b[ 0];
a[ 5]=b[ 5];
a[10]=b[10];
x=b[1]; a[1]=b[4]; a[4]=x;
x=b[2]; a[2]=b[8]; a[8]=x;
x=b[6]; a[6]=b[9]; a[9]=x;
// copy projection part
a[ 3]=b[ 3];
a[ 7]=b[ 7];
a[11]=b[11];
a[15]=b[15];
// convert origin: new_pos = - new_rotation_matrix * old_pos
x=(a[ 0]*b[12])+(a[ 4]*b[13])+(a[ 8]*b[14]);
y=(a[ 1]*b[12])+(a[ 5]*b[13])+(a[ 9]*b[14]);
z=(a[ 2]*b[12])+(a[ 6]*b[13])+(a[10]*b[14]);
a[12]=-x;
a[13]=-y;
a[14]=-z;
}
And compute the corners with perspective in mind as described above...
I used GLSL like vec3 but you can use any 3D math even own like float p0[3],.... You just need +,- and multiplying by constant.
Now the p0,p1,p2,p3 holds your znear camera screen corners in world coordinates.
[Edit1] example
I managed to put together simple example for this. Here support functiosn used first:
//---------------------------------------------------------------------------
void glutSolidTorus(float r,float R,int na,int nb) // render torus(r,R)
{
float *pnt=new float[(na+1)*(nb+1)*3*2]; if (pnt==NULL) return;
float *nor=pnt+((na+1)*(nb+1)*3);
float ca,sa,cb,sb,a,b,da,db,x,y,z,nx,ny,nz;
int ia,ib,i,j;
da=2.0*M_PI/float(na);
db=2.0*M_PI/float(nb);
glBegin(GL_LINES);
for (i=0,a=0.0,ia=0;ia<=na;ia++,a+=da){ ca=cos(a); sa=sin(a);
for ( b=0.0,ib=0;ib<=nb;ib++,b+=db){ cb=cos(b); sb=sin(b);
z=r*ca;
x=(R+z)*cb; nx=(x-(R*cb))/r;
y=(R+z)*sb; ny=(y-(R*sb))/r;
z=r*sa; nz=sa;
pnt[i]=x; nor[i]=nx; i++;
pnt[i]=y; nor[i]=ny; i++;
pnt[i]=z; nor[i]=nz; i++;
}}
glEnd();
for (ia=0;ia<na;ia++)
{
i=(ia+0)*(nb+1)*3;
j=(ia+1)*(nb+1)*3;
glBegin(GL_QUAD_STRIP);
for (ib=0;ib<=nb;ib++)
{
glNormal3fv(nor+i); glVertex3fv(pnt+i); i+=3;
glNormal3fv(nor+j); glVertex3fv(pnt+j); j+=3;
}
glEnd();
}
delete[] pnt;
}
//---------------------------------------------------------------------------
const int AABB_lin[]= // AABB lines
{
0,1,
1,2,
2,3,
3,0,
4,5,
5,6,
6,7,
7,4,
0,4,
1,5,
2,6,
3,7,
-1
};
const int AABB_fac[]= // AABB quads
{
3,2,1,0,
4,5,6,7,
0,1,5,4,
1,2,6,5,
2,3,7,6,
3,0,4,7,
-1
};
void AABBSolidTorus(vec3 *aabb,float r,float R) // aabb[8] = AABB of torus(r,R)
{
R+=r;
aabb[0]=vec3(-R,-R,-r);
aabb[1]=vec3(+R,-R,-r);
aabb[2]=vec3(+R,+R,-r);
aabb[3]=vec3(-R,+R,-r);
aabb[4]=vec3(-R,-R,+r);
aabb[5]=vec3(+R,-R,+r);
aabb[6]=vec3(+R,+R,+r);
aabb[7]=vec3(-R,+R,+r);
}
//---------------------------------------------------------------------------
void matrix_inv(float *a,float *b) // a[16] = Inverse(b[16])
{
float x,y,z;
// transpose of rotation matrix
a[ 0]=b[ 0];
a[ 5]=b[ 5];
a[10]=b[10];
x=b[1]; a[1]=b[4]; a[4]=x;
x=b[2]; a[2]=b[8]; a[8]=x;
x=b[6]; a[6]=b[9]; a[9]=x;
// copy projection part
a[ 3]=b[ 3];
a[ 7]=b[ 7];
a[11]=b[11];
a[15]=b[15];
// convert origin: new_pos = - new_rotation_matrix * old_pos
x=(a[ 0]*b[12])+(a[ 4]*b[13])+(a[ 8]*b[14]);
y=(a[ 1]*b[12])+(a[ 5]*b[13])+(a[ 9]*b[14]);
z=(a[ 2]*b[12])+(a[ 6]*b[13])+(a[10]*b[14]);
a[12]=-x;
a[13]=-y;
a[14]=-z;
