Python Reference

VapourSynth is separated into a core library and a Python module. This section explains how the core library is exposed through Python and some of the special things unique to Python scripting, such as slicing and output.

VapourSynth Structure

Most operations in the VapourSynth library are performed through the core object. This core may load plugins, which all end up in their own unit, or namespace, so to say, to avoid naming conflicts in the contained functions. For this reason you call a plugin function with core.unit.Function().

All arguments to functions have names that are lowercase and all function names are CamelCase. Unit names are also lowercase and usually short. This is good to remember.

Slicing and Other Syntactic Sugar

The VideoNode class (always called “clip” in practice) supports the full range of indexing and slicing operations in Python. If you do perform a slicing operation on a clip, you will get a new clip back with the desired frames. Note that frame numbers, like python arrays, start counting at 0. Here are a few examples.

Make a single frame clip containing frame number 5:

video = clip[5]

Make a clip containing frames 6 to 9 (unlike Trim, the end value of python slicing is not inclusive):

video = clip[6:10]

Select even numbered frames:

video = clip[::2]

Select odd numbered frames:

video = clip[1::2]

Negative step is also allowed, so this reverses a clip:

video = clip[::-1]

It may all be combined at once to confuse people, just like normal Python slicing:

video = clip[-400:-800:-5]

Filters can be chained with a dot, it mostly works like Avisynth:

clip = core.ffms2.Source("")
clip = clip.std.Trim(first=100, last=2000).std.FlipVertical()

The addition operator can be used to splice clips together:

clip4 = clip1 + clip2 + clip3

Which is equivalent to:

clip4 = core.std.Splice([core.std.Splice([clip1, clip2], mismatch=False), clip3], mismatch=False)

The multiplication operator can be used to loop a clip:

clip = clip * 42

Which is equivalent to:

clip = core.std.Loop(clip, times=42)

Note that multiplication by 0 is a special case that will repeat the clip up to the maximum frame count.

Python Keywords as Filter Arguments

If a filter’s argument happens to be a Python keyword, you may prepend an underscore to the argument’s name when invoking the filter. The Python module will strip one leading underscore from all filter arguments before passing them to the filters.

clip = core.plugin.Filter(clip, _lambda=1)

Another way to deal with such arguments is to place them in a dictionary:

args = { "lambda": 1 }
clip = core.plugin.Filter(clip, **args)

Windows File Paths (Strings With Backslashes)

If you have a string containing backslashes, you must either prefix the string with “r”, or duplicate every single backslash. The reason is that the backslash is an escape character in Python.

Incorrect; Python will think “\A” and “\G” are supposed to mean something special:

core.avs.LoadPlugin("B:\Avisynth plugins\GuavaComb.dll")

Correct; Python will think “\” means something special, namely a single backslash:

core.avs.LoadPlugin("B:\\Avisynth plugins\\GuavaComb.dll")

Correct; Python will not consider any combination of characters special:

core.avs.LoadPlugin(r"B:\Avisynth plugins\GuavaComb.dll")

Additionally, if the string is a Windows file path, it can also be written with forward slashes instead of backslashes:

core.avs.LoadPlugin("B:/Avisynth plugins/GuavaComb.dll")


The normal way of specifying the clip(s) to output is to call clip.set_output(). All standard VapourSynth components only use output index 0, but other tools may use something different. There are also other variables that can be set to control how a format is output. For example, setting enable_v210=True changes the packing of the YUV422P10 format to one that is common in professional software (like Adobe products). An example on how to get v210 output:

some_clip = core.resize.Bicubic(clip, format=vs.YUV422P10)
enable_v210 = True

Raw Access to Frame Data

The VideoFrame class simply contains one picture and all the metadata associated with it. It is possible to access the raw data using ctypes and some persistence. The three relevant functions are get_read_ptr(plane), get_write_ptr(plane), and get_stride(plane), all of which take the plane to access as an argument. Accessing the data is a bit trickier as get_read_ptr() and get_write_ptr() only return a pointer.

To get a frame simply call get_frame(n) on a clip. Should you desire to get all frames in a clip, use this code:

for frame in clip.frames():
    # Do stuff with your frame

Classes and Functions

Gets the singleton Core object. If it is the first time the function is called,
the Core will be instantiated with the default options. This is the preferred
way to reference the core.
get_core([threads = 0, add_cache = True, accept_lowercase = False])

Deprecated, use the core attribute instead.

Get the singleton Core object. If it is the first time the function is called, the Core will be instantiated with the given options. If the Core has already been instantiated, all options are ignored. Setting threads to a value greater than zero overrides the autodetection.


Sets a function to handle all debug output and fatal errors. The function should have the form handler(level, message), where level corresponds to the constants. Passing None restores the default handler, which prints to stderr.

get_output([index = 0])

Get a previously set output node. Throws an error if the index hasn’t been set.

clear_output([index = 0])

Clears a clip previously set for output.


Clears all clips set for output in the current environment.

construct_signature(signature[, injected=None])

Creates a inspect.Signature object for the given registration signature.

If injected is not None, the default of the first argument of the signature will be replaced with the value supplied with injected.

class Core

The Core class uses a singleton pattern. Use the core attribute to obtain an instance. All loaded plugins are exposed as attributes of the core object. These attributes in turn hold the functions contained in the plugin. Use get_plugins() to obtain a full list of all currently loaded plugins you may call this way.


The number of concurrent threads used by the core. Can be set to change the number. Setting to a value less than one makes it default to the number of hardware threads.


For debugging purposes only. When set to False no caches will be automatically inserted between filters.


When set to True function name lookups in the core are case insensitive. Don’t distribute scripts that need it to be set.


Set the upper framebuffer cache size after which memory is aggressively freed. The value is in megabytes.


