EX 3. To convince yourself, cover one eye and try to position yourself one metre from a wall by looking only at the wall. Then repeat the experiment with both of your eyes open.

1. Binocular cues:
   3D vision is a construction of your brain; you combine the two 2D images produced by both your eyes into one image. Since you have two eyes, you see double except at the point where you are focusing. Your eyes converge toward the point you are looking at (binocular convergence), which gives you an impression of depth once your brain has decoded the information.

EX 4. Stare at the tip of your nose and notice that objects are double.

EX 5. Draw a line in the middle of a piece of paper, lengthwise. Hold the paper in front of your eyes and stare at the pencil at the top of the sheet. The line seems to form a V. Then stare at the pencil in the middle of the sheet. The line forms an X.

FIGS IN 3D. These figures are all formed according to the same basic principle. Two drawings only a few millimeters apart are camouflaged (rule of good continuation) so that you can only see them with difficulty. Your binocular vision allows you to discern these images as 3D figures with depth.



Which surface of the cube is in front?

2. Monocular cues:
1. Linear perspective of texture:
   You don't perceive perspective; it is a constructed distance cue.



The train-tracks never touch in reality, and yet they seem to meet in this picture.



The plane is high up on the picture, so it looks further away. And yet the whole picture is at the same distance from your eyes.

2. Size of objects:
   You automatically suppose that similar objects are the same size.



You see David (the boy) as smaller than Goliath (the giant) because of perspective; yet in the drawing Goliath is definitely drawn smaller.

3. Interposition:
   When an object is cut off by another, you see the cut off figure as being further away than the one that hides it.



The circle seems to be in the back because it is cut by the square and the triangle.



The Penrose triangle is an eloquent example of an impossible shape because of interposition.

3. Relative parallax movement of objects:

EX 5. Try to notice this phenomenon in a car. Say that the scenery out of your window is composed, for example, of telephone posts in the foreground with a row of trees farther back and a mountain in the background. As the car rolls, the posts will seem to be going backward very quickly, the trees will also seem to be going backward but not as quickly, and the mountain will seem to be going forward, but very slowly. These ideas are commonly used in cartoons to give an impression of depth and of movement.



Is the branch in front of or behind the tree? It is impossible to decide because you have no information about relative movement; this is due to the fact that you are in a 2D support.

EX 6. Stare at a fluorescent pink drawing on a white wall for one minute and then turn off the light. You will still see the drawing.

EX 7. Try to put two pencils end to end with one eye closed.