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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Tcl/Tk - why does the canvas widget use a display list?

"Tcl/Tk uses a display list when you draw on a canvas widget." OK - so what does that really mean?

With most of the languages that we teach, any graphics you draw go straight into a frame buffer of a memory map that represents a frame buffer, so the earlier elements added to your graphic will be destructively overwritten by any later elements which happen to land on the same pixel. You have to be careful with drawing order, and if you need to remove a later element, you need to have a way of repairing the hole / repainting part of the screen when you do so

With Tcl/Tk, though, your elements are added into to a separate list in memory, in which all the graphic components are stored, and from which they can be replayed as and when required. This allows - you - for example to pick up and drag a component of your display without the need for a repaint.

Once again - Tcl and Tcl/Tk may be the oldest scripting language(s) that we teach, but they are very very neat and clever!
(written 2009-06-01)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T219 - Tcl/Tk - Drawing on Canvases
  [4460] Using Object Oriented Tcl and the Tk toolkit together - real life example - (2015-03-12)


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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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