This reading is also available in PDF.
Summary: We consider another model of image, one that involves
giving instructions based on the core tools associated with the GIMP.
Contents:
As we've noted in the past, in order to write algorithms, one must
also have a representation for the manipulated data and a collection of
primitive procedures with which to manipulate those data. For example,
for colors, we have built-in procedures to create colors (e.g.,
rgb.new), extract components (e.g., rgb.red),
and to modify them in fixed ways (e.g., rgb.darker).
But both the representation and the collection of related procedures
are motivated by a deeper issue, the model we use to think
about the data. For example, we have seen two simple ways to think about
images: We can think of images purely as grids of pixels or we can
think of images as collections of positions and colors, which can be
rendered on a grid of pixels in a variety of ways.
But there are certainly other ways to think about images. We might
also describe an image in terms of the sequence of actions used to
create the image. For example, most people who draw with the GIMP do
so by selecting appropriate tools (in sequence) and using those tools
and pointing device to draw the image. We might therefore represent
images by giving sequences of commands that a human might carry out,
but that a computer might also carry out. It is this model that we
consider in the reading.
In particular, in the GIMP, as in most painting programs, artists and
designers create new images by selecting appropriate tools and using those
tools to draw. Once a base image is created, they may also modify that
image using a variety of filters. Since we've already explored ways to
write filters, we'll take a step backwards and look at the basic tools
for creating images.
If we were using Scheme to communicate with other humans (a scary concept,
isn't it?), we might write something like the following to tell someone how
to draw a simple variant of our ubiquitous smiley face.
(define smiley (image.new 200 200))
(image.show smiley)
; Draw the primary circle
(image.select-ellipse! smiley selection.replace
10 10 180 180)
(envt.set-fgcolor! color.yellow)
(image.fill! smiley)
(envt.set-fgcolor! color.black)
(envt.set-brush! "Circle (09)")
(image.stroke! smiley)
; Draw the eyes
(image.select-ellipse! smiley selection.replace
50 60 30 20)
(image.select-ellipse! smiley selection.add
120 60 30 20)
(envt.set-fgcolor! color.white)
(image.fill! smiley)
(envt.set-fgcolor! color.black)
(envt.set-brush! "Circle Fuzzy (07)")
(image.stroke! smiley)
(image.select-ellipse! smiley selection.replace
60 60 10 20)
(image.select-ellipse! smiley selection.add
130 60 10 20)
(envt.set-fgcolor! "light steel blue")
(image.fill! smiley)
; Smile
(image.select-ellipse! smiley selection.replace
40 60 120 100)
(image.select-ellipse! smiley selection.subtract
40 45 120 100)
(envt.set-fgcolor! color.white)
(image.fill! smiley)
(envt.set-fgcolor! color.red)
(envt.set-brush! "Calligraphic Brush#3")
(image.stroke! smiley)
; Get ready to show
(image.select-nothing! smiley)
What's going on here? The model for much of the drawing in the GIMP
is that you select portions of the image and then either fill them
or stroke (trace the edges of) them. We draw the face by selecting
appropriate sections, colors, and brushes, and then stroking or filling,
as appropriate. We discuss each of these operations in the following
sections.
Before looking at the commands that place digital ink on the canvas,
As the example above suggests, before putting digital ink in the screen,
it is important to set contextual information that guides how the GIMP
interprets drawing actions. The most important contexts for drawing
are the foreground color, the background color, and the
brush.
When you draw in a painting program, the program often assumes that
you want to draw using selected brush. As is the case with physical brushes,
different digital brushes give very different effects. For example,
a square brush across the image will give you a very different kind of
line than will a fuzzy circle.
In DrFu, there are a few primary procedures for working with brushes.
-
(brush.list) gives a list of the names of all the available brushes.
-
(brush.list name) gives a list of the names of all the available brushes that include name. For example, (brush.list "circle") lists all the brushes whose name includes the word "circle".
-
(envt.set-brush! "brush-name") sets the working brush.
We recommend that you explore the strengths and weaknesses of each brush
for a variety of situations.
You've already encountered the foreground and background colors, which you
set with envt.set-fgcolor! and envt.set-bgcolor!.
While these procedures work best with RGB colors, you can also use them
with color names. (Behind the scenes, both procedures convert the name to
an RGB color.)
As the example suggested, a lot of simple drawing can be done by selecting
interesting regions and then stroking or filling those regions. There
are two basic operations for selecting regions, image.select-ellipse! and image.select-rectangle!. The two procedures take exactly
the same list of parameters:
-
image, the image in which to select a region;
-
operation, the way in which we want to update the selection;
-
left, the left edge of the selected region;
-
top, the top edge of the selected region;
-
width, the width of the selected region; and
-
height, the height of the selected region.
For ellipses, left and top represent the left
edge and top edge of the rectangle that bounds the ellipse.
Except for operation, all of the remaining parameters should
be obvious. The operation stems from an interesting, but useful, design
decision in the GIMP: Often, we build more complex images by selecting
more complex regions. However, if our only building blocks for describing
selections are rectangles and ovals, we need appropriate ways to combine
those building blocks. That's where the operation comes into play. The
operation can be
-
selection.replace, in which case the old selection is
replaced by the new selection;
-
selection.add, in which case the old selection is extended
by the new selection, even if the two seletions are non-contiguous;
-
selection.subtract, in which case any pixels in the the
old selection that are also in the new selection are de-selected; and
-
selection.intersect, in which case only pixels that
are in both the old and new selections remain selected.
There are also a few procedures that permit one to select more broadly:
-
(image.select-nothing! image) deselects everything.
-
(image.select-all! image) selects everything in the image.
Once you've made an appropriate selection, what can you do with that
selection? Only a few things. You can fill the interior of the image
using the current foreground color with (image.fill! image).
You can trace the exterior of the image with (image.stroke! image).
You've seen examples of both commands in the example above. The third
possibility is that you can clear
the selection with
image.cleaer-selection!. In simple images,
clearing a selection results in the selection getting filled by the
background color. In layered images, clearing the selection may reveal
images from lower layers.
Because designers may not want all of the changes to an image to appear
piecemeal, the GIMP does not automatically update the displayed version
of the image after one of these commands. The updates only become visible
after either (a) you click on the image or (b) you call
(envt.update-displays!).
While a lot of drawing can be done with selection, stroking, and filling,
it is also useful to have other tools available. DrFu provides two
other simple GIMP commands that focus on simple kinds of drawings.
(image.blot! image col row) draws a single
spot at the specified position using the current brush and foreground color.
(image.draw-line! image c1 r1 c2
r2) - draw a line from (c1,r1) to
(c2,r2) using the current brush and foreground color.
Neither of these procedures can easily be simulated with the selection
approach to drawing, so they provide a useful addition to the algorithmic
designer's toolbox.
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