On Fri, Jul 1, 2011 at 12:09 AM, John Riedl <ri...@cs.umn.edu> wrote: > Hello. I've been developing a simple turtle graphics package to let > my students play with drawing simple shapes, such as fractals. I've > been struggling with an approach that is efficient for large and > complex drawings. I've tried two approaches: > > 1. Keep a list of the lines that have been drawn as the model. After > each turtle move refresh-now to update the view by redrawing all the > lines. This approach works great, but slows down as the number of > lines grows large, such as for complex fractals. > > 2. Keep a backing bitmap into which the lines are drawn as the model. > After each turtle move refresh-now to copy the bitmap onto the canvas. > This approach also works great, but is always slow for large > canvases. (At least it doesn't get slower over time!) > > In the past I've used drawing packages -- including PLT! -- that had > an xor operator that worked as one would expect, which made the > solution easy: just draw with xor, and then redraw with xor to erase. > The 'xor style in Racket is the same as 'solid, so that doesn't work.
Yes, there is no 'xor anymore. The underlying GUI we use (cairo) doesn't support it anymore. > In the past I've used drawing packages that let me set the region to > be redrawn by the paint routine, which then allows for much more > efficient updating. I don't think that would speed up approach (1), > since all the lines would have to be drawn to compute the clipping, > but it might make approach (2) fast. I don't know how to do that in > Racket. I believe you can do that by just drawing directly into a canvas% object's dc the updated region (and, at the same time, draw into your backing bitmap to handle refresh requests). I'm not positive if that still works in the 5.1 world but I think that it does. the editor classes use this technique; when you're typing in drracket, they only redraw the online line that you're typing on, unless your edit spans multiple lines. > I can easily redraw the turtle with the background color, but that > erases some of the lines, which looks ugly. > > I could instead just copy the part of the bitmap that represents where > the turtle used to be to the canvas, effectively erasing it. With > this approach, I'd quit using refresh-now, and instead just draw > directly to the canvas. This approach seems a little less elegant -- > the model has to stay around for when refresh is actually needed, so > the lines will be drawn twice -- and I'm not sure how efficient it is > to copy part of a bitmap onto a canvas. > > Advice? > > Thanks, > John > > _________________________________________________ > For list-related administrative tasks: > http://lists.racket-lang.org/listinfo/users > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
