Don't let your eyes fool you:

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1 1 Don't let your eyes fool you: Your eyes can be fooled by an Optical Illusion. Webster's Dictionary defines an optical illusion as: "A visually perceived image that is deceptive or misleading." We can use Squeak Etoys to show when our eyes are being tricked. This project 2will give you a lot more experience with: Changing object color Naming objects Resizing objects Scripting The unique properties of a Playfield Changing an object's heading Making objects bounce That weird "obtrudes" tile 3 Drag a playfield from the supplies flap. A playfield is a great place to build an animation. Bring up the "Halo of Handles" with a right click of your mouse. Use the sizing handle to make your playfield into a rectangle. We'll be using the sizing handle a lot in the next few steps. 4

2 Drag a rectangle 4from the supplies flap and resize it to cover half the playfield. Now bring back the Halo of Handles and use the "Change Color" handle to make this rectangle and its border completely black. drag here to set border width 5 Change the color of the duplicate to white (don't forget to eliminate it's border by setting its border width to "0") and rename it. Then arrange it so it covers the other half of the playfield. Rename the rectangle (we named ours "Black") and use the "Duplicate" handle to make an exact copy. 6 Drag another rectangle from the supplies flap and resize it to a small square that fits easily into the larger black and white rectangles in the playfield. Change the color of this little square to gray and name it Square A. 7 Now duplicate Square A and call the duplicate Square B. It's very important that you use the "duplicate" handle for this operation because we want these two squares to be perfect twins

3 7 Place one 8square on the black rectangle and the other on the white. Now take a good look at these identical twins. What do your eyes tell you? The gray square on the white looks darker than the one on the black! Is it really darker? Switch the squares. Does the square on the white still appear darker? Let's animate these squares and see if they change or if our eyes are fooling us. Bring up the halo of handles on Square A and click on the "Viewer" 9handle. From the viewer drag out and drop the "Square A forward by" tile. When you drop it a new script is created. Set the "forward by" value to whatever you like, we left ours at "5". Now drag out a detailed watcher for Square A's Heading and drop it outside the playfield. 10 Click here to get a menu

4 Tear off a test page from the script box and drop it on the script. Open Square A's viewer and grab a "Square A obtrudes" tile from the "test" category. Drop it next to "Test" on the test page. When we do this we are getting ready to make Square A do something when it "obtrudes" upon or "imposes on" the boundary of the playfield. Test Pages What shall we make Square A do? Observe the detailed watcher for Square A's heading. Heading is direction of travel. If Square A's heading value is 90, which way will it travel? From the viewer, grab the heading value tile by the green "assignment arrow" and drop it on the "yes" section of the test page on your Square A script. 0 Dictionary.com says: ob-trude v. To impose on others with undue insistence or without invitation Now make a similar script for Square B. Try increasing the heading by What do you think will happen? What happens if you set the Square B heading to -90? Try it! When your scripts are ready, center Squares A and B on their respective rectangles. From the supplies flap drag out a set of "Stop, Step, Go" buttons to control both scripts. Use the green up/down arrows to select the statement "Square A's heading increase by". How much must Square A's heading increase in order to make Square A completely reverse direction?

5 1314 Hit "go". Watch squares A and B as they traverse the playfield and seem to change their tone as they move from the black background to white and back. What happens when they cross over each other? What happens when they separate again? Observe the heading watchers as the squares "obtrude" on the playfield boundary. What happens? 15 Now you know something about changing heading and about a special property of playfields. You've gained some cool insights into optical illusions. With all this new knowledge you ought to be able to get squares A and B to travel up and down on your screen. Can you do it?