In helping students with their projects, we've developed a number of techniques that other students may find useful. This document gathers some of those techniques.

As you may have noted, image-select-rectangle! and image-select-ellipse! will not work unless the width and height of the area selected are at least 1. > (image-select-rectangle! 80 REPLACE 100 200 0.5 0.5) image-select-rectangle!: width and height must be at least 1 When writing recursive procedures that select areas, you may end up with a width or height that is too small. You have two obvious solutions: (1) You can avoid the selection (and corresponding actions) altogether or (2) You can ensure that any selection you do has a width and height of at least one. You can use when to ensure that you only call the selection procedure (and anything else) when width and height are at least 1. For example, (when (and (>= width 1.0) (>= height 1.0)) (image-select-rectangle! canvas REPLACE left top width height) (image-fill! canvas) When we simply need to use 1 when width or height is smllaer than 1, we can use the old trick of using max. (image-select-rectangle! canvas REPLACE left top (max width 1) (max height 1)) (image-fill! canvas)

Right now, if you try to stroke or fill something, and there's nothing selected on the image, you get an error message or inappropriate behavior. > (image-select-nothing! canvas) > (image-stroke! canvas) gimp-edit-stroke: failed to execute What's the solution? You can check whether anything is selected with the image-has-selection? procedure. (when (image-has-selection? canvas) (image-stroke! canvas)

The undocumented selection-compute-pixels! procedure (which takes the same parameters as image-compute-pixels!) provides a handy way to build a color blend in a small section of an image. > (image-select-ellipse! canvas REPLACE 100 30 150 40) > (selection-compute-pixels! canvas (lambda (col row) (rgb-new col row col)))

We've found that it is quite easy to draw an N-sided regular polygon with a turtle. We simply repeat the steps of moving forward a particular amount and turning by 360/N. But turtles only draw the outline of a polygon. What if we want to select the polygon? We start by finding a list of the vertices in the polygon. We then use (image-select-polygon! image selection-type) to select them. Once the polygon is selected, we can do what we want with it. But how do we get the list of vertices? We can calculate the vertices. We can also use turtles to draw the polygon, and ask them to report back on their positions.

One technique is to compute the positions of the vertices of the polygon. It is easiest to compute those positions from the center of the polygon. In that case, we can just think of the angle to each vertex, use sine and cosine to break the angle into horizontal and vertical components, and scale and shift appropriately. But the angles should be obvious. For example, for a hexagon they are pi*1/3, pi*2/3, pi, pi*4/3, and pi*5/3. That may be a bit abstract, so here's a procedure to make a list of the positions in a hexagon. You can think about how to generalize it for other regular shapes. It's fairly straightforward to use the procedure.