Doc:Tutorials/Animation/BSoD/Principles of Animation/Tutorials/PDrift

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Blender Summer of Documentation: Contents | Manual | Blender Version 2.41

[edit] Introduction

[edit] About

In this simple tutorial we will reproduce a few optical illusion effects. It's a good introductory exercise to play with direct number input in Blender's transform tools.

[edit] File

1) Peripheral Drift Illusion.

[edit] The Illusion of Motion

Why do we perceive a succession of co-related still images, one slightly different from the other, as motion?

The best known explanation is called persistence of vision and states that we retain each image we see for a brief moment and mix it with the next one(s).

But this "theory" has been refuted many times by researchers in fields that study these phenomenons, as the articles linked at the end of the given link describe, in favor of different approaches investigating short range apparent motion.

Anyway, there are many optical illusion effects, that's a very interesting topic. And it's quite easy to create some of them. For this tutorial, to give a demonstration of short range apparent motion, we will reproduce two figures (two kinds of wheels) related to peripheral drift illusion (PDI).

[edit] Tutorial

[edit] Basic wheel

2) Disk with 64 vertices, 4 faces selected.

Both figures are based on the same pattern used for this first wheel shown above: slices repeating a sequence of tones: black, dark gray, white, light gray. The minimum number of slices also seems to be important, the illusion of motion works better with 64 than with 32, for example.

[edit] Creating a disk

1. Add a mesh circle with 64 vertices. From now on all steps are done in Mesh edit mode.
   • SPACE >> Add >> Mesh >> Circle, set Vertices to 64.
2. Extrude all vertices, but don't move them (if in edges select mode, choose "Only Edges" in the popup):
   • E , then confirm with  RMB or ENTER.
3. Now merge these new vertices at the center to complete the disk:
   • Alt M >> At Center.

3) Slice with 4 faces of the original disk

[edit] Applying materials

The naive way to add colors would be to select groups of faces and apply the right material to them, but there are faster ways. Since we have a repeating pattern, let's prepare a slice with the right colors and then recreate the whole disk from it.

1. Keep only a 4 faces slice of the disk:
   1. Select 4 contiguous faces.
   2. Invert the selection: press the Select Swap button in the Mesh Tools 1 panel of the mesh editing buttons window or use the menu in the header: Select >> Inverse.
   3. Delete selected faces: X >> Faces. The result should be a slice with 4 faces, like shown in the image.
2. Assign a different material to each face of the slice:
   • black: 0.0
   • dark gray: 0.42
   • white: 1.0
   • light gray: 0.85

We used tones of gray, so we only mentioned one value, that should be used for red, green and blue components. The materials were chosen from available entries in the palette in the color picker. You are welcome to try other values and see how that affects the illusion effect.

After applying the colors you should have something like the image of the slice above.

[edit] Finishing

Next step is recreating the wheel from this piece, which we do with the Spin Dup mesh tool.[1]

1. In the buttons window, mesh editing tab, panel "Mesh Tools", we define the following values for these buttons:
   1. Degr: 360 - a whole circle, 360º.
   2. Steps: 16 - the circle has 64 faces and our slice has 4, so we need 4x16 steps.
   3. Turns: 1.
2. Make sure the 3D cursor is at the center of the object, which is the center of the disk. If not already there, simply select the mesh in Object mode and move the 3D cursor to its center with Shift S >> Cursor-> Selection.
3. Besides the position of the 3D cursor, the spin dup tool also depends on the viewport, so the 3D View window where it will be applied must be a front view to the disk slice.
4. With the above values, press the Spin Dup button in that same panel to make the duplicates.
5. There are many uneeded vertices, 4 uneeded faces and many others to connect, which we can solve all at once simply by removing doubles: W >> Remove Doubles.

Ok, our first wheel is finished. It should look like the first image shown above.

4) Wheels with 32, 64 and 96 faces.

[edit] PDI Wheel

At the left we have a render with wheels having 32, 64 and 96 faces. This image should demonstrate the peripheral drift illusion effect -- some wheels in it seem to move as the viewer moves her or his eyes.

A good way to notice the apparent motion is to focus on specific parts of it for a short while.

Another method is to read something at the side of the image, like you are doing now. Blinking rapidly a few times was also a recommendation for the original wheel, not reproduced here (first article mentioned in the notes below).

From personal tests, the wheels with more faces (96) "work" much better than the others.

The image was rendered using the .blend file that accompanies this tutorial.

5) Enhanced PDI wheel.

