# Libavutil Documentation

# Table of Contents

# 1. Description

The libavutil library is a utility library to aid portable multimedia programming. It contains safe portable string functions, random number generators, data structures, additional mathematics functions, cryptography and multimedia related functionality (like enumerations for pixel and sample formats).

# 2. Syntax

This section documents the syntax and formats employed by the FFmpeg libraries and tools.

## 2.1 Quoting and escaping

FFmpeg adopts the following quoting and escaping mechanism, unless explicitly specified. The following rules are applied:

- `'` and `\` are special characters (respectively used for quoting and escaping). In addition to them, there might be other special characters depending on the specific syntax where the escaping and quoting are employed.
- A special character is escaped by prefixing it with a ’\’.
- All characters enclosed between ” are included literally in the parsed string. The quote character `'` itself cannot be quoted, so you may need to close the quote and escape it.
- Leading and trailing whitespaces, unless escaped or quoted, are removed from the parsed string.

Note that you may need to add a second level of escaping when using the command line or a script, which depends on the syntax of the adopted shell language.

The function `av_get_token` defined in ‘`libavutil/avstring.h`’ can be used to parse a token quoted or escaped according to the rules defined above.

The tool ‘`tools/ffescape`’ in the FFmpeg source tree can be used to automatically quote or escape a string in a script.

### 2.1.1 Examples

- Escape the string `Crime d'Amour` containing the `'` special character:

      Crime d\'Amour

- The string above contains a quote, so the `'` needs to be escaped when quoting it:

      'Crime d'\''Amour'

- Include leading or trailing whitespaces using quoting:

      '  this string starts and ends with whitespaces  '

- Escaping and quoting can be mixed together:

      ' The string '\'string\'' is a string '

- To include a literal `\` you can use either escaping or quoting:

      'c:\foo' can be written as c:\\foo

## 2.2 Date

The accepted syntax is:

    [(YYYY-MM-DD|YYYYMMDD)[T|t| ]]((HH:MM:SS[.m...]]])|(HHMMSS[.m...]]]))[Z]
    now

If the value is "now" it takes the current time.

Time is local time unless Z is appended, in which case it is interpreted as UTC. If the year-month-day part is not specified it takes the current year-month-day.

## 2.3 Time duration

The accepted syntax is:

    [-]HH:MM:SS[.m...]
    [-]S+[.m...]

HH expresses the number of hours, MM the number a of minutes and SS the number of seconds.

## 2.4 Video size

Specify the size of the sourced video, it may be a string of the form widthxheight, or the name of a size abbreviation.

The following abbreviations are recognized:

‘`sqcif`’

  128x96

‘`qcif`’

  176x144

‘`cif`’

  352x288

‘`4cif`’

  704x576

‘`16cif`’

  1408x1152

‘`qqvga`’

  160x120

‘`qvga`’

  320x240

‘`vga`’

  640x480

‘`svga`’

  800x600

‘`xga`’

  1024x768

‘`uxga`’

  1600x1200

‘`qxga`’

  2048x1536

‘`sxga`’

  1280x1024

‘`qsxga`’

  2560x2048

‘`hsxga`’

  5120x4096

‘`wvga`’

  852x480

‘`wxga`’

  1366x768

‘`wsxga`’

  1600x1024

‘`wuxga`’

  1920x1200

‘`woxga`’

  2560x1600

‘`wqsxga`’

  3200x2048

‘`wquxga`’

  3840x2400

‘`whsxga`’

  6400x4096

‘`whuxga`’

  7680x4800

‘`cga`’

  320x200

‘`ega`’

  640x350

‘`hd480`’

  852x480

‘`hd720`’

  1280x720

‘`hd1080`’

  1920x1080

## 2.5 Video rate

Specify the frame rate of a video, expressed as the number of frames generated per second. It has to be a string in the format frame_rate_num/frame_rate_den, an integer number, a float number or a valid video frame rate abbreviation.

The following abbreviations are recognized:

‘`ntsc`’

  30000/1001

‘`pal`’

  25/1

‘`qntsc`’

  30000/1

‘`qpal`’

  25/1

‘`sntsc`’

  30000/1

‘`spal`’

  25/1

‘`film`’

  24/1

‘`ntsc-film`’

  24000/1

## 2.6 Ratio

A ratio can be expressed as an expression, or in the form numerator:denominator.

Note that a ratio with infinite (1/0) or negative value is considered valid, so you should check on the returned value if you want to exclude those values.

The undefined value can be expressed using the "0:0" string.

## 2.7 Color

It can be the name of a color (case insensitive match) or a [0x|#]RRGGBB[AA] sequence, possibly followed by "@" and a string representing the alpha component.

The alpha component may be a string composed by "0x" followed by an hexadecimal number or a decimal number between 0.0 and 1.0, which represents the opacity value (0x00/0.0 means completely transparent, 0xff/1.0 completely opaque). If the alpha component is not specified then 0xff is assumed.

The string "random" will result in a random color.

# 3. Expression Evaluation

When evaluating an arithmetic expression, FFmpeg uses an internal formula evaluator, implemented through the ‘`libavutil/eval.h`’ interface.

An expression may contain unary, binary operators, constants, and functions.

Two expressions expr1 and expr2 can be combined to form another expression "expr1;expr2". expr1 and expr2 are evaluated in turn, and the new expression evaluates to the value of expr2.

The following binary operators are available: `+`, `-`, `*`, `/`, `^`.

The following unary operators are available: `+`, `-`.

The following functions are available:

‘`sinh(x)`’

  Compute hyperbolic sine of x.

‘`cosh(x)`’

  Compute hyperbolic cosine of x.

‘`tanh(x)`’

  Compute hyperbolic tangent of x.

‘`sin(x)`’

  Compute sine of x.