}
//---------------------------------------------------------------------------
const int QUAD_lin[]= // quad lines
{
0,1,
1,2,
2,3,
3,0,
-1
};
const int QUAD_fac[]= // quad quads
{
0,1,2,3,
-1
};
void get_perspective_znear(vec3 *quad) // quad[4] = world coordinates of 4 corners of screen at znear distance from camera
{
vec3 o,u,v; // 3D vectors
float im[16],m[16],znear,zfar,aspect,fovx;
// get stuff from perspective
glGetFloatv(GL_PROJECTION_MATRIX,m); // get perspective projection matrix
zfar =0.5*m[14]*(1.0-((m[10]-1.0)/(m[10]+1.0)));// compute zfar from perspective matrix
znear=zfar*(m[10]+1.0)/(m[10]-1.0); // compute znear from perspective matrix
aspect=m[5]/m[0];
fovx=2.0*atan(1.0/m[5])*aspect;
// get stuff from camera matrix (must be in right place in code before model transformations)
glGetFloatv(GL_MODELVIEW_MATRIX,im); // get camera inverse matrix
matrix_inv(m,im); // m = inverse(im)
u =vec3(m[ 0],m[ 1],m[ 2]); // x axis
v =vec3(m[ 4],m[ 5],m[ 6]); // y axis
o =vec3(m[12],m[13],m[14]); // origin
o-=vec3(m[ 8],m[ 9],m[10])*znear; // z axis offset
// scale by FOV
u*=znear*tan(0.5*fovx);
v*=znear*tan(0.5*fovx/aspect);
// get rectangle coorners
quad[0]=o-u-v;
quad[1]=o+u-v;
quad[2]=o+u+v;
quad[3]=o-u+v;
}
//---------------------------------------------------------------------------
bool collideLineQuad(vec3 *lin,vec3 *quad) // return if lin[2] is colliding quad[4]
{
float t,l,u,v;
vec3 p,p0,p1,dp;
vec3 U,V,W;
// quad (rectangle) basis vectors
U=quad[1]-quad[0]; u=length(U); u*=u;
V=quad[3]-quad[0]; v=length(V); v*=v;
W=normalize(cross(U,V));
// convert line from world coordinates to quad local ones
p0=lin[0]-quad[0]; p0=vec3(dot(p0,U)/u,dot(p0,V)/v,dot(p0,W));
p1=lin[1]-quad[0]; p1=vec3(dot(p1,U)/u,dot(p1,V)/v,dot(p1,W));
dp=p1-p0;
// test if crossing the plane
if (fabs(dp.z)<1e-10) return false;
t=-p0.z/dp.z;
p=p0+(t*dp);
// test inside 2D quad (rectangle)
if ((p.x<0.0)||(p.x>1.0)) return false;
if ((p.y<0.0)||(p.y>1.0)) return false;
// inside line
if ((t<0.0)||(t>1.0)) return false;
return true;
}
//---------------------------------------------------------------------------
bool collideQuadQuad(vec3 *quad0,vec3 *quad1) // return if quad0[4] is colliding quad1[4]
{
int i;
vec3 l[2];
// lines vs. quads
for (i=0;QUAD_lin[i]>=0;)
{
l[0]=quad0[QUAD_lin[i]]; i++;
l[1]=quad0[QUAD_lin[i]]; i++;
if (collideLineQuad(l,quad1)) return true;
}
for (i=0;QUAD_lin[i]>=0;)
{
l[0]=quad1[QUAD_lin[i]]; i++;
l[1]=quad1[QUAD_lin[i]]; i++;
if (collideLineQuad(l,quad0)) return true;
}
// ToDo coplanar quads tests (not needed for AABB test)
return false;
}
//---------------------------------------------------------------------------
bool collideAABBQuad(vec3 *aabb,vec3 *quad) // return if aabb[8] is colliding quad[4]
{
int i;
vec3 q[4],n,p;
// test all AABB faces (rectangle) for intersection with quad (rectangle)
for (i=0;AABB_fac[i]>=0;)
