Laboratory: Scripting the GIMP Tools

Here are the instructions for drawing a simple face from the reading.

(define smiley (image-new 200 200))
(image-show smiley)

; Draw the primary circle
(image-select-ellipse! smiley REPLACE 
                       10 10 180 180)
(context-set-fgcolor! "yellow")
(image-fill-selection! smiley)

(context-set-fgcolor! "black")
(context-set-brush! "Circle (09)")
(image-stroke-selection! smiley)

; Draw the eyes
(image-select-ellipse! smiley REPLACE 
                       50 60 30 20)
(image-select-ellipse! smiley ADD
                       120 60 30 20)
(context-set-fgcolor! "white")
(image-fill-selection! smiley)

(context-set-fgcolor! "black")
(context-set-brush! "Circle Fuzzy (07)")
(image-stroke-selection! smiley)

(image-select-ellipse! smiley REPLACE
                       60 60 10 20)
(image-select-ellipse! smiley ADD
                       130 60 10 20)
(context-set-fgcolor! "lightsteelblue")
(image-fill-selection! smiley)

; Smile
(image-select-ellipse! smiley REPLACE 
                       40 60 120 100)
(image-select-ellipse! smiley SUBTRACT
                       40 45 120 100)
(context-set-fgcolor! "white")
(image-fill-selection! smiley)

(context-set-fgcolor! "red")
(context-set-brush! "Calligraphic Brush#3")
(image-stroke-selection! smiley)

; Get ready to show
(image-select-nothing! smiley)
(context-update-displays!)

a. Copy and paste these instructions into your definitions pane. Read through the instructions to see what image you predict they will draw.

b. Unfortunately, the GIMP's list of brushes has changed since we created those instructions. Use (context-list-brushes) to see the names of the available brushes. Then, change any calls to context-set-brush to use one of those brushes.

c. Click Run and observe what happens. If all has gone well, you should see a new image with a smiling face.

d. In the interactions pane, write instructions for drawing a nose onto the image. Your instructions might look something like the following:

> (image-select-rectangle! smiley REPLACE 90 90 20 20)
> (image-select-ellipse! smiley INTERSECT 80 85 30 30)
> (context-set-fgcolor! "green")
> (image-fill-selection! smiley)

If the instructions don't seem to change the image, try clicking on the image window or typing (context-update-displays!).

e. It is possible to clear an image by selecting everything and then clearing the selection. Do so now.

> (image-select-all! smiley)
> (image-clear-selection! smiley)
> (context-update-displays!)

f. Sometimes it's more fun to see an image drawn little-by-little. Instead of clicking the Run button, you can copy and paste sections of the code from the definitions pane to the interactions pane.

Try this for the first few instructions in the image. We'd recommend that you copy sections that end with image-fill-selection! or image-stroke-selection!. You may have to call context-update-displays! to see the change.

a. Open a new DrRacket window or tab and write a series of instructions in the definitions pane to draw a face different than the sample we gave above. You should try a different shape of face, different brushes or colors, and different facial features.

b. What do you expect to happen if you change the size of the image in which you're drawing, such as a 100x100 image or a 300x300 image?

c. Check your answer experimentally.

d. If changing the size of the image yielded results that you found undesirable, summarize how you might fix them. (Don't fix them right now, just indicate how you might fix them.)

e. Now, suppose that we define two names, width and height. Describe how you might change your face code to take those values into account.

(define width 300)
(define height 200)
(define canvas (image-new width height))
...

(If you've learned procedures, you can also consider how to do this using a procedure.)

f. It's not worth the time to update all the parts of your face. Pick a few parts (say, the outline of the face and one eye) and update those parts to take the width and height of the image into account.

Suppose we've defined picname as a string naming an image.

(define picname "/home/rebelsky/glimmer/samples/rebelsky-stalkernet.jpg")

a. Write a series of instructions to load and show the picture.

b. Write a series of instructions to draw a diagonal line from the upper-left-hand-corner to the lower-right-hand corner of the image. Note that you'll need to use image-width and image-height to determine the width and height of the image.

c. Write a series of instructions to put some kind of border at the edges of the image. (E.g., select a rectangle ten pixels from each edge and then stroke that rectangle.)

a. Write a series of instructions that create a new image and then draw a house in in the image. (Yes, you are restricted in that you don't know how to make triangles. Deal with it.)

b. Suppose we start by defining two names, width and height. Describe how you might change your house code to take those values into account.

(define width 300)
(define height 200)
(define canvas (image-new width height))
...

(If you've learned procedures, you can also consider how to do this using a procedure.)

Revise your instructions from Exercise 2 so that the drawing adapts to the size of the image.

Recall that you can obtain the width and height of the image using the image-width and image-height procedures.