Hi,
Avoid 'minkowski' transformations. If a 'minkowsky' is not avoidable do it
in 2D and then extrude.
The 'hull' transformation can be very slow also.
If you have a composite object and you need view fluidity you may render the
modules that are completed an tested, starting those modules like this:

module foo() render convexity=x // x usually =2
{
code
}

However since cache saving in not an option in OpenSCAD yet, when initiating
a new editing session you will have to painfully wait  until all modules are
rendered again.
Good luck.

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Also if there are sub-parts that are repeatedly used (and stable with fixed
sizes) you can export them as STL files, and import the STL, thus the
geometry of the parts does not need to be calculated every time.

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...and don't use high $fn etc values for initial development.
Increase it when you are close to finished.
As sometimes different $fn can cause problems with some designs, you may
need a little further debugging on occasion. (mainly changes to how
cylinders/sphere join)

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Sorry, I forgot the 'render' round brackets.

module foo() render(convexity=x) // x usually 2
{
code
}

jpmendes

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Something went wrong with the initial posting.

The question was what can be done to speed up the rendering. Are there
programming rules for that?

http://forum.openscad.org/file/n14997/Bell_2.jpg

This is the object that I want to make: the upper side of the bell of a bass
clarinet.
What I do is draw the complete outside bell en substract the complete inside
bell using the difference function. I found out that difference is a lot
slower than union which I use in the test phase.

The bell I first have made as 150 cylinders with a specific diameter. Each
one rotated around a certian angel. But hat did not give a good result.
Now i make a 150 cylinder composed of a hull of very thin rotated cylinders.

I am afraid the difference and the hull function makes it slow.

Any idea for another way to draw this?

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