Object Selection

[David]

hi All,

Im having problems with object selection using the 'GL_SELECT' method, it works some of the time but sometimes selecting a blank area of the screen gets 'hits' which relate to a particular object, also sometimes clicking an object doesnt work.

Im using glPerspective projection with gluLookAt (i had real problems until I used gluLookAt), I specify these when drawing the objects and when using 'GL_SELECT'

Thanks

[cass]

Are you using gluPickMatrix(...) or something similar to shrink your view volume around the mouse position?

[David]

Yes, my area is 2x2 around the mouse pointer, just as an aside, is this 2x2 pixels?

[cass]

Hmm... maybe you could post some example code?

[David]

this is the mouse click code..


void mouseclick(int button, int state, int x1, int y1)
{
GLuint selectBuf[512];
GLint hits;
GLint viewport[4];

if (button != GLUT_LEFT_BUTTON | | state != GLUT_DOWN)
return;

glGetIntegerv(GL_VIEWPORT, viewport);

glSelectBuffer(512, selectBuf);
glRenderMode(GL_SELECT);

glInitNames();
glPushName(0);

glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();

// 2x2 picking region
gluPickMatrix((GLdouble) x1, (GLdouble) viewport[3]-y1, 5.0, 5.0, viewport);

gluPerspective(45,(GLfloat)800/(GLfloat)600,1,900);
gluLookAt(0,0,800,0,0,0,0,1,0);

env.DrawScene (GL_SELECT);

glPopMatrix();
glFlush();

hits=glRenderMode(GL_RENDER);

if (hits >0 )
env.Proces*hitss (hits,selectBuf);

env.MouseMove(button, state, x1, y1);
}

this is the process hits code.


environment::Proces****s (GLint hits,GLuint buffer[])
{
unsigned int i, j;
selected = NULL;
GLuint names, *ptr, ind;
brick *bptr;

ptr = (GLuint *) buffer;

names = *ptr;
bptr = start;
//selected = bptr;

ptr++;
ptr++;
ptr++;
//ptr++;
names = *ptr;

while(bptr != NULL)
{
ind = bptr->GetIndex ();

if (names != ind)
i=-1;
else
selected = bptr;

bptr = bptr->next ;
}
}

and this is the display code...


environment: :DrawScene (GLenum mode)
{

float *rgbv;
int scale,pos[3];
GLuint index;


glPushMatrix();
rgbv = start ->GetColour ();
scale = start ->GetSize ();
index = start ->GetIndex ();

start ->GetPosition (pos);
glTranslatef(pos[0]-100,pos[1],pos[2]);
glColor3fv(rgbv);
glScalef(scale, scale, scale);
if(mode == GL_SELECT)
glLoadName(1);


glCallList(index);
glPopMatrix();

current = start->next ;



glPushMatrix();

rgbv = current ->GetColour ();
scale = current ->GetSize ();
index = current ->GetIndex ();


current ->GetPosition (pos);
glTranslatef(pos[0],pos[1],pos[2]);
glColor3fv(rgbv);
glScalef(scale, scale, scale);
if(mode == GL_SELECT)
glLoadName(2);

glCallList(index );
glPopMatrix();

}

Hope you can help


[This message has been edited by David (edited 02-12-2000).]

[David]

oh, Ive just found out why,

I needed to set to MODELVIEW mode before calling the GL_SELECT command, it now works, but I got the code from the red book... Strange...

Thanks for the help anyway!!