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Transforming Pictures

This activity has the following desired goals:

  • Learning to transform pictures (A, M).

Step 1

Type in the following code and run it:

val pic = trans(100, 0) * rot(45) * penColor(blue) -> Picture.rectangle(100, 50)

Q1a. What does the above code do? Explain to a friend.

Step 2

Type in the following code and run it:

val pic = Picture.rectangle(100, 50)
pic.translate(100, 0)

Q2a. What does the above code do? How is it different from the code in step 1? Explain to a friend.


You can transform pictures in the following main ways (via a tranformation object/function or a transformation method/command):

Transformation Object/function Method/command
rotate rot(angle) -> pic pic.rotate(angle)
scale (to make bigger or smaller) scale(f) -> pic pic.scale(f)
translate (in its local coordinate system) trans(x, y) -> pic pic.translate(x, y)
translate (in its parent’s coordinate system) offset(x, y) -> pic pic.offset(x, y)
change pen color penColor(color) -> pic pic.setPenColor(color)
change pen thickness penThickness(t) -> pic pic.setPenThickness(t)
set no pen noPen -> pic pic.setNoPen()
change fill color fillColor(color) -> pic pic.setFillColor(color)
set opacity opac(o) -> pic pic.setOpacity(o)
position at a given location   pic.setPosition(x, y)
rotate to a particular heading   pic.setRotation(angle)
rotate to a particular heading (alternative way)   pic.setHeading(angle)
scale to a particular size   pic.setScale(scale)

As shown above, there are two distinct ways of doing picture transformations:

  • The object/function way, e.g., trans(100, 0) -> pic - which is useful while doing functional/compositional graphics. This way of doing transformations can be used only before a picture is drawn (and these transformations are applied when the picture is drawn).
  • The method/command way, e.g.. pic.translate(100, 0) - which is useful while doing generative art or gaming. This way of doing transformations can be used before and after a picture is drawn.

Multiple transformations can be combined in the following ways:

  • tranformation objects/functions are combined using *, e.g., trans(100, 0) * rot(45) -> pic
  • transformation methods/commands are combined by sequential calls, e.g., pic.translate(100, 0); pic.rotate(45)

Step 3

Type in the following code and run it


def square(n: Int) {
    repeat(4) {




Q3a. How can you make the second square not blue?

Now type in the following code and run it:


def sq = Picture.rectangle(100, 100)

val pic1 = fillColor(blue) -> sq
val pic2 = sq

val pics = picStack(pic1, trans(100, 0) -> pic2)


Q3b. What do you think the picStack instruction does? Is it a command or function?

Q3c. How is the second piece of code (which is picture-graphics based) in this section different from the first piece of code (which is turtle-graphics based)?
Hint. Think in terms of expression vs command oriented instructions.


Use all the picture transfomation functions listed above to create and transform one or more rectangles.

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