Collage

We've started to see that we can think of image making in many different ways. For example,

There are, of course, many other models of image making. In this section, we consider collage, a model of image making that relies on taking parts of other images and putting them together into a coherent (or semi-coherent) whole. Collage has regularly been used in a wide variety of situations, from kindergarten masterpieces to Warholesque pop art. It should therefore be part of your own toolbox.

Computers are certainly a natural domain for collage. The “cut”, “copy”, and “paste” operations are part of every modern computer application. We will consider the particulars of using these operations in Mediascheme.

Unfortunately, the designers of Mediascheme and gigls neglected to include copy and paste operations. Fortunately, Mediascheme and gigls are built upon a lower-level library, the Gimp Procedural Database, or PDB. (Mediascheme and gigls also depend on some other custom code, but a wide variety of operations are just nicer interfaces to PDB operations.) You can get a list of all of the PDB operations by selecting Procedure Browser from under the Help menu. You will then see a window that presents a list of procedure names. When you click on one, you will get documentation.

If we type “copy” in the search box, we see that there are nine procedures that relate to copying.

The two most relevant procedure are gimp-edit-copy and gimp-edit-copy-visible. The former takes a “drawable” as a parameter, the latter takes an image. What's a drawable? For those of you who have used graphics programs before, it's easiest to think of “drawable” as being another word for “layer”, but it's really a class of things that include layers. If you've never used a graphics programs before, layers can be thought of a stack of possible-transparent sheets that you stack on top of each other to make an image. In most cases, it will suffice to use gimp-edit-copy-visible, but it helps to know that other options are available.

We now know how to find out what procedures GIMP makes available and have started to figure out how to read them. Let's play with copying and pasting. We'll copy a portion of our favorite image.

> (define kitten (image-show (image-load "/home/rebelsky/Desktop/kitten.jpg")))
> (image-select-ellipse! kitten REPLACE 300 200 100 100)
3
> (gimp-edit-copy-visible kitten)
'(1)

That strange '(1) that gimp-edit-copy-visible returned is its way of marking success.

Here's what the screen looks like so far.

Both gimp-edit-copy and gimp-edit-copy-visible say that you can paste the copied selection with gimp-edit-paste. What does gimp-edit-paste take as parameters? The documentation says that we need to provide a “drawable” and an integer. As we noted above, the drawable is just the layer, which we can get with image-get-layer. The integer will normally be 1, which means “paste over whatever was there before”.

So, let's try pasting.

> (image-select-ellipse! kitten REPLACE 0 0 100 100)
3
> (gimp-edit-paste (image-get-layer kitten) 1)
'(12)

What's the '(12) that was returned? It's a list containing a drawable that corresponds to the newly pasted stuff. Why do we get a drawable? It's a long story. Suffice to say that it allows us to do some cool things, but makes our life a little bit more difficult.

Believe it or not, but the drawable is not yet “stuck” on the image yet. We need to eliminate our selection. We also need to “flatten” the drawable, which we do with the following instructions.

> (image-select-nothing! kitten)
3
> (gimp-image-flatten kitten)
'(22)

The '(22) at the end says that the one layer in kitten is identified as layer 22. (Yeah, GIMP likes to number everything, and the numbers aren't always obvious.)

While we've succeeded at copying and pasting, it feels like we should be able to do some more creative variations. We did note above that once you have a drawable, you can do some interesting other things with it. One possibility, as the title of this section suggests, is that you can scale a selection, making it larger or smaller.

How do you scale a selection? With the gimp-layer-scale procedure, which takes four parameters: the layer you want to scale, the new width you want, the new height you want, and a number to indicate what the scaling is around. (We normally use 1 to scale around the center of the selection.

Here's a series of operations that produce a similar image to above, but with the pasted carpet scaled horizontally by 200% and vertically by 150%.

> (define kitten (image-show (image-load "/home/rebelsky/Desktop/kitten.jpg")))
> (image-select-ellipse! kitten REPLACE 300 200 100 100)
6
> (gimp-edit-copy-visible kitten)
'(1)
> (image-select-ellipse! kitten REPLACE 0 0 100 100)
6
> (gimp-edit-paste (image-get-layer kitten) 1)
'(38)
> (image-select-nothing! kitten)
6
> (gimp-layer-scale 38 200 150 1)
'()
> (gimp-image-flatten kitten)
'(39)

And here's the image that results.

What else can we do? We can rotate selections that we've pasted using gimp-item-tranform-rotate, which takes five parameters. The first parameter is the identifier associated with the pasted image. The second parameter is the angle we want to rotate the selection. It's easiest to use 1 0 0 for the other three parameters, but you can read the documentation for more information.

> (define kitten (image-show (image-load "/home/rebelsky/Desktop/kitten.jpg")))
> (image-select-ellipse! kitten REPLACE 100 100 200 100)
7
> (gimp-edit-copy-visible kitten)
'(1)
> (gimp-edit-paste (image-get-layer kitten) 1)
'(43)
> (image-select-nothing! kitten)
7
> (gimp-layer-scale 43 150 75 1)
'()
> (gimp-item-transform-rotate 43 45 1 0 0)
> (gimp-image-flatten kitten)
'(44)

We'll let you imagine what image results.

Here's the normal list of steps for copying and pasting.