EECS 487, Winter 2007
Project 3: Sketch

Overview

In this project you will implement a simple sketch-based modeling system. The basic system supports two kinds of primitives (or basic shapes): boxes and cylinders. You can optionally add additional primitive types. Each type of primitive is matched to a set of input gestures, which are 2D strokes drawn by the user. The user can thus create a 3D scene interactively by "sketching" various primitives.

In addition to sketching shapes with the left mouse button, the user can move existing shapes around using the middle button. Every shape is given a constraint that limits how the shape moves when grabbed by the user via the middle mouse button. Initially, each object is constrained to move in the plane of the surface on which it was created. The user can interactively define new constraints to move the object in a different plane, or along a given direction. Primitives are organized into a hierarchy via nested transforms. When a new primitive is sketched on top of an existing one, it is added to the scene as a "child" of the existing one, meaning it is defined in the coordinate system of the existing primitive. When the existing one is moved, all of its children automatically move with it. But when a child is moved, it moves independently.

This project is loosely based on the SIGGRAPH 1996 paper: SKETCH: An Interface for Sketching 3D Scenes. (We will see the video in class.)

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Support Code

You are given support code for this project and will need to

1. link to the jot library (the same as for projects 1 and 2),
2. copy the project support code, follow the instructions in the README file to compile, and
3. implement the missing functionality.
   

For more information about jot, see the instructions given in project 1.

Note: Jot was updated a bit for this project. The previous versions should still work, but we recommend you use the newer version for improved rendering and better handling of input gestures. Pre-compiled jot on CAEN will be updated on 2/15 (after students turn in project 2).

Copy the project support code to your local machine.  (On windows, you can execute the following commands using cygwin, which we recommend):

  % scp -r red.engin.umich.edu:/afs/engin.umich.edu/class/perm/eecs487/proj3 .

(Note the 'dot' at the end of line.) The above command overwrites all proj3 files, including any files that you may have made changes to. To copy 'old' versions of files while leaving the new untouched (sync-ing operation):

  % rsync -avz red.engin.umich.edu:/afs/engin.umich.edu/class/perm/eecs487/proj3 .

(Do this from the parent directory containing the proj3 subdirectory.)

On (CAEN) machines, windows users can copy the files from:
    Z:\proj3
This assumes that you mounted the eecs487 directory on CAEN as explained in the project 3 README. If you mounted the network drive differently, change the above path accordingly.

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Tasks

The tasks and grading scheme for this project are as follows:

• Implement the correct transform needed to scale and position a box in the scene. (15 pts)
  
• Write code to generate mesh vertices and faces to form the shape of a canonical cylinder. (15 pts)
• Implement the correct transform needed to scale and position a cylinder in the scene. (15 pts)
  
• Fix the selection of 3D axes so that newly drawn axes align with the natural features of the selected shape. (15 points)
• Write code to let the user move objects in the scene by dragging with the middle button pressed. (10 pts)
  
• Write code to let the user define new constraints for a given object. (10 pts)
• Design an interesting scene and take one or more screen grabs. (10 pts)
• Use appropriate code design; use point, vector, line, and matrix classes appropriately, and document your code reasonably. (10 pts)
  

These tasks are explained in more detail below.

• Box transformation. The existing support code in sketch_pen.C matches the user's input gestures to 3 world-space directions to create a 3D "axis" (or line) for each straight line drawn by the user. When 3 perpendicular axes are drawn, the function SketchPen::create_box() is called to create a canonical box (a cube located at the origin with sides of length 1). Finish the implementation of this function to apply the correct transform to the box so that: the corner at (0,0,0) maps to the 3D location of the start of the first axis, and the directions and lengths of the 3 sides of the box are taken from the axes.
  
  
• Create canonical cylinder. When the user draws two parallel axes, SketchPen::create_cylinder() is invoked. The first piece of missing functionality you should implement is to create a mesh that represents a canonical cylinder whose base lies in the XZ plane centered at the origin, whose radius is 1, and whose height (rising along the positive Y direction) is 1. The comments in SketchPen::create_cylinder() provide more details.
  
  
• Cylinder transformation. Define the 4x4 matrix to transform the canonical cylinder so that its world-space location, radius, height, and orientation match the given axes. Most of the information comes from the 1st axis; the 2nd axis only determines the radius of the cylinder, as in the following examples. More details are given in the comments in SketchPen::create_cylinder().
  

  Left: a 2nd axis being drawn parallel to the red one. Right: the resulting cylinder.

  Left: a similar situation with axes drawn in the floor plane. Right: the resulting cylinder rests halfway inside the floor.

• 3D axis selection. Fix the implementation of SketchPen::rebuild_coord_sys() so that axes align with natural directions of the parent object. E.g., at a point on the side of a cylinder, one axis should point along the length of the cylinder, another should point along the surface normal, and the third should be perpendicular to the first two. The key to achieving this effect is to understand that in the parent's object space, the natural directions are determined by the canonical directions (1,0,0), (0,1,0), and (0,0,1). These directions are mapped to world space by the parent object's object-to-world transform.
  
  
• Moving objects. In node_manip.C, finish the implementation of NodeManip::move() so that when the user clicks on an object with the middle button and drags the mouse, the object moves according to its current constraint.
  
  
• Setting new constraints. Add code in NodeManip::down() to set a new constraint on an object if the user clicks on the object with the middle button while 1 or 2 axes are currently active. If one axis is active, set a line constraint so the object moves along the direction of the axis. If 2 axes are active, set a plane contraint, where the plane normal is the cross product of the two axis directions.
  

• Scene design. Use your system to create an interesting scene. Render the scene from an interesting viewpoint, grab a screen shot and and show the class by posting the image on the phorum as an attachment. Please use the thread dedicated to project 3 images. You can post multiple images. (Please include your images in the directory that you hand in as well.)
  

• Code quality:

1. Comment your code reasonably, particularly your design decisions and choice of approach.
2. Modularize your code: use a suitable number helper functions with descriptive names.
3. Use descriptive variable and function names: apply a suitable tradeoff between symbol-length and descriptiveness.
4. Learn about and use existing functionality in jot/mlib for working with points, vectors, lines, and matrices.
   

    Bottom line: Make your code readable! 10 points

Extended system (up to 15 bonus points):

The following are optional, but can be attempted by ambitious students:

• Custom rendering style. Add a custom rendering style (e.g. using GLSL) that makes sense for a sketch-based modeling system.
  
  
• Additional primitives. Implement additional primitives, mapping them to gestures of your choice. Possible ideas are: sphere, duct (i.e., a tube that follows the path of a curve in 3D), surface of revolution, and extruded shape defined from a planar shape. Corresponding gestures might be as follows. Sphere: draw a circle and tap the center. Duct: draw a circle, then draw a free-hand stroke from the center of the circle. Surface of revolution: draw an axis, then draw a "profile" curve indicating the shape to sweep around the axis. Extrude: first draw a planar shape by drawing its outline, then draw an axis perpendicular to it to define an inflated shape composed of 2 copies of the outline shape offset from each other, and joined with a ruled surface. For any primitive you contruct, consider assigning texture coordinates as well. These can be used in your renderer, either to apply conventional texture maps or for more elaborate effects like bump mapping (or both).
  
  
• Additional constraints. Add support for an additional type of motion constraint: rotation. If the user grabs a node with the middle button while a single axis is active in the scene, then the constraint should depend on the user's initial motion (the first few mouse motions while the cursor is still near the down point). If the motion is roughly parallel to the axis, use a line constraint as before. But if the motion is cross-wise to the axis, activate a rotation constraint that restricts the node to rotate around the rooted vector defined by the axis.