[edit] Spiraling wheel

The wheel done above causes a stronger PDI effect than the original, which can be seen in the paper referenced in the notes section below. But the effect can be further enhanced using this second model, which can be built with superimposed, slightly shrunk and rotated copies of the other.

6) Building a spiral from the PDI wheel...

7) Two more transformed copies.

All commands from now on are done in Object mode. The base wheel here has 96 faces, but was done just like the one above, except for the number of steps for the spin dup tool: (96 / 4) = 24 steps.

[edit] Duplicating and transforming

1. Copy the wheel object, but don't move the duplicate. We won't change the mesh, so a linked copy is the best choice.
   • Select the wheel and press Alt D, then confirm with  LMB or ENTER.
2. Move the copy a little above (positive Z axis) the original, 0.01 is enough:
   • G, Z then type .01.
3. Shrink it to 0.9 or so:
   • S, then type .9 or move the mouse, using Control to restrict to 0.1 changes.
4. Rotate it in Z:
   • R, Z, type 3.75, then confirm.

Why "3.75"? We want the rotation to move all faces one "step" clockwise. How many degrees does one step represent? Well, the disk has 96 faces, so the arc of one of them is (360 / 96) = 3.75.

Now we have two wheels superimposed, like shown in image #6. To complete the spiral we need to repeat this process over and over, but it doesn't need to be one copy at a time.

[edit] Repeating for a few more steps

1. Select both wheels, then duplicate them together:
   • Alt D, confirm without moving the new disks.
2. Move them a little in Z:
   • • G, Z then type .02, since the other disks are at 0.00 and 0.01.
3. Shrink them:
   • Do the same as above twice or do this directly: S, then type .81 (0.9x0.9) = 0.81.
4. Rotate them in Z:
   • R, Z, type 7.5 (two times 3.75), then confirm.

Figure #7 shows the four disks, which complete the cycle of rotations (the next rotation will put the black faces back where they started, for example). This means we can copy, move and shrink these four from now on, no need to rotate anymore.

8) The superimposed and transformed wheels in 3D.

[edit] Finishing

1. Select the four wheels, duplicate, move and shrink them:
   • linked copies: Alt D
   • moving: G, Z then type .04.
   • resizing: S, then type .6561 (it's 0.9^4). Alternative ways:
     • or repeat the 0.9 scaling four times.
     • or use Python Evaluation: type #.9**4 in the Transform Properties panel (N).

Repeat these steps a few times to complete the spiral. As soon as there's enough detail in the middle of the wheel to trick viewers to believe the spiral goes in forever, it's done.

That's it. Image #8 shows a render of this spiraling wheel made up of many transformed disks.

9) Enhanced PDI effect.

[edit] Spiraling PDI Wheel

As noted in Faubert and Herbert's article that introduced peripheral drift illusion:

• It happens in the visual periphery, you won't see it at a point where your eyes are staring.
• A resetting mechanism is necessary to continue noticing the apparent motion. This can be achieved with eye movement, blinks or displacements of the image.

[edit] Notes

[edit] Peripheral drift illusion literature

The original article:

• The peripheral drift illusion: A motion illusion in the visual periphery, by Jocelyn Faubert, Andrew M Herbert, Perception, 1999, volume 28, pages 617-621

The one where the figures shown here were introduced:

• Phenomenal Characteristics of the Peripheral Drift Illusion, by Akiyoshi KITAOKA and Hiroshi ASHIDA, VISION Vol. 15, Nº4, 261-262, 2003

Both articles can be found online easily, just search for their names.

[edit] Site

Professor Akioshi KITAOKA has a great site about these illusions.

[edit] Grid illusions

• Grid illusions

These are extras present in this tutorial's .blend file, two examples of another kind of optical illusion:

10) Hermann grid illusion.

11) Scintillating grid illusion.

The effect is related to the illusion of spots in the intersections. The models are just a black cube and a gray disk using two Array modifiers each.

[edit] More optical illusions

There are many other interesting optical illusion effects to know, some of which are also easy to reproduce in Blender.

• Optical Illusions

[edit] About the illusion of motion

These are other interesting articles to read:

• Beta movement
• Phi movement

[edit] Spin Dup tool

Interesting results can be achieved with the spin dup and other mesh tools, like screw, spin, extrude dup and so on. Hint: repeat the spin dup step above with the 3d cursor placed elsewhere.

Summer of documentation 2006 -- Willian 07:20, 5 July 2006 (CEST)

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