CAP 211 - Interactive Design and Game Development

Chapter 41 - Skinning Characters


This lesson discusses adding skin meshes to character bones sets. Objectives important to this lesson:

  1. Planning a character
  2. The Skin modifier
  3. Skin weights
  4. The Weight tool
  5. Skin Wrap and Skin Morph

Having looked at the chapters about bones and kinematics, we now turn to the chapter about applying skins to characters. The material on the first page should be familiar at this point:

  • Characters are given skin meshes to serve as their outside
  • The mesh is attached to a bones or biped system with the Skin modifier
  • The character is animated by moving the bones, which move the skin mesh.

The character you make should be designed with both the bones and the skin mesh in mind, since they have to work together. The mesh and the bones need to be designed to do what the character will do in the animation.

In the course of designing a character, try to reach a balance between too much detail and too little detail. The author observes that there is no need to add details to a mesh that will never be seen or used, but it is important to plan what will happen to the character to determine which details will be needed.

The chapter will discuss three skin modifiers:

  • the Skin modifier - this one is used as described above
  • the Skin Morph modifier - used to deform the skin mesh object
  • the Skin Wrap modifier - used to transfer a deformation from one figure to another
  • the text also mentions the Physique modifier, which was used before the Skin modifier was implemented

The process of skinning a character will typically involve creating a mesh, creating a skeleton system, and binding the mesh to the skeleton with the Skin modifier. The skin will then have a series of envelopes associated with it that represent the influence of the various bones on the skin itself.

Tutorial Notes and Questions

Tutorial 1 (Attaching skin to a biped):

  1. Open the indicated file. Note that the figure on the screen is currently composed of lots of objects. This will change in the next step. (For the next several steps, work in the Front viewport.)
  2. setting dialog box buttonSelect the pelvis object. Right-click it to get the quad menu and convert it to an Editable Poly. The setting dialog box button you are supposed to click next is not obvious. It is the button indicated by the red circle in the graphic on the right. Continue with the instructions to change the figure into one object with the original materials in their intended spots.
  3. Create a new biped object. It will be easier to drag from the feet of the skin mask to its head, then size it in the rest of the steps.
  4. The "future man model" is the Editable Poly you made in step 2. Select it and freeze it to avoid changing it as you manipulate the bones in the following steps.
  5. To select the biped object, it may be easier to press H to select by name, then choose the Bip01 object from the list. Once you have done so, click the Motion panel button.
    Find the Figure Mode button and the Body Horizontal button on the Motion panel. Click each of them. Once you have done so, align the biped pelvis with the pelvis of the model. You will note that the model's legs are longer than the biped's. You will correct this in the following steps. Note: you may find it useful to toggle full screen for the current viewport in this tutorial.

    Figure Mode Body Horizontal
    Figure Mode button Body Horizontal button

  6. Rotate and scale the bones as indicated. Remember, left is blue, right is green. (Undo is our friend.)
  7. The text says to Ctrl-click each of the spine objects. BEFORE you do so, de-select the last bone you changed.
    Question 1: If you Ctrl-clicked the spine bones before you let go of the right thigh, what should you do?
    The text tells you to select the right shoulder bone. It is actually a collar bone, called a clavicle. The text also tells you to use the Symmetrical button after clicking that bone.
    Question 2: Why do you want to click the Symmetrical button? Which of the buttons shown in the Body Horizontal button graphic above is the Symmetrical button?
  8. Switch to the views indicated to align the biped on another axis. Make the indicated changes to the various bones.
  9. When you are satisfied with the bones, unfreeze the skin as instructed. Select the skin model and use the menu command to add the Skin modifier to it.
  10. No, you are not done yet. You should now have a panel to add (bind) bones to the skin modifier. Follow the instructions to add all the bones.
  11. head with wireframeThis step was probably copied from another version of the book without a necessary edit. It tells you that the left shin bone is selected. It is not.

    It also tells you to examine the envelope for each bone. If you have followed the instructions in 3DS Max 2009, this leads to a very interesting image on the screen.

    In the graphic on the right, in 3DS Max 2009, I have chosen the head bone from the list of bones, and clicked Edit Envelopes. We see the burgundy envelope showing the influence of the head. The Envelope has gray handles to adjust its dimensions. Do this and look at it in different viewports. You will see that the envelope is shaped like a capsule with another capsule inside it. In this case, both capsules are shaped like elongated spheres.

