I am trying to learn opengl’s picking mechanism. I read the instructions in the Red Book and wrote a small demonstration code. But it doesn’t work the way I expected it to. Please help me find the fault.
The problem is that, no matter how small I make the picking area (by reduing ‘width’ and ‘height’ arguments of gluPickMatrix), when I click anywhere on the scene all the objects get picked.
I have drawn two small cubes reasonably separated from each other. I want the cube on which I click to be selected, but no matter where I click on the scene, both the cubes are being selected. Kindly look into the code,
#include<GL/glut.h>
#include<GL/gl.h>
float xrot=0,yrot=0;
void paintGL();
void drawScene(GLenum mode);
void drawCube(float size, float color1, float color2, float color3);
void arrows(int key, int x, int y);
void mousePress( int button, int state, int x, int y );
void proces****s(GLint hits, GLuint buffer[]);
void paintGL(void)
{
GLfloat x,y,z,angle,size=0;
int indi;
glClearColor(0.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT);
drawScene(GL_RENDER);
glFlush();
}
void drawScene(GLenum mode)
{
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-20.0,20.0,-20.0,20.0,-20.0,20.0);
glMatrixMode(GL_MODELVIEW);
// the scene is drawn here
glPushMatrix();
glTranslatef(5.0,0.0,0.0);
if(mode == GL_SELECT)
glPushName(8);
drawCube(1.0,1.0,0.0,0.0);
if(mode == GL_SELECT)
glPopName();
glPopMatrix();
glPushMatrix();
glTranslatef(-5.0,-5.0,0.0);
if(mode == GL_SELECT)
glPushName(5);
drawCube(1.0,0.0,1.0,0.0);
if(mode == GL_SELECT)
glPopName();
glPopMatrix();
}
void drawCube(float size, float color1, float color2, float color3)
{
glColor3f(color1,color2,color3);
glBegin(GL_QUAD_STRIP);
glVertex3f(0.0,0.0,0.0);
glVertex3f(0.0,size,0.0);
glVertex3f(size,0.0,0.0);
glVertex3f(size,size,0.0);
glVertex3f(size,0.0,-size);
glVertex3f(size,size,-size);
glVertex3f(0.0,0.0,-size);
glVertex3f(0.0,size,-size);
glVertex3f(0.0,0.0,0.0);
glVertex3f(0.0,size,0.0);
glEnd();
glBegin(GL_QUADS);
glVertex3f(0.0,size,0.0);
glVertex3f(0.0,size,-size);
glVertex3f(size,size,-size);
glVertex3f(size,size,0.0);
glEnd();
glBegin(GL_QUADS);
glVertex3f(0.0,0.0,0.0);
glVertex3f(0.0,0.0,-size);
glVertex3f(size,0.0,-size);
glVertex3f(size,0.0,0.0);
glEnd();
}
void arrows(int key, int x, int y)
{
switch(key){
case GLUT_KEY_DOWN:
xrot -= 10;
break;
case GLUT_KEY_UP:
xrot += 10;
break;
case GLUT_KEY_RIGHT:
yrot -= 10;
break;
case GLUT_KEY_LEFT:
yrot += 10;
break;
}
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glRotatef(xrot,1.0,0.0,0.0);
glRotatef(yrot,0.0,1.0,0.0);
paintGL();
}
void mousePress( int button, int state, int x, int y )
{
#define BUFSIZE 1024
GLuint selectBuf[BUFSIZE];
GLint hits;
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT,viewport);
glSelectBuffer(BUFSIZE,selectBuf);
(void) glRenderMode(GL_SELECT);
glInitNames();
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
gluPickMatrix((GLdouble) x,(GLdouble) y, 0.1, 0.1, viewport);
gluOrtho2D(0.0, 1.0,0.0,1.0);
drawScene(GL_SELECT);
glPopMatrix();
glFlush();
hits = glRenderMode(GL_RENDER);
proces****s (hits, selectBuf);
}
void proces****s(GLint hits, GLuint buffer[])
{
unsigned int i, j;
GLuint ii,jj,names,*ptr;
printf("hits = %d
", hits);
ptr = (GLuint*) buffer;
for(i=0; i<hits; i++)
{
names = *ptr;
printf("number of names for this hit = %d
“, names);
ptr++;
printf(“z1 is %u;”, *ptr); ptr++;
printf(” z2 is %u
",*ptr); ptr++;
printf("names are “);
for(j=0; j<names; j++)
{
printf(”%d
", *ptr);
ptr++;
}
}
}
main(int argc,char argv[])
{
glutInit(&argc , argv);
glutInitWindowPosition(30,30);
glutInitWindowSize(300,300);
glutInitDisplayMode(GLUT_SINGLE| GLUT_RGBA| GLUT_DEPTH);
glutCreateWindow(“polygon”);
glutDisplayFunc(paintGL);
glutSpecialFunc(arrows);
glutMouseFunc(mousePress);
glEnable(GL_DEPTH_TEST);
glClearColor(0.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT);
glutMainLoop();
}
The file is complete, and can be cut-copy-pasted into a file and run. Kindly help me find the problem.
Thanks for any help.