{
q[0]=aabb[AABB_fac[i]]; i++;
q[1]=aabb[AABB_fac[i]]; i++;
q[2]=aabb[AABB_fac[i]]; i++;
q[3]=aabb[AABB_fac[i]]; i++;
if (collideQuadQuad(q,quad)) return true;
}
// test if one point of quad is fully inside AABB
for (i=0;AABB_fac[i]>=0;i+=4)
{
n=cross(aabb[AABB_fac[i+1]]-aabb[AABB_fac[i+0]],
aabb[AABB_fac[i+2]]-aabb[AABB_fac[i+1]]);
if (dot(n,quad[0]-aabb[AABB_fac[i+0]])>0.0) return false;
}
return true;
}
//---------------------------------------------------------------------------
And here the usage (during rendering):
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
int i;
float m[16];
mat4 m0,m1;
vec4 v4;
float aspect=float(xs)/float(ys);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(60.0/aspect,aspect,0.1,20.0);
glMatrixMode(GL_TEXTURE);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
static float anim=180.0; anim+=0.1; if (anim>=360.0) anim-=360.0;
glEnable(GL_DEPTH_TEST);
glDisable(GL_CULL_FACE);
vec3 line[2],quad[4],aabb[8]; // 3D vectors
get_perspective_znear(quad);
// store view matrix for latter
glMatrixMode(GL_MODELVIEW);
glGetFloatv(GL_MODELVIEW_MATRIX,m);
m0=mat4(m[0],m[1],m[2],m[3],m[4],m[5],m[6],m[7],m[8],m[9],m[10],m[11],m[12],m[13],m[14],m[15]);
m0=inverse(m0);
// <<-- here should be for start that loop through your toruses
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
// set/animate torus position
glTranslatef(0.3,0.3,3.5*(-1.0-cos(anim)));
glRotatef(+75.0,0.5,0.5,0.0);
// get actual matrix and convert it to the change
glGetFloatv(GL_MODELVIEW_MATRIX,m);
m1=m0*mat4(m[0],m[1],m[2],m[3],m[4],m[5],m[6],m[7],m[8],m[9],m[10],m[11],m[12],m[13],m[14],m[15]);
// render torus and compute its AABB
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glColor3f(1.0,1.0,1.0);
glutSolidTorus(0.1,0.5,36,36);
AABBSolidTorus(aabb,0.1,0.5);
glDisable(GL_LIGHT0);
glDisable(GL_LIGHTING);
// convert AABB to the same coordinates as quad
for (i=0;i<8;i++) aabb[i]=(m1*vec4(aabb[i],1.0)).xyz;
// restore original view matrix
glPopMatrix();
// render wireframe AABB
glColor3f(0.0,1.0,0.0);
glBegin(GL_LINES);
for (i=0;AABB_lin[i]>=0;i++)
glVertex3fv(aabb[AABB_lin[i]].dat);
glEnd();
/*
// render filled AABB for debug
glBegin(GL_QUADS);
for (i=0;AABB_fac[i]>=0;i++)
glVertex3fv(aabb[AABB_fac[i]].dat);
glEnd();
// render quad for debug
glBegin(GL_QUADS);
glColor3f(1.0,1.0,1.0);
for (i=0;QUAD_fac[i]>=0;i++)
glVertex3fv(quad[QUAD_fac[i]].dat);
glEnd();
*/
// render X on colision
if (collideAABBQuad(aabb,quad))
{
glColor3f(1.0,0.0,0.0);
glBegin(GL_LINES);
glVertex3fv(quad[0].dat);
glVertex3fv(quad[2].dat);
glVertex3fv(quad[1].dat);
glVertex3fv(quad[3].dat);
glEnd();
}
// <<-- here should be end of the for that loop through your toruses
glFlush();
SwapBuffers(hdc);
just ignore the GLUT solid torus function as you already got it ... Here preview:
The red cross indicates collision with screen ...
