Think of stride length as a means to keep your character's feet from "slipping" on the floor during, for example, a walk cycle.  To make your character walk, you create a path, constrain the character to the path, and then load the walk action onto the character in your choreography.  (For more detail on this process, click here.)  At this point, you can choose to have stride length on or off.  Let's look at the differences between the two:

Stride length Off:
Let's say your walk cycle is 30 frames.  This means that your character will complete 2 steps every 30 frames, i.e. left foot forward, right foot forward, repeat.  If you render your action in the action window, you will see just that, the character taking 2 steps over 30 frames, or 1 step every 15 frames.

Now, create a path in your choreography window and constrain your character to the path.  Load the action onto the character with stride length set to "off."  You now have a 30-frame choreography where the character travels over the entire path in 30 frames, taking 2 steps to do it, as defined in the original walk action.

If you look at the animation closely, you will notice that the character appears to "skate" over the path, meaning that his feet slide along the path instead of remaining fixed on each downstep.  This is because the software needs to get the character from the beginning of the path to the end of the path as defined by the walk action.  Since the walk action is 30 frames long, the software moves the character along the path in equal increments over those 30 frames.  So with the advance of each successive frame, the character moves 1/30th further along the path until the end is reached.

This works well for characters flying or skating on a path, but does not look particularly normal for a walk action.  This is where stride length comes into play...

Stride length On:
By turning stride length on for your action, you are telling the software  to calculate the length of the path and have the character take as many steps as needed (based on the stride length) to get from one end of the path to the other.

So now let's assume we have the same 30-frame walk action, but you have set the stride length to be 50cm.  This means that one walk cycle (left foot, right foot) takes 100cm to complete.  Using the same choreography setup as before, but this time setting stride length on, the length of the path the character walks along now figures into the equation.  Let's assume the path length is 300cm.

The Project Workspace hasn't changed at all.  It still shows a 30-frame action being applied to the character.  However, if you run the animation, you will notice a distinct difference in the look of the action.  Now the character appears to walk MUCH faster than before.  Why is this?

This is because the software has determined that since the stride length of the character is 50cm and the path is 300cm long, the character must now take 6 steps over the 30 frames to get from the beginning to the end of the path.  300 divided by 50 equals 6.  Remember, with stride length off the character took 2 steps over 30 frames to travel the path, so now it appears as if he is walking 3 times as fast.  The character's feet are now planted on the ground as he walks instead of sliding along the path, but it looks like he's power-walking.

What do you do then, if you created the walk so that the pacing was just right?  How do you maintain the pace of the walk with stride length on?  You can do this two ways:

One way is to change the length of the path.  If you want the character to maintain the pace of the action window, in this case 1 step every 15 frames, then you can shorten the path from 300cm to 100cm.  This will cause the character to perform the 30-frame walk action over 100cm, and since stride length is 50cm, he will take 2 steps over the 30 frames, or 1 step every 15 frames.

This is fine for some cases, but most of the time the path is defined by the animator to exist in a fixed amount of space, depending on the scene.  If the path cannot be altered, you must then alter the number of frames over which the action will take place.  The default in the choreography is to have the action occur over the number of frames as originally defined in the action window, in this case 30.  We know that with stride length on our character will take 6 steps.  For the pace to look right we need 1 step every 15 frames, so we will need 90 (6*15) frames.

NOTE: In both of the above cases, what you are effectively doing is altering the "Ease" of the path constraint. This can be done very easily by manipulating the Ease Channel. I am merely describing the mathematics behind it.

Now we have everything the way we want it.  We have our path, which is 300cm long, and our character walking along that path over 90 frames, taking 1 step every 15 frames, with each step taking 50cm.  The stride length is correct, the feet don't slip, and the pace is perfect.

Rolling Objects
Even objects that don't have feet can have stride length.  For rolling objects like wheels and balls, it is essential to use stride length so that as the distance traveled changes depending on the speed of the roll.  Calculating the stride length for a round object is a piece of cake, or shall we say, easy as "pi."

Remember that the circumference of a round object like a wheel or a ball is 2*pi*radius.  To create a rolling wheel for example, lathe a torus so that it's bottom is at Y=0cm and it's top is at Y=20cm.  It's radius is then 10cm, since the radius is half of the diameter.  Create a bone parallel to the X-axis, starting on the right side of the wheel, going through the middle, and ending on the left side.

 

Create your roll action.  For a forward roll, open the left view in your action window and move the roll handle of the bone to the right.  This rolls the wheel forward in the Z direction.  An easy way to complete the action is to create 4 keyframes, moving the roll handle around 90 degrees at each keyframe to complete a full 360 degree roll.

Click on the action in the Project Workspace and click the checkbox for stride length.  Enter 62.834cm as the stride length, which is just 2*10*pi.

Let's look at this wheel in a choreography now.  As before, open a choreography and create a path that you want the wheel to travel along.  Constrain the wheel to the path, and apply the roll action to the wheel.  Make sure that "Use stride length" checkbox is active.

By adjusting the "ease" channel for the path constraint, you can change how far along the wheel is on the path at any given frame.  In the example here, I've adjusted the ease so that the wheel starts off slow, accelerates, and then slows down again before stopping.  While you wait for the Quicktime example to download, think about what it should look like with stride length set.

With stride length, we saw the wheel start out slowly, and it covered a little bit of distance as it crept forward.  As it sped up, the wheel covered more distance and rolled faster, and then slowed down again as it covered the last few inches before stopping.  That's exactly how we expect it to behave in the real world.

Wrap-Up
Hopefully you now have a better understanding of stride length and its uses.  Try experimenting with it on your own, and try to think of your own uses for it to create various effects.
 
 

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