1. Mouse-Based Viewing & Navigation Glenn G. Chappell CHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 381 Lecture Notes Monday, November 3, 2003

2. 3 Nov 2003CS 3812 Review: Advanced Viewing [1/4] In most of the programs we have dealt with, we have constructed the model/view transformation from scratch each display cycle. This becomes inconvenient when we implement some of the more advanced viewing interfaces (driving, flying, etc.). In these interfaces we often want to add an increasing number of transformations at the end of the sequence (= beginning of the code). Solution Save the viewing transformation in a (global) variable. When the transformation needs to be changed, modify this variable. Use the variable in the display function. Remember, the saved matrix should be treated like other display-related globals in a GLUT program. Cue music for flashback …

3. 3 Nov 2003CS 3813 Blast from the Past: Back in mid-September … To add keypress-based display changes to a GLUT program: A global variable is needed to hold the current state of whatever part of the display is to change. Declare the variable. Initialize this variable to an appropriate value somewhere. In its declaration? In the init function? In the display function: Use the value of this variable. Draw whatever should be drawn, according to the current value of the variable. Do not change the variable in the display function. In the keyboard function, when the appropriate key is pressed: Change the value of the variable. Call “ glutPostRedisplay(); ”. Do not call the display function.

4. 3 Nov 2003CS 3814 Review: Advanced Viewing [2/4] We handle matrix globals similarly to other GLUT globals: Declare the variable. GLdouble your_matrix_variable [16]; Initialize the variable. In the init function, often. Or maybe in a “reset” function, to be called during initialization, and also when the user does an “I’m lost” keypress. glMatrixMode(GL_MODELVIEW); glPushMatrix(); ***** Transformation commands go here? ***** glGetDoublev(GL_MODELVIEW_MATRIX, your_matrix_variable ); glPopMatrix();

5. 3 Nov 2003CS 3815 Review: Advanced Viewing [3/4] In the display function: Use the variable. glPushMatrix(); // Do NOT forget the push-pop pairs! glTranslated(0., 0., -3.); // MAYBE this goes here. // MAYBE it goes in init. glMultMatrixd( your_matrix_variable ); ***** Draw something here ***** glPopmatrix(); More on that glTranslate * command “ MAYBE ” stuff in a bit.

6. 3 Nov 2003CS 3816 Review: Advanced Viewing [4/4] Where appropriate (keyboard? special? mouse? motion?): Change the value of the variable. Call “ glutPostRedisplay(); ”. glPushMatrix(); ***** Transformation commands go here? ***** glMultMatrixd( your_matrix_variable ); glGetDoublev(GL_MODELVIEW_MATRIX, your_matrix_variable ); glPopMatrix(); glutPostRedisplay();

7. 3 Nov 2003CS 3817 More on Advanced Viewing: Two Kinds of Interfaces Now, about that translation: In our “zoom & pan”, “driving”, the translation was done in the display function, not the saved matrix. But in “flying” the translation was in the saved matrix. Advanced interfaces can be divided into two categories: “View the world” interfaces. We look at the scene. Rotation and scaling are centered on the point we are looking at. So the translation does not go in the saved matrix. Examples Driving from above. Object manipulation. “Move in the world” interfaces. In these, we are explicitly inside the scene. Rotation is centered on the camera. So the translation goes in the saved matrix. Examples Flying. Driving, where we appear to be inside the car.

8. 3 Nov 2003CS 3818 Mouse-Based Viewing: Overview Today we will look at two mouse-based viewing interfaces. Click-and-drag rotation. View an object and rotate it in an intuitive way with the mouse (click-and-drag). Flying with the mouse. Fly forward on mouse-button press. Turn in the direction of the mouse position. Both of these will require the saved-matrix techniques we looked at last time. Sample code will not be given on the web page yet. However, hints and code snippets are in these slides. Feel free to use them.

9. 3 Nov 2003CS 3819 Mouse-Based Viewing: Click-and-Drag Rotation [1/3] Interface Description We view an object, centered in the window. The object is normally motionless. When we click the left mouse button and drag, the object rotates in a “reasonable” way. When the mouse stops moving, or the mouse button is released, the object stops moving. Questions Is this “view the world” or “move in the world”? How do we determine what axis to rotate the object about? Does our new rotation come before or after the old, in the code? How much do we rotate the object?