‘`cos(x)`’

  Compute cosine of x.

‘`tan(x)`’

  Compute tangent of x.

‘`atan(x)`’

  Compute arctangent of x.

‘`asin(x)`’

  Compute arcsine of x.

‘`acos(x)`’

  Compute arccosine of x.

‘`exp(x)`’

  Compute exponential of x (with base `e`, the Euler’s number).

‘`log(x)`’

  Compute natural logarithm of x.

‘`abs(x)`’

  Compute absolute value of x.

‘`squish(x)`’

  Compute expression `1/(1 + exp(4*x))`.

‘`gauss(x)`’

  Compute Gauss function of x, corresponding to `exp(-x*x/2) / sqrt(2*PI)`.

‘`isinf(x)`’

  Return 1.0 if x is +/-INFINITY, 0.0 otherwise.

‘`isnan(x)`’

  Return 1.0 if x is NAN, 0.0 otherwise.

‘`mod(x, y)`’

  Compute the remainder of division of x by y.

‘`max(x, y)`’

  Return the maximum between x and y.

‘`min(x, y)`’

  Return the maximum between x and y.

‘`eq(x, y)`’

  Return 1 if x and y are equivalent, 0 otherwise.

‘`gte(x, y)`’

  Return 1 if x is greater than or equal to y, 0 otherwise.

‘`gt(x, y)`’

  Return 1 if x is greater than y, 0 otherwise.

‘`lte(x, y)`’

  Return 1 if x is lesser than or equal to y, 0 otherwise.

‘`lt(x, y)`’

  Return 1 if x is lesser than y, 0 otherwise.

‘`st(var, expr)`’

  Allow to store the value of the expression expr in an internal variable. var specifies the number of the variable where to store the value, and it is a value ranging from 0 to 9. The function returns the value stored in the internal variable. Note, Variables are currently not shared between expressions.

‘`ld(var)`’

  Allow to load the value of the internal variable with number var, which was previously stored with st(var, expr). The function returns the loaded value.

‘`while(cond, expr)`’

  Evaluate expression expr while the expression cond is non-zero, and returns the value of the last expr evaluation, or NAN if cond was always false.

‘`ceil(expr)`’

  Round the value of expression expr upwards to the nearest integer. For example, "ceil(1.5)" is "2.0".

‘`floor(expr)`’

  Round the value of expression expr downwards to the nearest integer. For example, "floor(-1.5)" is "-2.0".

‘`trunc(expr)`’

  Round the value of expression expr towards zero to the nearest integer. For example, "trunc(-1.5)" is "-1.0".

‘`sqrt(expr)`’

  Compute the square root of expr. This is equivalent to "(expr)^.5".

‘`not(expr)`’

  Return 1.0 if expr is zero, 0.0 otherwise.

‘`pow(x, y)`’

  Compute the power of x elevated y, it is equivalent to "(x)^(y)".

‘`random(x)`’

  Return a pseudo random value between 0.0 and 1.0. x is the index of the internal variable which will be used to save the seed/state.

‘`hypot(x, y)`’

  This function is similar to the C function with the same name; it returns "sqrt(x*x + y*y)", the length of the hypotenuse of a right triangle with sides of length x and y, or the distance of the point (x, y) from the origin.

‘`gcd(x, y)`’

  Return the greatest common divisor of x and y. If both x and y are 0 or either or both are less than zero then behavior is undefined.

‘`if(x, y)`’

  Evaluate x, and if the result is non-zero return the result of the evaluation of y, return 0 otherwise.

‘`ifnot(x, y)`’

  Evaluate x, and if the result is zero return the result of the evaluation of y, return 0 otherwise.

‘`taylor(expr, x) taylor(expr, x, id)`’

  Evaluate a taylor series at x. expr represents the LD(id)-th derivates of f(x) at 0. If id is not specified then 0 is assumed. note, when you have the derivatives at y instead of 0 taylor(expr, x-y) can be used When the series does not converge the results are undefined.

‘`root(expr, max)`’

  Finds x where f(x)=0 in the interval 0..max. f() must be continuous or the result is undefined.

The following constants are available:

‘`PI`’

  area of the unit disc, approximately 3.14

‘`E`’

  exp(1) (Euler’s number), approximately 2.718

‘`PHI`’

  golden ratio (1+sqrt(5))/2, approximately 1.618

Assuming that an expression is considered "true" if it has a non-zero value, note that:

`*` works like AND

`+` works like OR

and the construct:

    if A then B else C

is equivalent to

    if(A,B) + ifnot(A,C)

In your C code, you can extend the list of unary and binary functions, and define recognized constants, so that they are available for your expressions.

The evaluator also recognizes the International System number postfixes. If ’i’ is appended after the postfix, powers of 2 are used instead of powers of 10. The ’B’ postfix multiplies the value for 8, and can be appended after another postfix or used alone. This allows using for example ’KB’, ’MiB’, ’G’ and ’B’ as postfix.

Follows the list of available International System postfixes, with indication of the corresponding powers of 10 and of 2.

‘`y`’

  -24 / -80

‘`z`’

  -21 / -70

‘`a`’

  -18 / -60

‘`f`’

  -15 / -50

‘`p`’

  -12 / -40

‘`n`’

  -9 / -30

‘`u`’

  -6 / -20

‘`m`’

  -3 / -10

‘`c`’

  -2

‘`d`’

  -1

‘`h`’

  2

‘`k`’

  3 / 10

‘`K`’

  3 / 10

‘`M`’

  6 / 20

‘`G`’

  9 / 30

‘`T`’

  12 / 40

‘`P`’

  15 / 40

‘`E`’

  18 / 50

‘`Z`’

  21 / 60

‘`Y`’

  24 / 70