Alright, I'm trying to recreate the old classic, Missile Command, using OpenGL in C++. This is my first foray into OpenGL, although I feel fairly comfortable with C++ at this point.
I figured my first task was to figure out how to move 2d objects around the screen, seemed like it would be fairly simple. I created two quick method calls to make either triangles or quads:
void makeTriangle(color3f theColor, vertex2f &p1, vertex2f &p2, vertex2f &p3,
int &xOffset, int &yOffset)
{
//a triangle
glBegin(GL_POLYGON);
glColor3f(theColor.red, theColor.green, theColor.blue);
glVertex2f(p1.x, p1.y);
glVertex2f(p2.x, p2.y);
glVertex2f(p3.x, p3.y);
glEnd();
}
void makeQuad(color3f theColor, vertex2f &p1, vertex2f &p2, vertex2f &p3,
vertex2f &p4, int &xOffset, int &yOffset)
{
//a rectangle
glBegin(GL_POLYGON);
glColor3f(theColor.red, theColor.green, theColor.blue);
glVertex2f(p1.x, p1.y);
glVertex2f(p2.x, p2.y);
glVertex2f(p3.x, p3.y);
glVertex2f(p4.x, p4.y);
glEnd();
}
color3f and vertex2f are simple classes:
class vertex2f
{
public:
float x, y;
vertex2f(float a, float b){x=a; y=b;}
};
class color3f
{
public:
float red, green, blue;
color3f(float a, float b, float c){red=a; green=b; blue=c;}
};
And here is my main file:
#include <iostream>
#include "Shapes.hpp"
using namespace std;
int xOffset = 0, yOffset = 0;
bool done = false;
void keyboard(unsigned char key, int x, int y)
{
if( key == 'q' || key == 'Q')
{
exit(0);
done = true;
}
if( key == 'a' )
xOffset = -10;
if( key == 'd' )
xOffset = 10;
if( key == 's' )
yOffset = -10;
if( key == 'w' )
yOffset = 10;
}
void init(void)
{
//Set color of display window to white
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
//Set parameters for world-coordiante clipping window
glMatrixMode(GL_PROJECTION);
gluOrtho2D(-400.0,400.0,-300.0,300.0);
}
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);
color3f aGreen(0.0, 1.0, 0.0);
vertex2f pa(-400,-200);
vertex2f pb(-400,-300);
vertex2f pc(400,-300);
vertex2f pd(400,-200);
makeQuad(aGreen,pa,pb,pc,pd,xOffset,yOffset);
color3f aRed(1.0, 0.0, 0.0);
vertex2f p1(-50.0,-25.0);
vertex2f p2(50.0,-25.0);
vertex2f p3(0.0,50.0);
makeTriangle(aRed,p1,p2,p3,xOffset,yOffset);
glFlush();
}
int main(int argc, char** argv)
{
// Create Window.
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowSize(WINDOW_WIDTH, WINDOW_HEIGHT);
glutCreateWindow("test");
// Some initialization.
init();
while(!done)
{
//display functions
glutDisplayFunc(display);
glutKeyboardFunc(keyboard);
// Start event loop.
glutMainLoop();
}
return 0;
}
A quad is defined as the "background" for the time being and consists of just a green rectangle along the bottom of the screen. The red triangle is the "object" that I wish to move. On a keypress, an offset is saved in the direction indicated.
