Turtles and Colors

You’ve already seen the turtle, quietly sitting in the center of the Graphics panel, patiently waiting for you to tell it what to do. Well, the waiting is over! You’re about to enter the fascinating world of turtle graphics, a special way of drawing interesting, exciting, and colorful graphics on your computer screen by telling a turtle what to do. Not a real turtle, of course. The Logo turtle is a special graphics cursor that can move forward or back, rotate right or left, change color, change size, change shape and even disappear. There’s even more, but let’s do something now.

Type FORWARD 100 and watch what happens. The turtle moves in the direction that its head is pointing; the line it draws is 100 turtle steps long (that’s how turtles measure distance).

Now, type RIGHT 90 and you’ll see the turtle rotate like it’s peeking around a corner. Turns are measured in degrees, but they’re not called turtle degrees because that’s how turns are measured for just about everything, from boats and planes to spaceships.

If you type these two instructions again and again and again, you’ll get pretty tired of typing! Here’s a shortcut: type FD 100 RT 90 all on the same line before you press Enter. FD is short for FORWARD and RT is short for RIGHT. Next, just press the up-arrow key to recall the command line into the input panel and then press Enter. What could be easier? You’ll see.

Repeat

There is a very simple way to repeat instructions without typing the same thing over and over again. It’s the REPEAT command. But before you can use it, you need to learn a little bit about a special type of Logo data called a list. A list starts with ‘[’ and ends with ‘]’. In between the brackets is where you put the instructions you want repeated. Lists are used a lot in Logo but that’s enough to know for now.

The REPEAT command needs a number and a list. The number tells it how many times to repeat and the list tells it what to repeat. So, to repeat the earlier drawing, type REPEAT 4 [FD 100 RT 90]. That’s a lot easier than typing the instructions four times.

Don’t forget about the up-arrow key: it’s still worth using if you make a mistake or if you just want to change a previous line of instructions. Let’s say you want to change the REPEAT we just used so that the turtle does the opposite of what it did last time. The opposite of FD is BK (short for BACK) and the opposite of RT is LT (short for LEFT). See what happens when you change both instructions to their opposite and then change just one at a time. Is this a pain? It should be. Try this next: REPEAT 3 [PRINT “HA].

If your Graphics panel is getting a little messy, just type CS to clean it up and start drawing again. (CS is short for CLEARSCREEN.) You can clean up the Listener panel by typing CT (short for CLEARTEXT).

More About the Turtle

There are many Logo commands for the turtle. You can’t learn them all at once, but it makes sense to learn them in pairs or groups. Commands that are opposites are easy to learn at the same time. You already know the FORWARD and BACK opposites and the RIGHT and LEFT opposites. Here are some others you can try out:

PENUP or PU - the turtle won’t draw lines now
PENDOWN or PD - the turtle will draw lines again

Where’s the pen? It’s right in the middle of the underside of the turtle. If the pen didn’t go up and down, you’d have a hard time making your drawings look nice. Try it with a pencil and some paper and you’ll see what I mean. Does your pencil have an eraser? The turtle’s pen does.

PENERASE or PE - erases a drawing

Erasing in the Graphics panel is not exactly the same as erasing pencil lines from paper but the effect is the same - what was there is now gone. To see this happen quickly, try FD 100 PE BK 100 but don’t forget to put the pen back down when you’re done! Here’s one more pen trick.

PENREVERSE or PX - inverts the colors under the pen

This special pen flips colors when it passes over them. In computer speak, the color’s bits are inverted; black becomes white, red becomes yellow, and more. Try to create a drawing, set the turtle’s pen width to, say 10, and draw over your drawing to see interesting color effects!

If the turtle is moving too quickly to follow, type SLOWTURTLE to slow it down. Type SETSPEED 1 to restore normal speed.

The Turtle Continued

When you finish a drawing, do you leave your pencil in it?

HIDETURTLE or HT - the turtle disappears
SHOWTURTLE or ST - the turtle reappears

Hiding the turtle makes drawing faster and gets the turtle out of the way. Showing the turtle helps you see which way it’s heading and where it is. If your turtle gets lost, type HOME to quickly get the turtle back to the center of the Graphics panel. If you still don’t see it, type ST for SHOWTURTLE.

