• Background
• Instructions
• Illustration
• Quiz

## Background

Deletion is the gradual occlusion of a moving object as it passes behind another object. Accretion is the gradual reappearance of a moving object as it emerges from behind another object. Think about being in a library. You watch someone emerge from behind one bookshelf and then disappear behind another bookshelf. When the person first becomes visible, you note that the bookshelf was not moving, but the person suddenly emerged. This provides information about relative depth. The person must be behind the shelf. Similarly when the person is “deleted” as she moves behind the next shelf, you again see from her movement the relative positions of her and the furniture. Thus, we can formalize this in the following way. The object that is being deleted and then later accreted is the object that is farther away than the object we can see continuously, which is therefore closer.

In this activity, observe this depth cue in a moving illustration. You can also add some other pictorial depth cues to see how they might add to the perception of depth when all behind the barier that blocks the moving object.

## Instructions

### Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

### Settings

On the Illustration tab, you can adjust these parameters:

Animate: Select this check box to start the motion of the dot.

Number of barriers: Select the number of bariers that the dot will be blocked from view by. You can select from 0, or none, to 4 barriers.

You can select from the following pictorial depth cues (select them by clicking or pressing the cue to activate the checkbox):

Relative Height: Change the height in the image of the dot to simulate the depth cue of relative height.
Relative Size: Change the size of the dot to simulate the depth cue of relative size.
Texture Gradient: Add a gradient at the bottom of the drawing area that can indicate a floor. Check the check box first and then adjust the texture gradient floor.

### Reset

Pressing this button restores the settings to their default values and allows you to adjust speed and relative size. It also resets the counter before you can indicate if you have the objects arriving at the same time.