general question about CAD/drafting programs

Special K

Diamond Member
Jun 18, 2000
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I have used several drawing or CAD programs that for one reason or another, require you to manually refresh the screen every so often to reflect changes made to your design. For example, if I am using a program to create a schematic and drag a part across the screen and over other wires, parts, etc, parts of those wires will appear to have been "erased" by the part that was drug across them. It will then look like this until I manually issue a refresh command through the program, which will then redraw the whole screen and reflect any changes I have made. My question is why can't the screen just update in real time? Programs that I have noticed this in include Orcad layout and Cadsoft EAGLE (PCB layout software) and Pro/CDM (2D version of Pro/E). All of these programs have relatively simple 2D interfaces, and I have run all of them on systems with relatively fast video cards, yet they all exhibit the behavior described above. Why does this happen when we are able to create 3D games that run at 100+ fps at high resolutions with no noticeable choppiness?