linuxsampler-devel

[Linuxsampler-devel] Understanding LV2 Plugin Sourcecode

[Nils T. <nil...@gm...>]

Hi,

because of bug #235 I'm trying to understand how the LV2 Plugin works. 
I'm a wannabe programmer but maybe I have found two things.

1. The function LV2_State_Status Restore() has the parameter flags in 
PluginLv2.h but rflags in .cpp. I think that's a typo?!
Why is there actually a flags parameter when we want to retrieve it 
afterwards (or rflags when we don't use it).

2. LV2_State_Status Save() stores NS_LS "state-file" with flag 
LV2_STATE_IS_PORTABLE but
http://lv2plug.in/doc/html/state_8h.html#afb5cc1033410d51fdcdbfdd09fc7d808
says "Portable values MUST NOT contain filenames."

And two questions:
1. Why don't we store the absolute path? 
http://lv2plug.in/ns/ext/atom/#Path --> "effectively any Path sent to or 
received from a plugin instance MUST be absolute."

2. Why are the old MapPath and MakePath restored after saving and 
restoring the state?

Best regards,
Nils

Re: [Linuxsampler-devel] Understanding LV2 Plugin Sourcecode

[Christian S. <sch...@li...>]

On Friday 16 January 2015 00:13:02 Nils Tonnätt wrote:
> because of bug #235 I'm trying to understand how the LV2 Plugin works.
> I'm a wannabe programmer but maybe I have found two things.

Disclaimer: I did not write the LV2 plugin, Andreas wrote it. And I am not 
into the details of the LV2 API either. I just had a glimpse at the code ...

> 1. The function LV2_State_Status Restore() has the parameter flags in
> PluginLv2.h but rflags in .cpp. I think that's a typo?!

Looks intentional. The "flag"/"rflags" argument is there, because Andreas 
resembled the method prototype with the respective LV2 save/restore function 
callback signatures:

	http://lv2plug.in/doc/html/structLV2__State__Interface.html

So yes, the "flags" argument is not used, but he retained it for clarity with 
the LV2 API. And he renamed the argument "flags" (.h) -> "rflags" in the cpp 
file, because he added a local variable "flags" in the method body which would 
clash with the argument's name. That's no issue. You can name the argument in 
the .cpp file whatever you want. It does not affect the compiled behavior.

> Why is there actually a flags parameter when we want to retrieve it
> afterwards (or rflags when we don't use it).

See above.

> 2. LV2_State_Status Save() stores NS_LS "state-file" with flag
> LV2_STATE_IS_PORTABLE but
> http://lv2plug.in/doc/html/state_8h.html#afb5cc1033410d51fdcdbfdd09fc7d808
> says "Portable values MUST NOT contain filenames."

Well, no idea about the LV2 aspects here. I mean I currently don't know what 
the actual use case of "portable" data in a LV2 session data is for. Or is 
there any LV2 plugin/host out there that allows to store a plugin state on one 
machine and allows to restore it on another machine (with different OS)?

> And two questions:
> 1. Why don't we store the absolute path?
> http://lv2plug.in/ns/ext/atom/#Path --> "effectively any Path sent to or
> received from a plugin instance MUST be absolute."

Because it seems MapPath is used to transform the "abstract" relative paths 
back to absolute paths. Looks valid to me.

> 2. Why are the old MapPath and MakePath restored after saving and
> restoring the state?

No idea about that one. :) Maybe Andreas can bring some more clues into it.

CU
Christian

Re: [Linuxsampler-devel] Understanding LV2 Plugin Sourcecode

[Andreas P. <and...@ou...>]

Christian Schoenebeck skrev den 2015-01-16 00:45:
> On Friday 16 January 2015 00:13:02 Nils Tonnätt wrote:
>> because of bug #235 I'm trying to understand how the LV2 Plugin works.
>> I'm a wannabe programmer but maybe I have found two things.
> 
> Disclaimer: I did not write the LV2 plugin, Andreas wrote it. And I am not 
> into the details of the LV2 API either. I just had a glimpse at the code ...

I wrote the LV2 plugin initially, but the current Save and Restore code
comes from patches by David Robillard (who happens to be the main
inventor of the LV2 standard, so I guess he knows what he's doing...)

>> 2. LV2_State_Status Save() stores NS_LS "state-file" with flag
>> LV2_STATE_IS_PORTABLE but
>> http://lv2plug.in/doc/html/state_8h.html#afb5cc1033410d51fdcdbfdd09fc7d808
>> says "Portable values MUST NOT contain filenames."
> 
> Well, no idea about the LV2 aspects here. I mean I currently don't know what 
> the actual use case of "portable" data in a LV2 session data is for. Or is 
> there any LV2 plugin/host out there that allows to store a plugin state on one 
> machine and allows to restore it on another machine (with different OS)?

I haven't thought about this much, but I guess you want to be able to
save your Ardour song (which includes plugin state) and give it to a
friend who can load it on his/her computer.

>> 2. Why are the old MapPath and MakePath restored after saving and
>> restoring the state?
> 
> No idea about that one. :) Maybe Andreas can bring some more clues into it.

Sorry, I don't understand that either. That's David's code.

/Andreas