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Rotation and Pivot Points in HFX

Rotation and Pivot Points in HFX By Le Papy - Last updated on Sunday, January 15, 2012
11313 Views :: 1 Comments :: Article Rating (Total Votes: 10)
Category: Hollywood FX Tutorials / Theory  -  Previous tutorials
Published on Friday, July 22, 2011
Created on Tuesday, November 30, 2010 8:11 AM
 

Rotations and pivot points aren't seen very often in HFX software. I have long wondered why these parameters were used so little.

I found two explanations that, in my opinion, are the cause:

  1. The documentation provided with the software is very brief;
  2. Part of the documentation is not entirely accurate and the alternating use of the words "rotation" and "pivot" in the same sentence does not make it easy to understand.

I am therefore going to review the use of these parameters in detail in order for you to understand them and show how you can use them once you understand their properties.

Step 1 : The Rotation

Since rotation and pivot points are intertwined they should be studied at the same time.

HFX allows you to change each angle separately (X,Y,Z) and it also allows you to change the pivot points separately. It is therefore possible to rotate an object in 2D or 3D from any point around all 3 axes X, Y and Z. By default, the pivot point is located at the center of the object. Since the object defaults to the center of the screen, it follows that the point of rotation of the object coincides with the center of the screen.
Like almost all the parameter changes in HFX, rotation can be done by entering the parameters directly or by using the mouse.

How to rotate an object around an axis?
  1. Select the object to animate in the HFX tree;
  2. Rotate the dial with the mouse or by entering a parameter in the "Rotation" fields, X, Y and Z;
  3. Or, after selecting the correct motion button, use the mouse directly on the object in the preview window.
It goes without saying that entering a parameter directly is the quickest and most accurate method.

I suggest you create an effect just so you can understand the principle. Here's how. To refresh your memory, you can review our tutorial "Tuto-5 Creating effects".

Inserting a new object in the effect
  1. Create a new effect with the menu "File /New" or Ctrl + N;
  2. Search in the "Objects" library folder and click on the subfolder "Simple";
  3. Drag and drop a "Flat 01" object under the camera in the HFX Tree.

The Flat 01 object is displayed fullscreen. To better understand the rest of the tutorial resize it by entering a value of 0.700 in "Resize" in X, Y and Z. To do this you can see the detailed explanation in the tutorial "The keyframes types in HFX".

Now consider the three axes of rotation.

  1. Rotation around the X axis :
Rotation around the X axis
  1. Select the Flat 01 object in the HFX Tree;
  2. Enter a setting of 30° in the X rotation. The field located above it shows the number of revolutions with a maximum of plus or minus 5 full turns and 200°. Which still allows you to make a little over 11 turns of an object, -5 turns and -200° to +5 turns and 200° or 11 turns and 40°;
  3. If you prefer to work directly with the dial using your mouse, the setting will be less accurate.

The pivot point is indicated by a light blue X shape for the selected object (I highlighted it with a yellow ellipse on the screenshot).

Although a precise adjustment is more difficult, you can also act directly on the object itself in the preview window. Select the "Rotate" tool button and use the left mouse button to rotate around the X axis.

  • If you use the left mouse button and you move the pointer from up to down, you apply a positive rotation X.
  • If you use the left mouse button and you move the pointer from down to up, you apply a negative rotation in X.

Warning, if you deviate from the vertical direction with the mouse movement, you will apply a simultaneous rotation in X and Y..

X rotation of the object using the mouse in the preview window
  1. Rotation around the Y axis :
Rotation around the Y axis
  1. Select the Flat 01 object in the HFX Tree;
  2. Enter a setting of 30° for the Y rotation;
  3. If you prefer to work directly with the dial using your mouse, the setting will be less accurate.

Although a precise adjustment is more difficult, you can also act directly on the object itself in the preview window. Select the "Rotate" tool button and use the left mouse button to rotate around the X axis.

  • If you use the left mouse button and you move the pointer from left to right, you rotate positive Y.
  • If you use the left mouse button and you move the pointer from right to left, you rotate positive Y.

