Open GL Super Bible:Interactive Graphics















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Open GL Super Bible


(Publisher: Macmillan Computer Publishing)

Author(s): Waite group Press

ISBN: 1571690735

Publication Date: 08/01/96

 




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An Example
An excellent use of feedback is to obtain window coordinate information regarding any objects that you render. You can then use this information to place controls near the objects in the window, or other windows around them.

To demonstrate feedback, we will use selection to determine which of two objects on the screen have been clicked on by the user. Then we will enter feedback mode and render the scene again to obtain the vertex information in window coordinates. Using this data, we will determine the minimum and maximum x and y values for the object, and use those values to draw a focus rectangle around the object. The end result is graphical selection and deselection of one or both objects.
Label the Objects for Feedback
Listing 19-6 shows the rendering code for our example program, SELECT. Donłt confuse this with a demonstration of selection mode! Even though selection mode is employed in our example to select an object on the screen, we are demonstrating the process of getting enough information about that objectusing feedbackto draw a rectangle around it using normal Windows GDI commands. Notice the use of glPassThrough to label the objects in the feedback buffer, right after the calls to glLoadName to label the objects in the selection buffer.
Listing 19-6 Rendering code for the SELECT example program

#define CUBE1
#define SPHERE2

// Called to draw scene
void RenderScene(void)
{
// Clear the window with current clearing color
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// Save the matrix state and do the rotations
glMatrixMode(GL_MODELVIEW);
glPushMatrix();

// Translate the whole scene out and into view
glTranslatef(-80.0f, 0.0f, -300.0f);

// Initialize the names stack
glInitNames();
glPushName(0);

// Set material color, Yellow
// Cube
glRGB(255, 255, 0);
glLoadName(CUBE);
glPassThrough((GLfloat)CUBE);
auxSolidCube(75.0f);

// Draw Sphere
glRGB(128,0,0);
glTranslatef(130.0f, 0.0f, 0.0f);
glLoadName(SPHERE);
glPassThrough((GLfloat)SPHERE);
auxSolidSphere(50.0f);

// Restore the matrix state
glPopMatrix(); // Modelview matrix

// Flush drawing commands
glFlush();
}

Step 1: Select the Object
Figure 19-6 shows the output from this rendering code, displaying a cube and a sphere. When the user clicks on one of the objects, the function ProcessSelection is called (Listing 19-7). This is very similar to the selection code in the previous two examples.


Figure 19-6  Output from the SELECT program after the sphere has been clicked
Listing 19-7 Selection processing for the SELECT example program

// Process the selection, which is triggered by a right mouse
// click at (xPos, yPos).
#define BUFFER_LENGTH 64
void ProcessSelection(int xPos, int yPos)
{
// Space for selection buffer
GLuint selectBuff[BUFFER_LENGTH];

// Hit counter and viewport storage
GLint hits, viewport[4];

// Set up selection buffer
glSelectBuffer(BUFFER_LENGTH, selectBuff);

// Get the viewport
glGetIntegerv(GL_VIEWPORT, viewport);

// Switch to projection and save the matrix
glMatrixMode(GL_PROJECTION);
glPushMatrix();

// Change render mode
glRenderMode(GL_SELECT);

// Establish new clipping volume to be unit cube around
// mouse cursor point (xPos, yPos) and extending two pixels
// in the vertical and horizontal direction
glLoadIdentity();
gluPickMatrix(xPos, yPos, 2,2, viewport);

// Apply perspective matrix
gluPerspective(60.0f, fAspect, 1.0, 425.0);

// Draw the scene
RenderScene();

// Collect the hits
hits = glRenderMode(GL_RENDER);

// Restore the projection matrix
glMatrixMode(GL_PROJECTION);
glPopMatrix();

// Go back to modelview for normal rendering
glMatrixMode(GL_MODELVIEW);

// If a single hit occurred, display the info.
if(hits == 1)
MakeSelection(selectBuff[3]);
}





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