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6028 :: FFI questions - VapourSynth

• mantielero (orginal) [2020-03-03T23:15:45+01:00] view original

  I am just playing around with FFI just for my learning (I am a newbie). I have the code here. I will put in this thread my questions.

  My first question is about how to make an struct with methods:

  • C
  • Nim

  Is this OK?

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• yglukhov (orginal) [2020-03-04T11:30:47+01:00] view original

  Proc types are {.closure.} by default i.e. fat pointer. You want {.cdecl.} instead:

  
  type:
     VSAPI*  {.importc: "struct VSAPI", header: headerVapourSynth, bycopy.} = object
         createCore*: proc(threads:cint):ptr VSCore {.cdecl.}
         ....
  Run

  Also as long as you exactly match the struct layout to that of C, you don't need importc and header pragmas for the type, and thus don't depend on the headers at all.

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• mratsim (orginal) [2020-03-04T12:14:13+01:00] view original

  It's been a while since I've done video encoding but I would love to have a VapourSynth wrapper

  • for reasons I explain here: https://github.com/abhiTronix/vidgear/issues/86
  • to use it as preprocessing for deep learning on videos

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• mantielero (orginal) [2020-03-04T22:28:27+01:00] view original

  Thanks @yglukhov.

  The reason why I am playing with VapoursSynth is because I already wrapped it in Julia.

  More questions, it is not clear to me if I should define the functions as types (like nimterop does):

  type
     VSGetVapourSynthAPI* {.impVapourSynth.} = proc(version: cint): VSAPI {.cdecl.}
  Run

  or like in most examples:

  proc VSGetVapourSynthAPI*(version: cint): VSAPI {.impVapourSynth, cdecl.}
  Run

  What is the difference?

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• mantielero (orginal) [2020-03-05T19:00:05+01:00] view original

  Good progress. It looks like the wrapper works fine now:

  $ ./ex01
  ptr 0x557ede9361a8 --> [versionString = 0x7f0e3ea18860"VapourSynth Video Processing Library\10"
  "Copyright (c) 2012-2019 Fredrik Mellbin\10"
  "Core R48\10"
  "API R3.6\10"
  "Options: -\10"
  "",
  core = 48,
  api = 196614,
  numThreads = 8,
  maxFramebufferSize = 4294967296,
  usedFramebufferSize = 0]

  Still some work to do.

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