Warning, if you deviate from the horizontal trajectory with the movement of the mouse, you will apply a simultaneous rotation in Y and X.

Y rotation of the object using the mouse in the preview window
  1. Rotation around the Z axis :
Rotation around the Z axis
  1. Select the Flat 01 object in the HFX Tree;
  2. Enter a setting of 30° in the Z rotation;
  3. If you prefer to work directly with the dial using your mouse, the setting will be less accurate.

Although a precise adjustment is more difficult, you can also act directly on the object itself in the preview window. Select the "Rotate" tool button and use the right mouse button to rotate around the Z axis.

  • If you use the right mouse button and you move the pointer from down to up, you apply a positive rotation in Z.
  • If you use the right mouse button and you move the pointer from up to down you apply a negative rotation in Z.
Z rotation of the object using the mouse in the preview window

For clarity, we studied the rotation axis by axis. But you can change the 3 axes simultaneously by entering values in the rotation fields or by using the mouse alternately on each dial, or by clicking the correct motion button and directly manipulating the object with the right and left mouse buttons with different vertical and horizontal movements of the pointer. The effects obtained are often interesting, remember the video that accompanied this tutorial ...

Step 2 : Changing the Pivot Point

The term "Pivot Point" is actually not very accurate in the following segment, I prefer to use the term "Axis of rotation" whenever I can because I think it is more accurate than "Pivot Point" .

To begin, remember that the "Rotate" tool button is not acting on the pivot parameters, it acts only on the rotation parameters. There is no tool button whose function changes pivot points (or axes of rotation). One could then believe that the change of the axes of rotation can be done only by entering parameters in the "Pivot Point" fields in X, Y and Z or by using the sliders. But the help file says "Use the left mouse button to move the pivot point along the X and Y axis. Use the right mouse button to move the pivot point along the Z axis". That instruction fails to mention that before doing so, you must click on the "Move" tool button to be able to do this operation. In fact, you do not move the axis of rotation, you move the object to realign its axis of rotation.

On this screenshot that represents an orange 2D object :

  1. I added a horizontal axis at the center of the screen;
  2. I added vertical axis at the center of the screen;
  3. When all parameters are zero, the 3 axes of rotation of the object (light blue on the screenshot) all pass through the center of the object. And because the object is centered by default, these three axes also pass through the center of the screen.
Initially, the axis of rotation coincides with the center of the screen

When changing the position of the axis of rotation of an object, this change is relative to the coordinates of the object itself. It follows, therefore, a displacement of the object so that the axis of rotation of the object continues to coincide with the center of the screen.xxxx

The coordinates of the axes of rotation included within an unscaled object range from :

  • -200 to 200 on the X axis
  • -150 to 150 on the Y axis

On a resized object, the values above are multiplied by the value of scaling.

But we will see later that the axis of rotation may be located outside of the object.

On the picture to the right (not a screenshot), I blew up the black area around the unscaled object to write in the coordinates. The value of the "pivot point" is 0 in X and Y. If we apply a rotation to the object, it will rotate around the center of the screen as well as X and Y.

The pivots values on an object

In this screenshot, a value of +100 was applied to the X "Pivot point" on an object that is not resized.

To make the rotation point visible in light blue, the object is selected and rotated by an angle of 4 ° in the Y coordinate.

You can see that the object has been moved to the left of the screen but that its axis of rotation remains the center of the screen.

Application of a pivot 100 in X

In this screenshot, a value of +200 was applied to the X "Pivot point" on an object that is not resized.

To make the rotation point visible in light blue, the object is selected and rotated by an angle of 4 ° in the Y coordinate.

You can see that the object has been moved accordingly to the left of the screen but that its axis of rotation remains the center of the screen.

Application of a pivot 200 in X

In this screenshot, a value of 300 was applied to the X "Pivot point" in X on an object that is not resized (pivot outside the object).

To make the rotation point visible in light blue, the object is selected and rotated by an angle of 4 ° in the Y coordinate.

