1. Picking in
OpenGL
Yingcai Xiao

2. Picking
Identifying objects in 3D
Two approaches
HW: ID-Buffer
SW: Selection Mode

3. HW: ID-Buffer
Use a memory buffer of the same resolution as the frame buffer.
Assign each object an ID (name)
Update the ID buffer with the object ID whenever the frame buffer is being updated (after checking and updating the z-buffer)
Return the ID at the cursor location when the user clicks the left button

4. SW: Selection Mode
Same idea as the ID-buffer but without using the buffer.
Switch render mode to the selection mode
Set a small picking area around the cursor (e.g. 5x5) as the viewport
“Render” each object after pushing its name on the stack
All objects in the small viewport are added into a hit-list.
The ID, zmin and zmax of each hit-object are stored into a predefined selection buffer not
Switch back to regular render mode
Process the hit-list (e.g. display the selected object with highlight)

5. Implement picking in OpenGL
gluPickMatrix(x, y, width, height, viewport);
x, y should be set as the mouse location: xMouse and yMouse
width, height are the size of the picking box
glGetIntegerv(GL_VIEWPORT, viewport);
glInitNames();
glPushName(ID);

6. Example 1: void pick(xPos, yPos) { glSelectBuffer(LENGTH, selectBuff);
glRenderMode(GL_SELECTION);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPickMatrix(x, y, 2, 2, viewport);
gluPerspective(fovy, aspect, near, far);
RenderScene();
hits = glRenderMode(GL_RENDER);
Cout << we pick << selectBuff[3] << object; }

7. Example 2: void ObjectManager::selectObject(float x, float y) {
GLuint buff[64] = {0};
GLint hits, view[4];
glSelectBuffer(64, buff);
glGetIntegerv(GL_VIEWPORT, view);
glRenderMode(GL_SELECT);
glInitNames();
glPushName(0);
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
gluPickMatrix(x, y, 0.05,0.05 , view);
gluOrtho2D(0, 800, 0, 600);

8. Example 2: glutSwapBuffers(); drawObjects();
glMatrixMode(GL_PROJECTION);
glPopMatrix();
hits = glRenderMode(GL_RENDER);
list_hits(hits, buff);
glMatrixMode(GL_MODELVIEW); }

9. Example 2: void ObjectManager::drawObjects() {
for (list<GameObject*>::iterator ii = Objects.begin();
ii != Objects.end(); ++ii) {
glLoadName((*ii)->getId());
glPushMatrix();
(*ii)->drawObject();
glPopMatrix(); }

10. Example 2: void ObjectManager::list_hits(GLint hits, GLuint *names) {
for (int i = 0;i<hits;i++) {
for (list<GameObject*>::iterator ii = Objects.begin();
ii != Objects.end(); ++ii) {
if ((GLubyte)names[i * 4 + 3] == (*ii)->getId())
(*ii)->onMouse(); }

11. Example 2: printf("%d hits:\n", hits); for (i = 0; i < hits; i++)
printf( "Number: %d\n"
"Min Z: %d\n"
"Max Z: %d\n"
"Name on stack: %d\n",
(GLubyte)names[i * 4],
(GLubyte)names[i * 4 + 1],
(GLubyte)names[i * 4 + 2],
(GLubyte)names[i * 4 + 3] );
printf("\n"); }