Another Look at Camera Control Karan Singh†, Cindy Grimm, Nisha Sudarsanan Media and Machines Lab Department of Computer Science and Engineering Washington University in St. Louis †University of Toronto

What is camera control? Manipulation of camera parameters Projection of 3D geometry into 2D plane Applications Interactively navigate around a scene (games) Create a fixed camera path (movies) Use different viewpoints to understand structure (visualization) Take pictures Cameras

Cameras in different fields Computer graphics From, at, up, field of view View matrix Cameras

Cameras in different fields (cont.) Film, photography Camera is physical object Describe movement of camera itself Dolly in/out, tilt, pan, roll Zoom in/out Graphics systems use same vocabulary Through-the-lens Exterior views Camera-centric Reposition the camera Object-centric Reposition the camera relative to an object Cameras

Cameras in different fields (cont.) Mathematics 4X4 matrix 11 degrees of freedom Straight lines to straight lines Computer vision 3X4 matrix Don’t keep depth Graphics maps 11 dof to “useful” parameters J. C. Michener, I. B. Carlbom, Natural and efficient viewing parameters, SIGGRAPH 80 Six extrinsic parameters (position, orientation) 5 intrinsic parameters (center-of-projection, focal length, skew, aspect ratio) Cameras

Cameras in different fields (cont.) Artists have a qualitative vocabulary Describe relationship of camera to object in the scene Perspective effects Vanishing point One point Horizon line Three point Two point Cameras

Goals Visualization of camera parameters in the 2D image E.g., feed back on perspective distortion Manipulate current projection New position indicates desired projection change Changes appropriate camera parameters May change multiple parameters Minimal mouse, keyboard use Current approaches use entire right button Click-through interface Cameras

Related work Original camera paper Trackball manipulation J. C. Michener, I. B. Carlbom, Natural and efficient viewing parameters, SIGGRAPH 80 Trackball manipulation Michael Chen, S. Joy Mountford, and Abigail Sellen, A Study in Interactive 3D Rotation using 2D Input Devices , SIGGRAPH K. Henriksen, J. Sporring, and K. Hornbaek Virtual trackballs revisited, IEEE TVCG Cameras

Related work Through-the-lens camera control Use image constraints to change camera parameters Not very stable Jim Blinn, Where am I? What am I Looking at?, IEEE CG&A, 1988 Michael Gleicher and Andrew Witkin, Through-the-lens camera control, SIGGRAPH 92 Cameras

The IBar A cube centered along the look vector Changing the rendering of the cube changes the camera in a corresponding way Different segments move limbs simultaneously Cyan = top and bottom left limbs Red = left and right bottom limbs Rendering of cube reflects projection parameters Cameras

Demo (traditional) Rotate left-right up-down spin Allows framing Pan (camera-centric) Pan (object-centric) Dolly in and out Zoom (camera-centric) Zoom (object-centric) Cameras

Demo (Perspective change) Dolly + zoom Dolly in and out Center of projection (horizontal – vertical) Cameras

Demo (Just Weird) Skew Aspect ratio Cameras

Camera- versus object-centric Camera-centric Allows “framing” of objects Position cube in relation to scene Object-centric Traditional camera-moves-with-mouse Nice to have both Map different limbs E.g., zoom using the left handle is camera-centric, the right handle is object-centric Cameras

Some implementation details R W v0 u0 V L T f H Camera parameters T – Eye position L – Look vector V – Up vector W,H – width, height f – focal length u0,v0 – center of projection d – distance to object V L T f H q d Cameras

Drawing the IBar Cube edge is centered on look vector Adjust for center of projection Size of cube adjusted so is always sc high on screen Cameras

Drawing the IBar (cont.) Draw horizontal bar at horizon line t 1-t p1 p2 Cameras

Drawing the IBar (cont.) Feedback IBar highlights when mouse is over active part Indicate selected segment with circle Cameras

Manipulating the IBar Relative, not absolute Determine which limb, and which segment selected Determine ratio/percentage moved Change corresponding parameter(s) by ratio E.g., Multiply zoom by ratio of length change Pan by mouse movement Cameras

Manipulating the IBar Limb movement constrained to vertical (or horizontal) Shift key unconstrains Left-right movement rotates up or down Up-down movement changes center of projection Pan unconstrained Shift key constrains Stem Shift chooses aspect ratio or skew Cameras

Changing parameters simultaneously Dolly plus zoom Calculate dolly in Find zoom that keeps everything at a distance d away the same size Center of projection Translate in reverse direction Cameras

Camera- versus object-centric Camera-centric Render cube with new camera, scene with original camera Object-centric Render both cube and scene with new camera Changing parameters Invert operation (i.e., pan in the opposite direction) Cameras

Centering the IBar on an object Allows rotation around arbitrary point User selects point p in scene Determines d Render at p Rotation Translate p to origin (and camera) Rotate Translate back Cameras

In practice In use in short animated film, Ryan Used for dramatic perspective changes Cameras

Summary Visualization of COP, horizon line, perspective distortion Also at arbitrary points in the scene One mouse button for all 11 parameters Shift key chooses less-common action Click through interface Toggle key for disabling Usable perspective manipulation Simultaneous editing of parameters Cameras

Drawbacks Remembering which parameters go where Visual clutter User study comparing IBar to Maya camera Primary conclusion: camera manipulation is hard for both naïve and knowledgeable users Unable to manipulate camera to match a target scene IBar helped people to “learn” camera manipulation Cameras

Future work Reducing number of parameters on widget Multiple widgets, quick swap between them Similar handles Pre-viewing of manipulation effects What does this handle do? Bookmarking and camera paths Visualizing in scene May be out of scene Cameras