Filter SDK/InvertNeg

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I'll start off with a complete, working Avisynth plugin. It's called "InvertNeg", and it produces a photo-negative of the input clip.

Here's InvertNeg.cpp:

#include <windows.h>
#include "avisynth.h"

class InvertNeg : public GenericVideoFilter {
public:
   InvertNeg(PClip _child, IScriptEnvironment* env);
   PVideoFrame __stdcall GetFrame(int n, IScriptEnvironment* env);
};

InvertNeg::InvertNeg(PClip _child, IScriptEnvironment* env) :
   GenericVideoFilter(_child) {
   if (!vi.IsPlanar() || !vi.IsYUV()) {
      env->ThrowError("InvertNeg: planar YUV data only!");
   }
}

PVideoFrame __stdcall InvertNeg::GetFrame(int n, IScriptEnvironment* env) {

   PVideoFrame src = child->GetFrame(n, env);
   PVideoFrame dst = env->NewVideoFrame(vi);

   const unsigned char* srcp;
   unsigned char* dstp;
   int src_pitch, dst_pitch, row_size, height;
   int p, x, y;

   int planes[] = {PLANAR_Y, PLANAR_V, PLANAR_U};
	
   for (p=0; p<3; p++) {
      srcp = src->GetReadPtr(planes[p]);
      dstp = dst->GetWritePtr(planes[p]);

      src_pitch = src->GetPitch(planes[p]);
      dst_pitch = dst->GetPitch(planes[p]);
      row_size = dst->GetRowSize(planes[p]);
      height = dst->GetHeight(planes[p]);

      for (y = 0; y < height; y++) {
         for (x = 0; x < row_size; x++) {
            dstp[x] = srcp[x] ^ 255;
         }
         srcp += src_pitch; // or srcp = srcp + src_pitch;
         dstp += dst_pitch; // or dstp = dstp + dst_pitch;
      }
   }
   return dst;
}

AVSValue __cdecl Create_InvertNeg(AVSValue args, void* user_data, IScriptEnvironment* env) {
   return new InvertNeg(args[0].AsClip(), env);
}

const AVS_Linkage *AVS_linkage = 0;

extern "C" __declspec(dllexport) const char* __stdcall AvisynthPluginInit3(IScriptEnvironment* env, const AVS_Linkage* const vectors) {
   AVS_linkage = vectors;
   env->AddFunction("InvertNeg", "c", Create_InvertNeg, 0);
   return "InvertNeg sample plugin";
}

Compile this file into a DLL named InvertNeg.dll. See compiling instructions. Now create an Avisynth script which looks something like this:

LoadPlugin("d:\path\InvertNeg.dll")
clip = BlankClip().ConvertToYV12()
return clip.InvertNeg()

Line by line breakdown

Here's a line-by-line breakdown of InvertNeg.cpp:

#include "avisynth.h"

This header declares all the classes and miscellaneous constants that you might need when writing a plugin. All external plugins should #include it.

External plugins do not link with avisynth.dll, so they can't directly access functions that are defined in the main Avisynth source code. Therefore, every important function in avisynth.h is either defined inline or declared as virtual. The virtual functions act as callbacks for external DLLs.

class InvertNeg : public GenericVideoFilter {

An Avisynth filter is simply a C++ class implementing the IClip interface. More about classes can be found in this tutorial. IClip has four pure virtual methods: GetVideoInfo, GetFrame, GetParity, and GetAudio.

The class GenericVideoFilter is a simple do-nothing filter defined in avisynth.h. It derives from IClip and implements all four methods. Most filters can inherit from GenericVideoFilter rather than directly from IClip; this saves you from having to implement methods that you don't care about, like GetAudio in this example. More about inheritance can be found in this tutorial. In our case two functions are defined in the class: the constructor (InvertNeg()) and GetFrame.

InvertNeg(PClip _child, IScriptEnvironment* env);

This is our constructor. It can be used to initialize variables, create look up tables (LUTs) and to check whether the source clips (if there is one) have the desired properties. In our example it is taking a single clip, does some property checking and then simply passes through to its output.

A PClip is a "smart pointer" to an IClip. It maintains a reference count on the IClip object and automagically deletes it when the last PClip referencing it goes away. For obvious reasons, you should always use PClip rather than IClip* to refer to clips.