10. 3 Nov 2003CS 38110 Mouse-Based Viewing: Click-and-Drag Rotation [2/3] My Answers Is this “view the world” or “move in the world”? This is “view the world”. Therefore, the big translation that moves the object to where we can see it, should not be in the saved matrix. How do we determine what axis to rotate the object about? Rotate about an axis perpendicular to the recent mouse motion. The “recent motion” is the difference between the last two mouse positions. Does our new rotation come before or after the old, in the code? Before. Yes, we are rotating an object (which suggests and answer of “after”). However, we are not rotating in the object’s own natural coordinate system, but in the world’s. How much do we rotate the object? Rotate the object by an amount proportional to the length of the most recent mouse motion (the distance between the last two positions). Note There are other ways to design & implement a mouse-based object- manipulation interface. Try to make a better one!

11. 3 Nov 2003CS 38111 Mouse-Based Viewing: Perpendicular Rotation If we rotate (x, y) 90 counterclockwise, we get (–y, x). Based on this idea, here is some useful code. // perprot // Rotates about an axis in the x,y-plane, perpendicular to the given // vector (vx, vy). Rotation amount is proportional to the length of // the vector and to rotmultiplier. void perprot(double rotmultiplier, double vx, double vy) { double len = sqrt(vx*vx + vy*vy); glPushMatrix(); glRotated(rotmultiplier*len, -vy,vx,0.); glMultMatrixd( your_matrix_variable ); glGetDoublev(GL_MODELVIEW_MATRIX, your_matrix_variable ); glPopMatrix(); glutPostRedisplay(); }

12. 3 Nov 2003CS 38112 Mouse-Based Viewing: Click-and-Drag Rotation [3/3] A few thoughts about implementing this interface: GLUT and OpenGL handle y-coordinates differently. Save the window dimensions in the reshape function. Then subtract GLUT’s mouse y to get a number useful to OpenGL. Save the mouse position whenever the left button is down. Are we saving it in GLUT or OpenGL format? Be consistent! Remember: mouse & motion are called for other buttons, too. Save the state of the left button in the mouse function. Test this state in the motion function. If the left button is up, do not alter the scene. That said, all that is left is for motion to call perprot with the difference between the current and previous mouse positions. Change rotfactor to speed up or slow down the rotation. Now write it!

13. 3 Nov 2003CS 38113 Mouse-Based Viewing: Flying with the Mouse [1/3] Interface Description We are inside the scene, moving and turning. If the mouse is centered in the window, we do not rotate. Otherwise, we rotate toward the mouse point. If the left mouse button is down, we fly forward. Otherwise, we are stationary (although we may be rotating). Questions Is this “view the world” or “move in the world”? How do we determine what axis to rotate about? Does our new rotation come before or after the old, in the code? How much do we rotate?

14. 3 Nov 2003CS 38114 Mouse-Based Viewing: Flying with the Mouse [2/3] My Answers Is this “view the world” or “move in the world”? This is “move in the world”. Therefore, the big translation should be in the saved matrix. How do we determine what axis to rotate about? Rotate about an axis perpendicular to the difference between the mouse position and the center of the window. Does our new rotation come before or after the old, in the code? Before. (This is easier to figure out, right?) How much do we rotate? Rotate the object by an amount proportional to the distance between the mouse position and the center of the window. So, again, we have a vector. We rotate about an axis perpendicular to it, by an amount proportional to its length. It looks like we can use perprot again. We probably need to change rotfactor. Note Again, can you do better?

15. 3 Nov 2003CS 38115 Mouse-Based Viewing: Flying with the Mouse [3/3] A few thoughts about implementing this interface: We want to rotate when the mouse button is not pressed. So we need to use the passive motion function. We want the scene to change when the mouse is not moving. So the scene changing should go in the idle function. This means that all the mouse, motion, and passive motion functions need to do is to save the button state and mouse position. Idle does the rest. We still need the window dimensions from reshape, in order to find the center of the window. Now write it! Another thought: we need a “roll” function. Otherwise, we end up flying sideways much of the time.

16. 3 Nov 2003CS 38116 Mouse-Based Viewing: Another Flying Interface In class I implemented what I call a “spaceship-style” flying interface. In such an interface, there is no meaningful “up” direction. You can fly in any direction & orientation you want. It is very easy to get lost or disoriented.  Another alternative is an “airplane-style” interface. Here, there is a well-defined “up” direction. Turning left and right would rotate about the world’s up- vector, not the camera’s. I guess … is there a better way? To think about: How would an airplane-style flying interface be implemented?