An alias for setting max_cache_size. Kept for compatibility with older scripts.


Returns a dict containing all loaded plugins and their functions.


Works similar to get_plugins() but returns a human-readable string.

register_format(color_family, sample_type, bits_per_sample, subsampling_w, subsampling_h)

Register a new Format object or obtain a reference to an existing one if it has already been registered. Invalid formats throw an exception.


Retrieve a Format object corresponding to the specified id. Returns None if there is no format with that id.


Returns version information as a string.


Returns the core version as a number.

class VideoNode

Represents a video clip. The class itself supports indexing and slicing to perform trim, reverse and selectevery operations. Several operators are also defined for the VideoNode class: addition appends clips and multiplication repeats them. Note that slicing and indexing always return a new VideoNode object and not a VideoFrame.


A Format object describing the frame data. If the format can change between frames, this value is None.


The width of the video. This value will be 0 if the width and height can change between frames.


The height of the video. This value will be 0 if the width and height can change between frames.


The number of frames in the clip.


The framerate represented as a Fraction. It is 0/1 when the clip has a variable framerate.


Deprecated, use fps.numerator instead

The numerator of the framerate. If the clip has variable framerate, the value will be 0.


Deprecated, use fps.denominator instead

The denominator of the framerate. If the clip has variable framerate, the value will be 0.


Special flags set for this clip. This attribute should normally be ignored.


Returns a VideoFrame from position n.


Returns a concurrent.futures.Future-object which result will be a VideoFrame instance or sets the exception thrown when rendering the frame.

The future will always be in the running or completed state

get_frame_async_raw(n, cb: callable)

First form of this method. It will call the callback from another thread as soon as the frame is rendered.

The result-value passed to the callback will either be a VideoFrame-instance on success or a Error-instance on failure.

This method is intended for glue code. For normal use, use get_frame_async instead.

  • n – The frame number
  • cb – A callback in the form cb(node, n, result)
get_frame_async_raw(n, cb: Future[, wrapper: callable = None])

Second form of this method. It will take a Future-like object (including asyncio.Future or similar) and set its result or exception according to the result of the function.

The optional wrapper-parameter is intended for calls like asyncio.EventLoop.call_soon_threadsafe in which all calls to its future-object must be wrapped.

This method is intended for glue code. For normal use, use get_frame_async instead.

  • n – The frame number
  • cb – The future-object whose result will be set.
  • wrapper – A wrapper-callback which is responsible for moving the result across thread boundaries. If not given, the result of the future will be set in a random thread.
set_output(index = 0)

Set the clip to be accessible for output. This is the standard way to specify which clip(s) to output. All VapourSynth tools (vsvfw, vsfs, vspipe) use the clip in index 0.

output(fileobj[, y4m = False, prefetch = 0, progress_update = None])

Write the whole clip to the specified file handle. It is possible to pipe to stdout by specifying sys.stdout as the file. YUV4MPEG2 headers will be added when y4m is true. The current progress can be reported by passing a callback function of the form func(current_frame, total_frames) to progress_update. The prefetch argument is only for debugging purposes and should never need to be changed.

class VideoFrame
This class represents a video frame and all metadata attached to it.

A Format object describing the frame data.


The width of the frame.


The height of the frame.


If readonly is True, the frame data and properties cannot be modified.


This attribute holds all the frame’s properties mapped as sub-attributes.


Returns a writable copy of the frame.


Returns a pointer to the raw frame data. The data may not be modified.


Returns a memoryview of the frame data that’s only valid as long as the VideoFrame object exists. The data may not be modified.


Returns a pointer to the raw frame data. It may be modified using ctypes or some other similar python package.


Returns a memoryview of the frame data that’s only valid as long as the VideoFrame object exists.


Returns the stride between lines in a plane.

class Format

This class represents all information needed to describe a frame format. It holds the general color type, subsampling, number of planes and so on. The names map directly to the C API so consult it for more detailed information.


A unique id identifying the format.


A human readable name of the format.


Which group of colorspaces the format describes.


If the format is integer or floating point based.


How many bits are used to store one sample in one plane.


The actual storage is padded up to 2^n bytes for efficiency.


The subsampling for the second and third plane in the horizontal direction.


The subsampling for the second and third plane in the vertical direction.


The number of planes the format has.

class Plugin

Plugin is a class that represents a loaded plugin and its namespace.


The namespace of the plugin.


Returns a dict containing all the functions in the plugin. You can access it by calling core.std.get_functions(). Replace std with the namespace of the plugin you want to query.


Works similar to get_functions() but returns a human-readable string.

class Function

Function is a simple wrapper class for a function provided by a VapourSynth plugin. Its main purpose is to be called and nothing else.


The function name. Identical to the string used to register the function.


The Plugin object the function belongs to.


Raw function signature string. Identical to the string used to register the function.


Refers to the Plugin-instance which registered this function.

class Func

Func is a simple wrapper class for VapourSynth VSFunc objects. Its main purpose is to be called and manage reference counting.

exception Error

The standard exception class. This exception is thrown on most errors encountered in VapourSynth.

Color Family Constants

The color family constants describe groups of formats and the basic way their color information is stored. You should be familiar with all of them apart from maybe YCOCG and COMPAT. The latter is a special junk category for non-planar formats. These are the declared constants in the module:


Format Constants

Format constants exactly describe a format. All common and even more uncommon formats have handy constants predefined so in practice no one should really need to register one of their own. These values are mostly used by the resizers to specify which format to convert to. The naming system is quite simple. First the color family, then the subsampling (only YUV has it) and after that how many bits per sample in one plane. The exception to this rule is RGB, which has the bits for all 3 planes added together. The long list of values:












Sample Type Constants