EECS487 PA4: Textures, Splines, and Animation

The assignment is due on Saturday, Dec. 12th, 2015 at 12 noon.


In this assignment you will extend the view3D from PA2 to render animated textured scenes.

The given support code parses an input X3D file, partially constructs a scene graph, inserting various X3D nodes into it, and then passes the scene to OpenGL for rendering. The X3D file format is described in the X3D Specification. Most tasks require modifying the scene.cpp file (as usual, look for ``YOUR CODE HERE''). As part of your assignment you will need to create an example X3D scene file that will show an animated humanoid character moving through a scene.


  The first three tasks of this assignment familiarize you with how texturing fits into a scene graph. The first two tasks should be easy peasy for you by now. The third one introduces you to texture transform. When you build the provided code as is, you will likely get warnings about the index variable being not used in various functions in scene.cpp. You can ignore these warnings for now. The index variable will be used by your code. As is, view3D will display scenes/aclock.x3d as shown on the right.

  1. Texture Setup. The provided view3D application can load texture image from a JPEG or PNG file and store it as an object of Image class (image_ points to the image). To enable texturing, you need to generate a new OpenGL texture handle (ID) and set up OpenGL for texturing. Modify the SetupTexture method of the X3ImageTexture class. You MUST enable mipmapping to avoid aliasing effects. Now scenes/aclock.x3d should look like on the right. Also check that fish-textured.x3d shows the fins striped. Image files used for texturing MUST be in the same folder as your scene file.  5 points

  2. Textured Cylinder. When rendering geometry, you need to specify texture coordinates for each vertex. Modify the X3Cylinder::Render() method to render a textured cylinder. Texture coordinates MUST be assigned as specified in the X3Cylinder spec. You may want to consult the XBox::Render() and XCone::Render() methods for examples. Now scenes/aclock.x3d should look like on the right (notice the upside down clock face). You should also be able to view scenes/cylhead.x3d now.  10 points

  3. Texture Transforms. OpenGL can apply tranformations to specified texture coordinates before using them. Modify the X3TextureTransform::Render() method to enable texture coordinates transformations. You MUST implement it as described in the X3TextureTransform spec. This is similar to what happened in the X3Transform node; however, there is no hierarchy and no need to use the matrix stack. Just override the transforms every time X3TextureTransform is needed. Now scenes/aclock.x3d should look like on the right (notice the corrected clock face). Also check that scenes/wall.x3d shows the correct mountain panorama.  10 points

  4. Linear interpolation. The provided support code has some basic animation capability in the Timer, Interpolator, and Link nodes. Each Link specifies which Timer's time to feed into which Interpolator, to update which field of which node (see the section on Field Update Mechanism below).

Study the provided code that implements piecewise linear interpolation for the X3ScalarInterpolator and the X3PositionInterpolator classes. Given an array of keys, a corresponding array of values, and a time value, the code calls the X3InterpolatorNode::FindKeyInterval() method to determine the index between which two keys the time falls. The code computes an in-between time distance between the two keys and returns a linear interpolation of the values associated with the two keys. Due to polymorphism, the code for X3ScalarInterpolator::LinearInerpolation() and X3PositionInterpolator::LinearInerpolation() are identical. For rotations, angles and axes must be interpolated separately. Implement linear interpolation for X3OrientationInterpolator that linearly interpolates the axes and angles separately and returns the two together as a rotation_t. View scenes/spline-linear.x3d. Hit SPACEBAR to start the animation. All four objects in the scene should animate, each using a separate Interpolator. Study the file scenes/spline-linear.x3d to see which object uses which Interpolator. The scene scenes/aclock.x3d also animates. This task is mainly about getting you to spend the time to read the spec on Field Update Mechanism and to study the codes that implement linear interpolation and to study the scene file so that you're familiar with the support code structure for animation. Make sure you're thoroughly familiar with the spec and code and scene file before you move on to the next two tasks.  5 points

  5. Spline Interpolation. Implement spline interpolation for the X3ScalarInterpolator and X3PositionInterpolator classes to enable animated content rendering--with smoothly varying parameters, as well as to enable rendering of smooth X3Curve. Use the Catmull-Rom splines (with tension zero) to produce smooth interpolation. You only need to implement smooth interpolation for the scalar and position values, not rotations (which would require use of quaternions). For easier debugging, use the X3Curve node to draw a curve in space (X3Curve is not part of standard X3D). The X3Curve has an X3PositionInterpolator as a child. It draws a curve in space that samples the X3PositionInterpolator's trajectory. Once you have these implemented, you should be able to view all objects in the scene scenes/spline-smooth.x3d animated as in the previous task, but smoother.  15 points

  6. Humanoid Walking. Create an X3D scene file that renders an animated humanoid walking through some scene. For full credit, the character should be textured, have two legs (comprised of upper legs, lower legs, and feet), two arms, a body, a head, and exhibit a proper walk cycle. The walking should propagate the character through the scene at the appropriate rate without sliding. It is acceptable to use cylinders and boxes for rendering the character. This is 55% of the assignment; be sure to treat it as such!