Like a genuine pointer, a PClip is only four bytes long, so you can pass it around by value. Also like a pointer, a PClip can be assigned a null value (0), which is often useful as a sentinel. Unlike a pointer, PClip is initialized to 0 by default.

You'll need to make sure your class doesn't contain any circular PClip references, or any PClips sitting in dynamically allocated memory that you forget to delete. Other than that, you don't have to worry about the reference-counting machinery.

Avisynth filters have a standardized output channel via IClip, but (unlike for example VirtualDub filters) no standardized input channel. Each filter is responsible for obtaining its own source material -- usually (as in this case) from another clip, but sometimes from several different clips, or from a file.

We will override the GetFrame method to do something more useful, while leaving the other three methods as-is to pass through aspects of the clip that we don't need to change.

PVideoFrame InvertNeg::GetFrame(int n, IScriptEnvironment* env) {

This method is called to make our filter produce frame n of its output. The second argument, env, is for our purposes simply a callback suite. It is actually implemented in Avisynth by a class called ScriptEnvironment. One instance of this class is created for each opened AVS script, so there may sometimes be several instances active at once. It is important that the callback functions be called through the proper instance. A particular instance of your class will only be used in one ScriptEnvironment, but different instances might be used in different ScriptEnvironments.

InvertNeg::InvertNeg(PClip _child, IScriptEnvironment* env) :
   GenericVideoFilter(_child) {

This is the class constructor and it is called when a filter instance is being created (see Create_InvertNeg()).

  if (!vi.IsPlanar() || !vi.IsYUV()) {
     env->ThrowError("InvertNeg: planar YUV data only!");
  }

"vi" is a protected member of GenericVideoFilter. It is a structure of type VideoInfo, and it contains information about the clip (like frame size, frame rate, pixel format, audio sample rate, etc.). In our example, when the image format of the clip is not planar or the colorspace of the clip is not YUV, an error is returned using ThrowError. As of writing this document there is no planar RGB, but that might very well change in the future.

  PVideoFrame src = child->GetFrame(n, env);

This method returns a PVideoFrame, which is a smart pointer like PClip.

"child" is the other protected member of GenericVideoFilter, of type PClip. It contains the clip that was passed to the constructor. For our filter to produce frame n we need the corresponding frame of the input. If you need a different frame from the input, all you have to do is pass a different frame number to child->GetFrame.

GetFrame calls are usually intercepted by Avisynth's internal caching code, so the frame request may never actually reach the child filter.

  PVideoFrame dst = env->NewVideoFrame(vi);

The NewVideoFrame callback allocates space for a video frame of the supplied size. In this case it will hold our filter's output. The frame buffer is uninitialized raw memory (except that in the debug build it gets filled with the repeating byte pattern 0A 11 0C A7 ED, which is easy to recognize because it looks like "ALLOCATED").

As already explained, "vi" contains information about the properties of the clip. NewVideoFrame uses the information in this structure to return a frame buffer of the appropriate size.

Frame buffers are reused once all the PVideoFrame references to them go away. So usually NewVideoFrame won't actually need to allocate any memory from the heap.

  const unsigned char* srcp;
  unsigned char* dstp;
  int src_pitch, dst_pitch, row_size, height;
  int p, x, y;

  int planes[] = {PLANAR_Y, PLANAR_V, PLANAR_U};

This is the declaration of the used variables. Note that we will modify one color component (instead of an entire pixel) at the same time. Such a component (for example an U sample) has a value between 0 and 255 (exactly one byte). Hence srcp and dstp needs to be declared as unsigned char (or BYTE would have been possible too).

Note that in the constructor we excluded the formats which are not planar or not YUV. For filter writers this means that they can write one simple function that is called three times, one for each color channel, assuming that the operations are channel-independent (which is not always the case). This is what I have done here. Note it also works for Y8 since these functions return zero if the plane (PLANAR_U, PLANAR_V) doesn't exist. In our case the operations are channel-independent so it doesn't matter which plane is processed first.

   for (p=0; p<3; p++) {
      srcp = src->GetReadPtr(planes[p]);
      dstp = dst->GetWritePtr(planes[p]);

All frame buffers are readable, but not all are writable.

The rule about writability is this: A buffer is writable if and only if there is exactly one PVideoFrame pointing to it. In other words, you can only write to a buffer if no one else might be reading it. This rule guarantees that as long as you hold on to a PVideoFrame and don't write to it yourself, that frame will remain unchanged. The only drawback is that you can't have two PVideoFrames pointing to a writable buffer. This can sometimes be an inconvenience, as I'll explain later.