When the point of your pencil gets dull, the lines get wider. The turtle’s pen never gets dull but you can change its width from 1 to 99. Width 1 is the default. You may not even see the turtle with a really wide pen!

SETWIDTH or SETW - changes the pen width (1-99)

With all the commands you know so far, you can make some interesting special effects. The star illustration looks like a really complicated thing to create. In fact, the star was drawn with SETW 10 and then erased with SETW 4.

PD SETW 10 REPEAT 5 [FD 100 RT 144]
PE SETW 4 REPEAT 5 [FD 100 RT 144]

See what you can do. Just play around.

Add Some Color

Coloring with the turtle is simple and fun! You can choose from endless colors, but let’s keep it simple for now.

SETPC - sets the pen color (0-137)

SETPC needs a number from 0 to

1. Try any color you want. Pick your favorite.
   
   What about SETPC 15? Try it. When Logo starts up, the Graphics screen is white. If you draw with the same color as the background, it’s like not drawing at all. What if you want to draw in white?
   
   SETBG - sets the background color (0-137)
   
   SETBG needs a number from 0 to 137 just like SETPC.
   Do you want a green square? Or, a blue star? Just change the pen color and then use the up-arrow key to recall a previous command line and press Enter.
   
   Can you make a red diamond? A diamond is really just a square that’s turned sideways a little.

A Logo color is a three-element list. Each element is a value between 0 and 255, which stands for the intensity of one of the colors red, green, and blue. The first element is the Red value, the second is the Green value, and the third is the Blue value. This color model is often also called the RGB model. Guess why?

The color black would, therefore, be [0 0 0] (all base colors turned off), and white is [255 255 255] all three colors at full strength). A bright red would be [255 0 0]. You get the idea?

This page contains a list of the standard colors, their names, their indexes, and their RGB values.

Luckily, all color commands accept either a color index, an RGB list, or a color name. So, these commands are fully equivalent:

SETPC 60
SETPC [255 215 0]
SETPC "GOLD

To ease the conversion, Logo has the COLOR command, which reports the RGB color, the COLORINDEX command which reports a color index, and the COLORNAME command, which reports a color name. Just try this:

COLOR “GOLD Result: [255 215 0 1]

But wait! Why are we getting four instead of three values?

Well, Logo can do much more with colors! The fourth value is the alpha or transparency value. This is a value between 0 and 1 that indicates how transparent the color is. 0 means that the color is so transparent that it is invisible, and 1 means no transparency at all. If you do not use that value, Logo assumes a fully opaque color (the value 1).

You can set the value that Logo should use if you do not specify an alpha value in the color list with the SETALPHA command. This lets you use color names or indexes even with transparency. Now imagine how much fun you can have if you start blending colors together! The overlapping circles to the left are easily created with these commands:

PU SETX -30 PD SETPC (LIST 255 0 0 0.3) (STAMPOVAL 50 50 TRUE)
PU SETX 30 PD SETPC (LIST 0 255 0 0.3) (STAMPOVAL 50 50 TRUE)
PU SETXY [0 50] PD SETPC (LIST 0 0 255 0.3) (STAMPOVAL 50 50 TRUE)

This extended RGB color model is usually called RGBA. Surprise!

The Toolbox panel has a “Colors” panel. From the Colors panel, you can use your mouse to drag a color and drop it on the Graphics panel to set the background color, or you can drop a color on a turtle to set it’s pen color.

Turtle Shadows and Colors

Every turtle has a shadow. It is initially invisible, but you can turn it on. A turtle’s SHADOW property determines how far a turtle’s shadow is away from the turtle itself. Usually, this is the value 0, which means that the shadow is invisible. If you, for example, set the turtle’s shadow to be, say, five pixels apart, you would enter this command:

SETSHADOW 5

And all of a sudden, the turtle would have a shadow!

Instead of a shadow, you could also make the turtle “glow” by setting its GLOW property together with the SHADOW property:

PPROP 0 "GLOW TRUE

And the turtle would “glow” with a grey glow.