I've tried using glTranslatef(xOffset,yOffset,0); but the problem with that is that it moves both elements on the screen and not just the red triangle. I attempted to put the whole call to draw the triangle between a push and pop matrix operation:
PushMatrix();
glTranslatef(xOffset,yOffset,0);
glBegin(GL_POLYGON);
glColor3f(theColor.red, theColor.green, theColor.blue);
glVertex2f(p1.x, p1.y);
glVertex2f(p2.x, p2.y);
glVertex2f(p3.x, p3.y);
glEnd();
PopMatrix();
As far as I can tell, that destroys any changes that the translation was doing beforehand.
I've also tried just changing the values of the x and y coordinates before calling the draw, but that just causes a brief flicker before leaving the triangle in its original position:
p1.x += xOffset;
p2.x += xOffset;
p3.x += xOffset;
p1.y += yOffset;
p2.y += yOffset;
p3.y += yOffset;
There has to be a nice simple way of doing this, and I'm just overlooking it. Could someone offer a suggestion please?
EDIT:
My actual problem was that I was never refreshing the screen after an initial draw. What I needed was to specify an idle function inside my main loop:
glutIdleFunc(IdleFunc);
Where the actual IdleFunc looks like:
GLvoid IdleFunc(GLvoid)
{
glutPostRedisplay();
}
Instead of using glFlush() inside my draw function, I should have been using glutSwapBuffers(). By doing that, the code I had first come up with:
p1.x += xOffset;
p2.x += xOffset;
p3.x += xOffset;
p1.y += yOffset;
p2.y += yOffset;
p3.y += yOffset;
Works fine for my purposes. I didn't have a need to translate the matrix, I just needed to draw the element in a different position from one scene to the next.
GL_MODELVIEW is what you need.
From the OpenGL FAQ, 2.1: http://www.opengl.org/resources/faq/technical/gettingstarted.htm
program_entrypoint
{
// Determine which depth or pixel format should be used.
// Create a window with the desired format.
// Create a rendering context and make it current with the window.
// Set up initial OpenGL state.
// Set up callback routines for window resize and window refresh.
}
handle_resize
{
glViewport(...);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
// Set projection transform with glOrtho, glFrustum, gluOrtho2D, gluPerspective, etc.
}
handle_refresh
{
glClear(...);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
// Set view transform with gluLookAt or equivalent
// For each object (i) in the scene that needs to be rendered:
// Push relevant stacks, e.g., glPushMatrix, glPushAttrib.
// Set OpenGL state specific to object (i).
// Set model transform for object (i) using glTranslatef, glScalef, glRotatef, and/or equivalent.
// Issue rendering commands for object (i).
// Pop relevant stacks, (e.g., glPopMatrix, glPopAttrib.)
// End for loop.
// Swap buffers.
}
You answer your own question, that is the solution:
glPushMatrix();
glTranslatef(xOffset,yOffset,0);
glBegin(GL_POLYGON);
glColor3f(theColor.red, theColor.green, theColor.blue);
glVertex2f(p1.x, p1.y);
glVertex2f(p2.x, p2.y);
glVertex2f(p3.x, p3.y);
glEnd();
glPopMatrix();
That will change the modelview matrix while the rectangle is drawn, then it will revert the modelview matrix back to what it were before. Did you actualy tried that? What whent wrong?
If I'm reading your code right, you want to only rotate one element right? If so, do this:
Call glPushMatrix();
then do your rotation
Store how much you've rotated
then draw your rotated item
then call glPopMatrix();
That will only rotate the one object.
EDIT:
I see that doing that "destroys" the previous rotation. Could you elaborate? That is the correct way to translate/rotate one object.
I also notice that you aren't initializing the Modelview Matrix. You should initialize the Modelview Matrix after you setup your PROJECTION matrix. You also need to make sure that you are initializing both matrices to the identity. And finally, make sure that you are initializing both matrices EVERY time the screen refreshes. To test this, set a breakpoint on your matrix initialization and see if it gets hit only once or every frame.
I want to know how to draw a spiral.
I wrote this code:
void RenderScene(void)
{
glClear(GL_COLOR_BUFFER_BIT);
GLfloat x,y,z = -50,angle;
glBegin(GL_POINTS);
for(angle = 0; angle < 360; angle += 1)
{
x = 50 * cos(angle);
y = 50 * sin(angle);
glVertex3f(x,y,z);
z+=1;
}
glEnd();
glutSwapBuffers();
}
If I don't include the z terms I get a perfect circle but when I include z, then I get 3 dots that's it. What might have happened?
