[GEM-dev] tutorials for extending Gem

[IOhannes m zmoelnig]

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On 2013-10-29 13:39, Claude Heiland-Allen wrote:
> Hi,
> 
> I want to write a new Geo.
> 
> Looking for a tutorial, I found the documentation at 
> gem/doc/manual/WriteCode.html which essentially says "use the
> source, Luke".  But 140k lines of code are quite daunting...

ahja, this document has not been touched since i took over Gem 13 or
so years ago...

> 
> Where should I start?

derive your object-class from GemBasse


> I need 3 methods, respectively called

> after OpenGL context creation,

virtual void startRendering();

> at render time

virtual void render(GemState *state);


> and before context destruction.

virtual void stopRendering();


to there's also a
 virtual bool isRunnable();
that get's called (usually once) for each context so you can add some
checks whether the current openGL-context provides everything you
need: if isRunnable() returns FALSE, the object will not be part of
the rendering chain. (this is mainly to prevent continuous triggers of
invalid calls)


start/stopRendering is not really tied to context creation/destruction
but to turning rendering on/off (which has been enough for most
use-case so far)
once the multi-context support is finished, stopRendering() will also
be called if a context is destroyed.


in order to make your object multi-context aware, you should use
gem::ContextData for any per-context variable.

something like:
  gem::ContextData<GLenum>m_id;

in most cases, you can use `m_id` just as if it was a GLenum (that is:
the template-argument type), but it will hold a different value for
each context.


hope this helps.

fgmasdr
IOhannes
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