# VapourSynth C API Reference

See the example filters in the sdk dir. Reading simplefilters.c, which contains several built-in functions, can also be very helpful.

## Public Headers

- VapourSynth4.h
- VSHelper4.h
- VSScript4.h

## Common Pitfalls

There are several minor pitfalls related to the threading and design that have to be taken into consideration. Most of them usually aren’t a problem but here’s a small checklist of things you have to watch out for sometimes.

### General API

You may not pass objects (clips, functions and so on) owned by one core as arguments to filters in another core. A manual full deep copy of the data you want to pass on is required. This is generally not a problem since you should never need more than one core per filter graph.

### Plugins

Plugin code may run more multithreaded than it initially appears. VapourSynthPluginInit is the only function always guaranteed to not run in parallel. This means that the constructor and destructor of a filter may be run in parallel for several instances. Use proper synchronization if you need to initialize shared data.

The GetFrame function is a bit more complicated so see the reference of the constants. Do however note that the parallelism is per instance. Even if a filter is fmUnordered or fmSerial other instances may enter GetFrame simultaneously.

There are two common misconseptions about which mode should be used. A simple rule is that fmSerial should never be used. And source filters (those returning a frame on arInitial) that need locking should use fmUnordered.

## Reserved Frame Properties

All frames contain a map of key–value pairs. It is recommended that these properties are named using only a-z, A-Z, 0-9 using CamelCase. There is a special category of keys starting with _ which have strictly defined meanings specified below. It is acceptable to not set any of these keys if they are unknown. It is also a fatal error to set them to a value not specified below.

int _ChromaLocation

> Chroma sample position in YUV formats.

> 0=left, 1=center, 2=topleft, 3=top, 4=bottomleft, 5=bottom.

int _ColorRange

> Full or limited range (PC/TV range). Primarily used with YUV formats.

> 0=full range, 1=limited range.

int _Primaries

> Color primaries as specified in ITU-T H.265 Table E.3.

int _Matrix

> Matrix coefficients as specified in ITU-T H.265 Table E.5.

int _Transfer

> Transfer characteristics as specified in ITU-T H.265 Table E.4.

int _FieldBased

> If the frame is composed of two independent fields (interlaced).

> 0=frame based (progressive), 1=bottom field first, 2=top field first.

float _AbsoluteTime

> The frame’s absolute timestamp in seconds if reported by the source filter. Should only be set by the source filter and not be modified. Use durations for all operations that depend on frame length.

int _DurationNum, int _DurationDen

> The frame’s duration in seconds as a rational number. Filters that modify the framerate should also change these values.

> This fraction should always be normalized.

bint _Combed

> Whether or not the frame needs postprocessing, usually hinted from field matching filters.

int _Field

> If the frame was produced by something like core.std.SeparateFields, this property signals which field was used to generate this frame.

> 0=from bottom field, 1=from top field.

string _PictType

> A single character describing the frame type. It uses the common IPB letters but other letters may also be used for formats with additional frame types.

int _SARNum, int _SARDen

> Pixel (sample) aspect ratio as a rational number.

bint _SceneChangeNext

> If 1, this frame is the last frame of the current scene. The next frame starts a new scene.

bint _SceneChangePrev

> If 1, this frame starts a new scene.

frame _Alpha

> A clip’s alpha channel can be attached to the clip one frame at a time using this property.