For 2021 - online Python 3 training - see ((here)).
Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.
Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
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 articlesT219 - Tcl/Tk - Drawing on Canvases 
Using Object Oriented Tcl and the Tk toolkit together - real life example - (2015-03-12)
Some other Articles
If nothing, make it nothing.Global Index to help you find resourcesKeyboard reading in Perl - character by character not line by lineDripping taps, Java threads and the single thread modelTcl/Tk - why does the canvas widget use a display list?MySQL server replicationMonitoring and Tuning your MySQL installationA beautiful coastlineNorth Antrim Coast - a jewel thats well worth a visitSpot the odd one out
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