The animated humanoid must be modeled as an articulated hierarchical character model. You must tie together several Interpolation nodes with proper rotation transformations of the skeletal links to produce a walking figure. Once a basic periodic walk cycle is created, you can propagate it through a scene with a translation transform. (As an optional extension, i.e., not required, walk your character on a curved path, which would mean changing its orientation also). Texture your character with some images to help indicate that you have control over your scene viewer (i.e., the head should point forward and knees bend backward). Do not worry about being overly realistic; think of your character as a puppet/robot (though the character parts should not fly off in different directions during animation). You MUST name your scene file walk.x3d. On the right is an example. Be creative and don't create a scene similar to this one! The best way to figure out enough X3D syntax to complete this task is to study spline-smooth.x3d. Some students have also found this optional X3D tutorial useful (if you do consult it, try not to spend too much time on it!).  55 points

Field Update Mechanism


We use a simplified custom mechanism for animating node properties. A special scenegraph node, Link, is introduced to bind together three components required for animation, namely, timers, interpolators, and fields to be updated. Specifically, each Link stores the pointer to the field to be updated, and it references the Timer and Interpolator nodes. In each frame, before the rendering pass, each Link updates the value of the field by the value produced by the Interpolator. Each Interpolator node has its <key, value> sequence as attributes read from the X3D file.

Each Interpolator may be used in multiple instances: for example, a pendulum orientation interpolator may be reused in several different clock gadgets present in the scene. All of these gadgets would go through the same sequence of orientations, even if the gadgets have different speeds and are not synchronized with each other. The gadgets use the same Interpolator but different Timers to drive the Interpolator. Thus, for each gadget a separate Link node will be used to refer to the same Interpolator but different Timers. The role of the Timer is to take a current global time and perform a timing transformation on it. Schematically, a single update operation for a Link, L, can be represented as: L.field_pointer = L.interpolator.Evaluate(L.timer.ConvertTime(t)), where t is the current global time. Global time runs in sync with the system clock, however it may be paused and restarted by the application.


Timer nodes define time conversion from global time to a warped time used as a parameter to the interpolator. A timer has two attributes: period and shift.

Timers with positive periods are periodic, they produce values between zero and one. Therefore, in order to produce periodic motion, one should define Interpolators whose key sequence starts at zero and ends at one. To avoid abrupt transitions at the end of each cycle, the periodic Interpolator should have the same starting and ending values in its value sequence.

A Timer with a negative period attribute is non-periodic, it simply scales the input time by 1/(-period). This can be used with Interpolators that have no restrictions on their key sequence (as long as it is increasing).

Please refer to X3Timer::ConvertTime() method for more information.


An Interpolator maps its input time into a value. The parameters of the mapping are the key and value sequences specified in the corresponding attributes in the X3D file. Depending on the output value, we define three interpolator classes: Depending of the value of the smooth flag, the Interpolators implement piecewise linear or cubic Catmull-Rom spline interpolation. Orientation Interpolators will not use smooth interpolation (which requires use of quaternions).

Link nodes

A Link node can appear anywhere within the scene, and has four required attributes: (The Link node is not part of the X3D standard. It is introduced in this assignment to define update relations.) The diagram below shows an example arrangement with two links using two different Timers, two Interpolators, and two updated fields.

In some situations, we can reuse Interpolators or Timers. For example, in the following diagram, the two Links use the same Interpolator but different Timers. Therefore, the same sequence of 3D vector values will be placed into the two updated fields, possibly with some delays. Or perhaps the values in the translation field of the transformation node will change more quickly (since it is using the "timer_fast" Timer which may indicate a shorter period value). In a similar fashion, a single Timer can be used to drive two different Interpolators.

Support Code and Required Libraries

Download the assignment archive from /afs/

The program view3D should be called with one command-line argument--the name of an X3D scene file. We have included a few in the scenes sub folder. The code needs to be linked against the png, jpeg, expat and the usual OpenGL, GLU, and GLUT libraries; and its compilation requires the corresponding header files. For instructions on how to install these libraries, please see the course note. Your code must not require other external libraries or include files other than the ones included in the support code or listed in this spec (e.g., do not include xmat.h, you don't need it).

The following keyboard shortcuts are defined for view3D (you may bind other operations to keys in view3D.cpp:kbd(), but do not change these predefined keys):

You can rotate the object around by left-click-and-hold while dragging the mouse about. Dollying in and out can be done by right-click-and-hold while dragging the mouse about.

Submission Guidelines

As with PA1, to incorporate publicly available code in your solution is considered cheating in this course. To pass off the implementation of an algorithm as that of another is also considered cheating. For example, if the assignment asks you to implement sort using heap sort and you turn in a working program that uses insertion sort in place of the heap sort, it will be considered cheating. If you can not implement a required algorithm, you must inform the teaching staff when turning in your assignment, e.g., by documenting it in your writeup.

Test the compilation!
Code that does not compile will be heavily penalized.

The creative portion of this project, Task 6, will take some time to do right if you are interested in getting a good grade. It involves a fair amount of trial and error.

Create a writeup in text format that discusses:

  1. Your platform: Linux, Mac OS X, Windows, or any other.
  2. Anything noteworty about your implementation, e.g., if you implemented quaternions.
  3. Feedback on the assignment.
  4. Name the file writeup-uniqname.txt.
    For example, the person with uniqname tarukmakto would create writeup-tarukmakto.txt.

Your "PA4 files" then consists only of your writeup-uniqname.txt, scene.cpp, your humanoid walking scene file from Task 6 (which MUST be named walk.x3d), and new texture file(s) you use as part of Task 6.

To turn in your PA4, upload a zipped or gzipped tarball of your PA4 files to the CTools Drop Box. Keep your own backup copy! The timestamp on your uploaded file will be your time of submission. If this is past the deadline, your submission will be considered late. You are allowed multiple "submissions" without late-policy implications as long as you respect the deadline.

Turn in ONLY the files you have modified. Do not turn in support code we provided that you haven't modified. Do not turn in any binary files (object, executable, dll, library, or image files) with your assignment. Your code must not require other external libraries and include files other than the ones listed in the Makefile.

Do remove all printf()'s or cout's and cerr's you've added for debugging purposes.

General Information

The General Information section from PA1 applies. Please review it if you haven't read it or would like to refresh your memory.