I set the viewport using glviewport(0,0,w,h)
To include z should i do anything to set viewport in z direction?
You see points because you are drawing points with glBegin(GL_POINTS).
Try replacing it by glBegin(GL_LINE_STRIP).
NOTE: when you saw the circle you also drew only points, but drawn close enough to appear as a connected circle.
Also, you may have not setup the depth buffer to accept values in the range z = [-50, 310] that you use. These arguments should be provided as zNear and zFar clipping planes in your gluPerspective, glOrtho() or glFrustum() call.
NOTE: this would explain why with z value you only see a few points: the other points are clipped because they are outside the z-buffer range.
UPDATE AFTER YOU HAVE SHOWN YOUR CODE:
glOrtho(-100*aspectratio,100*aspectratio,-100,100,1,-1); would only allow z-values in the [-1, 1] range, which is why only the three points with z = -1, z = 0 and z = 1 will be drawn (thus 3 points).
Finally, you're probably viewing the spiral from the top, looking directly in the direction of the rotation axis. If you are not using a perspective projection (but an isometric one), the spiral will still show up as a circle. You might want to change your view with gluLookAt().
EXAMPLE OF SETTING UP PERSPECTIVE
The following code is taken from the excellent OpenGL tutorials by NeHe:
glViewport(0, 0, width, height);
glMatrixMode(GL_PROJECTION); // Select The Projection Matrix
glLoadIdentity(); // Reset The Projection Matrix
// Calculate The Aspect Ratio Of The Window
gluPerspective(45.0f,(GLfloat)width/(GLfloat)height,0.1f,100.0f);
glMatrixMode(GL_MODELVIEW); // Select The Modelview Matrix
glLoadIdentity(); // Reset The Modelview Matrix
Then, in your draw loop would look something like this:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // Clear The Screen And The Depth Buffer
glLoadIdentity();
glTranslatef(-1.5f,0.0f,-6.0f); // Move Left 1.5 Units And Into The Screen 6.0
glBegin(GL_TRIANGLES); // Drawing Using Triangles
glVertex3f( 0.0f, 1.0f, 0.0f); // Top
glVertex3f(-1.0f,-1.0f, 0.0f); // Bottom Left
glVertex3f( 1.0f,-1.0f, 0.0f); // Bottom Right
glEnd();
Of course, you should alter this example code your needs.
catchmeifyoutry provides a perfectly capable method, but will not draw a spatially accurate 3D spiral, as any render call using a GL_LINE primitive type will rasterize to fixed pixel width. This means that as you change your perspective / view, the lines will not change width. In order to accomplish this, use a geometry shader in combination with GL_LINE_STRIP_ADJACENCY to create 3D geometry that can be rasterized like any other 3D geometry. (This does require that you use the post fixed-function pipeline however)
I recommended you to try catchmeifyoutry's method first as it will be much simpler. If you are not satisfied, try the method I described. You can use the following post as guidance:
http://prideout.net/blog/?tag=opengl-tron
Here is my Spiral function in C. The points are saved into a list which can be easily drawn by OpenGL (e.g. connect adjacent points in list with GL_LINES).
cx,cy ... spiral centre x and y coordinates
r ... max spiral radius
num_segments ... number of segments the spiral will have
SOME_LIST* UniformSpiralPoints(float cx, float cy, float r, int num_segments)
{
SOME_LIST *sl = newSomeList();
int i;
for(i = 0; i < num_segments; i++)
{
float theta = 2.0f * 3.1415926f * i / num_segments; //the current angle
float x = (r/num_segments)*i * cosf(theta); //the x component
float y = (r/num_segments)*i * sinf(theta); //the y component
//add (x + cx, y + cy) to list sl
}
return sl;
}
An example image with r = 1, num_segments = 1024:
P.S. There is difference in using cos(double) and cosf(float).
You use a float variable for a double function cos.