    Choose several different bones, and toggle wireframe view on and off for each.

    head with smoothing With smoothing turned on, we also see red, yellow, and blue bands of color. This has more to do with weight concept in the next few pages. The text tells us to look for skin vertices that are red if they are controlled only by this envelope, green for shared control between two envelopes, and gray for minimal or no control from this envelope. In 3DS Max 2009, it appears that red still means controlled by this bone only, yellow means overlapping control, and gray still means not controlled by this bone. Blue would appear to mean greatly controlled by another bone, but inside this envelope.

The envelopes will need editing.

envelope propertiesSo, why are there two envelopes for each bone? See the top of page 959, to learn that we can engage Absolute or Relative envelope properties. The difference being that Absolute properties make all vertices within the outer envelope fully weighted. Relative properties mean that only vertices inside the inner envelope are fully weighted, while vertices between the envelopes are partially weighted.
Question 3: What does the forward slash button do?

The text goes on to explain that different vertices on the skin can be given different weights with respect to different bones. The effect is expressed as a number from 0 (no effect) to 1 (maximum effect). Since vertices can be affected by more than one bone, the weight can be set for each bone having an influence.
Question 4: The note on page 959 provides a list of colors associated with various weight values. Write the list of weights and colors as the answer to this question.

The next tutorial tries to fix the leg problem illustrated back on page 957. You will probably have to make a few attempts to get this working the way you want it to work.

Tutorial 2 (Applying skin weights):

  1. You may want to use your file from the first tutorial instead of the prepared file.
  2. It is important to know when to select a bone from the "press H" list, and when to select it from the skin mesh's list. In this case, select the skin mesh first, open the Modify panel, look for the bone list, and choose the left foot. Click the Edit Envelopes button. (That's your clue: selecting the bone object won't let you edit an envelope.) Zoom and turn off wireframe so the screen is something like the picture on page 963.
  3. In the remaining steps, read the instructions carefully to make sure you are selecting the right things in the right order. As you adjust the cross sections of the envelope for the foot, note that there are two cross sections to adjust. Change them both.
  4. The book tells you to select the vertices on the lower part of the boot. Try region selecting them, from about the instep down. Watch what you are doing in more than one viewport: do not select vertices on another part of the skin.
  5. The icon on the Weight Tool button is a wrench. Click the buttons as instructed.
    Question 5: What color do the selected icons turn when you click the 25% (,25) button?
    Question 6: What color effect does the Blend button have?
  6. As you test the improved (not finished) version, watch in more than one viewport. This is a 3D problem. Its attempted solution may create more of them.
  7. Practice with the Paint Weights tool. It is like the reverse of the process above, in which you selected vertices, then applied a weight to them.
  8. This completes one round of what you see as a cycle. After you build something, you test it, correct it, and test again. Continue testing and correcting until it works for the scene.

The text moves on to try mirroring a solution. Stands to reason: if we fix one leg, we should fix the other one with mirrored symmetry. There is a short tutorial on this subject, but it only has three steps, none of which need explanation. Do this tutorial in class to try the method described.

The next major topic is the Skin Wrap modifier. The text explains that this modifier is useful when the mesh for a character is too complex for the basic Skin modifier. Think of it as giving us a simpler control set (the Skin Wrap) for a complex object (the character mesh). In the tutorial, we also see that it is useful for an object that bones do not apply to.

Tutorial 4 (Making a simple squirt bottle walk):

  1. Open the indicated file. Note that the bottle has not been animated, but a box in the scene has. (Run the animation to check.)
  2. Apply a Skin Wrap modifier to the bottle, as instructed.
  3. Add the box object to the skin wrap's parameters as instructed. This will teach the box's animation to the skin wrap.
  4. Hide the box object. It is still there, so its animation is still there.
  5. Play the animation again.
    Question 7: Compare the animation you saw for the box in step 1 to the animation you see in step 5.

The text moves on to discuss the Skin Morph modifier, which can be used to simulate muscle bulges in a moving skin mesh by deforming faces and/or vertices of the skin. We apply the Skin Morph on top of a Skin modifier.

Tutorial 5 (Bulging arm muscles):

  1. Open the indicated file. Note that the model already has a skin modifier and a bone system.
  2. Select the skin and add a Skin Morph modifier to it.
  3. Add the bone that will be the trigger for the event to the Skin Morph parameters.
  4. This step begins the artwork. Use the picture on page 968 as a guide to rotating the bone. The skin bulge will not appear yet.
  5. You may need a couple of tries to get the skin to bulge the way you want. Remember the undo button.
  6. Test the new feature.