Any buffer you get from NewVideoFrame is guaranteed to be writable (as long as you only assign it to one PVideoFrame!). Our filter's dst came from NewVideoFrame, so we can safely call dst->GetWritePtr(). However, frames you get from other clips via GetFrame may not be writable, in which case GetWritePtr() will return a null pointer.

There is an IsWritable() method which you can call to find out if a buffer is writable or not, and there's a MakeWritable callback to ensure that it is.

     src_pitch = src->GetPitch(planes[p]);
     dst_pitch = dst->GetPitch(planes[p]);

Just as in VirtualDub, the "pitch" of a frame buffer is the offset (in bytes) from the beginning of one scan line to the beginning of the next. The source and destination buffers won't necessarily have the same pitch.

Buffers created by NewVideoFrame are always quadword (8-byte) aligned and always have a pitch that is a multiple of 8. (i think it's 16 now, need to check ???)

     row_size = dst->GetRowSize(planes[p]);

The row size is the length of each row in bytes (not pixels) of plane p. It's usually equal to the pitch or slightly less, but it may be significantly less if the frame in question has been through Crop.

Since our source and destination frames have the same width and pixel format, they will always have the same row size. Thus I only need one row_size variable, and I could just as well have called src->GetRowSize().

     height = dst->GetHeight(planes[p]);

The height is the height in samples of the plane p. Again, for our filter this is the same for the source and the destination.

     for (y = 0; y < height; y++) {
        for (x = 0; x < row_size; x++) {
           dstp[x] = srcp[x] ^ 255;
        }
        srcp += src_pitch;
        dstp += dst_pitch;
     }

This is the code that does the actual work. The "srcp += src_pitch; dstp += dst_pitch;" idiom is a useful way of dealing with potentially differing pitches without too much grief.

  return dst;

GetFrame returns the newly-created frame. Our own references to this frame and to the source frame will go away with the src and dst variables. Our caller will become sole owner of the destination frame (which therefore will still be writable), and the source frame will be retained in the cache and eventually recycled. All through the magic of C++ classes.

AVSValue __cdecl Create_InvertNeg(AVSValue args, void* user_data, IScriptEnvironment* env) {

In order to use our new filter, we need a scripting-language function which creates an instance of it. This is that function.

Script functions written in C++ take three arguments. args contains all the arguments passed to the function by the script. user_data contains the void pointer which you passed to AddFunction (see below). Usually you won't need this. env contains the same IScriptEnvironment pointer that will later be passed to GetFrame.

AVSValue is a variant type which can hold any one of the following: a boolean value (true/false); an integer; a floating-point number; a string; a video clip (PClip); an array of AVSValues; or nothing ("undefined"). You can test which one it is with the methods IsBool(), IsInt(), IsFloat(), IsString(), IsClip(), IsArray(), and Defined() (which returns true if the AVSValue is not "undefined"). You can get the value with AsBool(), AsInt(), etc. For arrays, you can use the ArraySize() method to get the number of elements, and [] indexing to get the elements themselves. For convenience, IsFloat() and AsFloat() will work with integers also. But boolean values are not treated as numeric (unlike C).

The name "Create_InvertNeg" is arbitrary. This function will actually be known as "InvertNeg" in scripts, because that's the name we pass to AddFunction below.

    return new InvertNeg(args[0].AsClip(), env);

The args argument passed to a script function will always be an array. The return value should be any one of the other types (never an array).

The types of the values in the args array are guaranteed to match one of the function signatures that you pass to AddFunction, just as in VirtualDub. Therefore, there's no need to worry about IsClip here.

Create_InvertNeg simply creates and returns a filter instance; it is automatically converted to an AVSValue via the constructor AVSValue(IClip*).

const AVS_Linkage *AVS_linkage = 0;

This declares and initializes the server pointers static storage AVS_Linkage.

extern "C" __declspec(dllexport) const char* __stdcall AvisynthPluginInit3(IScriptEnvironment* env, const AVS_Linkage* const vectors) {

This is the only function which gets exported from the DLL. It is called by the script function LoadPlugin the first time this plugin in loaded in a particular script. If several scripts are open at once and more than one of them loads this plugin, AvisynthPluginInit3 may be called more than once with different IScriptEnvironments. Therefore:

  • You should not save the env parameter in a global variable.
  • If you need to initialize any static data, you should do it in DLLMain, not in this function.
  AVS_linkage = vectors;

This saves the server pointers.