You can see that the object has been moved accordingly to the left of the screen but that its axis of rotation remains the center of the screen.

Application of a pivot 300 in X

In this screenshot, a value of -100 was applied to the Y "Pivot point" on an object that is not resized.

To make the rotation point visible in light blue, the object is selected and rotated by an angle of 4 ° in the Y coordinate.

You can see that the object has been moved to the top of the screen, but that its axis of rotation remains at the center of the screen.

Application of a pivot -100 in Y

In this screenshot, a value of 100 was applied to the Y "Pivot point" and a value of 300 was applied to the X "Pivot point" (pivot point outside the object).

To make the rotation point visible in light blue, the object is selected and rotated by an angle of 4 ° in the Y coordinate.

You can see that the object has been moved down and left of the screen, but that its axis of rotation remains at the center of the screen.

Application of a pivot 100 in Y and 300 in X

To see this view :

  1. Click on the "Perspective" button in order to show two different angles simultaneously.

On this screenshot, a value of 600 was applied to the Z "Pivot point" (the maximum possible, the object is now out of range of the camera).

To make the rotation point visible in light blue, the object is selected and rotated by an angle of 4 ° in the Y coordinate.

You can see that the object has been moved to the background. Away from the camera, the object becomes smaller without being resized.

Keep this "Perspective" view and, using your mouse, change the Y rotation from 60 to -60... OK?

Application of a pivot 600 in Z

On this screenshot, a value of -389 was applied to the Z "Pivot point" (the maximum possible, the object passes behind the camera).

The rotation point is no longer visible. You can see just the top of the crossbar of the letter "A" at the bottom of the screen.

You can see that the object has been moved to the extreme foreground (it affects the camera lens). In approaching the camera, the object becomes larger without being resized.

Keep this "Perspective" view and with the mouse change the X rotation from 10 to -10... OK?

Application of a pivot -389 in Z

Demo video...

Step 3 : Change the "Pivot Point" Parameters without Changing the Position of the Object

You saw in the previous step that changing the "Pivot point" parameters changes the position of the object. This is perhaps the most difficult point to understand in this tutorial.

If the modification applied by changing the "Pivot point" parameters is not desired, you need to be able to return the object to its original position. Of course you can use the "Move" tool button to do this but the move will not be very accurate and it will not give the desired results later on when animating the object. After a little experimenting, I have come up with a method I'd like to share with you.

Once you've determined the pivot point, you can reposition the object to its exact original position by applying the following rule to calculate the position of X, Y, and Z.
The position parameter for X, Y or Z is equal to the pivot point value * resizing value / 100

Example :

Start a new effect.

Flat 02 is an object loaded into the HFX tree and is scaled to 0.400 in X, Y and Z space.

The Flat 02 object is scaled to 0.400

A "Pivot point" parameter of Y = 300 is added to this object.

It is automatically moved to the bottom of the screen. Its axis of rotation remains the center of the screen.

But you want this object to always be at the center of the screen. You have to change its Y position:

  • either with the mouse,
  • or by computing the Y value of its future position

Y = 300 * 0.400 /100 = 1.200 and entering this value in the Y "Position" field.

A parameter "Pivot" Y = 300 is added to the Flat 02 object .

The Y position of the object is set to 1,200.

Now it is centered and its axis of rotation has moved so it is no longer at the center of the screen.

The Y position of the Flat 02 object is set to 1,200.

An X rotation of 40° animates the object around its axis of rotation located near the top of the screen.

For best viewing, vary the X rotation from -70 to 100 with your mouse for example... OK?

An X rotation of 40° animates theFlat 02 object around its axis of rotation
Step 4 : Bonus

Without the use of the "Pivot point" and "Rotation" parameters, a transition such as 226-Afficheur, for example, would not be possible.

After trying to digest all that information, try to take a well-deserved break ....

{Thanks to Loosecannon for the fine tuning}

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Comments
Gert Oskar Reitner By # Gert Oskar Reitner @ Friday, July 22, 2011 9:15 AM
WOW ! Good Job , however this one I will play with on a WINTERY day !

 
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