The main purpose of the AvisynthPluginInit3 function is to call env->AddFunction.

  env->AddFunction("InvertNeg", "c", Create_InvertNeg, 0);

As promised, we now call AddFunction to let Avisynth know of the existence of our filter. This function takes four arguments: the name of the new script function; the parameter-type string; the C++ function implementing the script function; and the user_data cookie.

The parameter-type string can be specified as follows:

  • No return type is given. Function return values are not type-checked; you can return anything you like.
  • The following types are available:
    • c - clip
    • i - integer
    • f - float
    • s - string
    • b - boolean
  • You can follow any type with '*' or '+' to indicate "zero or more" or "one or more" respectively. In this case all the matching arguments will be gathered into a sub-array.
For example, if your type string is "is+f", then the integer argument will be args[0], the string arguments will be args[1][0], args[1][1], etc. (and there will be args[1].ArraySize() of them), and the float argument will be args[2].
  • '.' matches a single argument of any type. To match multiple arguments of any type, use ".*" or ".+".
  • You can have named arguments, by specifying the name in [brackets] before the type. Named arguments are also optional arguments; if the user omits them, they will be of the undefined type instead of the type you specify. For convenience, AVSValue offers a set of As...() functions which take default values. See avisynth.h.
  return "InvertNeg sample plugin";

The return value of AvisynthPluginInit3 is a string which can contain any message you like, such as a notice identifying the version and author of the plugin. This string becomes the return value of LoadPlugin, and will almost always be ignored. You can also just return 0 if you prefer.

A variation as an in place filter

The Invert filter could easily do its work in a single buffer, rather than copying from one buffer to another. Here's a new implementation of GetFrame that does this:

#include <windows.h>
#include "avisynth.h"

class InvertNeg : public GenericVideoFilter {
public:
   InvertNeg(PClip _child, IScriptEnvironment* env);
   PVideoFrame __stdcall GetFrame(int n, IScriptEnvironment* env);
};

InvertNeg::InvertNeg(PClip _child, IScriptEnvironment* env) :
   GenericVideoFilter(_child) {
   if (!vi.IsPlanar() || !vi.IsYUV()) {
      env->ThrowError("InvertNeg: planar YUV data only!");
   }
}

PVideoFrame __stdcall InvertNeg::GetFrame(int n, IScriptEnvironment* env) {

   PVideoFrame src = child->GetFrame(n, env);
   env->MakeWritable(&src);

   unsigned char* srcp;
   int src_pitch, row_size, height;
   int p, x, y;

   int planes[] = {PLANAR_Y, PLANAR_V, PLANAR_U};
	
   for (p=0; p<3; p++) {
      srcp = src->GetWritePtr(planes[p]);

      src_pitch = src->GetPitch(planes[p]);
      row_size = src->GetRowSize(planes[p]);
      height = src->GetHeight(planes[p]);

      for (y = 0; y < height; y++) {
         for (x = 0; x < row_size; x++) {
            srcp[x] = srcp[x] ^ 255;
            // even shorter would be srcp[x] ^= 255;
         }
         srcp += src_pitch;
      }
   }
   return src;
}

AVSValue __cdecl Create_InvertNeg(AVSValue args, void* user_data, IScriptEnvironment* env) {
   return new InvertNeg(args[0].AsClip(), env);
}

const AVS_Linkage *AVS_linkage = 0;

extern "C" __declspec(dllexport) const char* __stdcall AvisynthPluginInit3(IScriptEnvironment* env, const AVS_Linkage* const vectors) {
   AVS_linkage = vectors;
   env->AddFunction("InvertNeg", "c", Create_InvertNeg, 0);
   return "InvertNeg sample plugin";
}

The key difference between this version of the function and the original version is the presence of the MakeWritable callback. This is necessary because this time "we don't know where that source frame has been." Someone else in the filter chain may be holding a reference to it, in which case we won't be allowed to write to it.

Old versions

Ben's AviSynth Docs is the documentation written for AviSynth 1.0 by Ben Rudiak-Gould, in its original form.

See more about the modifications for AviSynth 2.5 in the AviSynth Two-Five SDK. (need to adjust the two-five version using the code above ...)

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