| @chapter Expression Evaluation |
| @c man begin EXPRESSION EVALUATION |
| |
| When evaluating an arithemetic expression, FFmpeg uses an internal |
| formula evaluator, implemented through the @file{libavutil/eval.h} |
| interface. |
| |
| An expression may contain unary, binary operators, constants, and |
| functions. |
| |
| Two expressions @var{expr1} and @var{expr2} can be combined to form |
| another expression "@var{expr1};@var{expr2}". |
| @var{expr1} and @var{expr2} are evaluated in turn, and the new |
| expression evaluates to the value of @var{expr2}. |
| |
| The following binary operators are available: @code{+}, @code{-}, |
| @code{*}, @code{/}, @code{^}. |
| |
| The following unary operators are available: @code{+}, @code{-}. |
| |
| The following functions are available: |
| @table @option |
| @item sinh(x) |
| @item cosh(x) |
| @item tanh(x) |
| @item sin(x) |
| @item cos(x) |
| @item tan(x) |
| @item atan(x) |
| @item asin(x) |
| @item acos(x) |
| @item exp(x) |
| @item log(x) |
| @item abs(x) |
| @item squish(x) |
| @item gauss(x) |
| @item isnan(x) |
| Return 1.0 if @var{x} is NAN, 0.0 otherwise. |
| |
| @item mod(x, y) |
| @item max(x, y) |
| @item min(x, y) |
| @item eq(x, y) |
| @item gte(x, y) |
| @item gt(x, y) |
| @item lte(x, y) |
| @item lt(x, y) |
| @item st(var, expr) |
| Allow to store the value of the expression @var{expr} in an internal |
| variable. @var{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. |
| |
| @item ld(var) |
| Allow to load the value of the internal variable with number |
| @var{var}, which was previosly stored with st(@var{var}, @var{expr}). |
| The function returns the loaded value. |
| |
| @item while(cond, expr) |
| Evaluate expression @var{expr} while the expression @var{cond} is |
| non-zero, and returns the value of the last @var{expr} evaluation, or |
| NAN if @var{cond} was always false. |
| |
| @item ceil(expr) |
| Round the value of expression @var{expr} upwards to the nearest |
| integer. For example, "ceil(1.5)" is "2.0". |
| |
| @item floor(expr) |
| Round the value of expression @var{expr} downwards to the nearest |
| integer. For example, "floor(-1.5)" is "-2.0". |
| |
| @item trunc(expr) |
| Round the value of expression @var{expr} towards zero to the nearest |
| integer. For example, "trunc(-1.5)" is "-1.0". |
| |
| @item sqrt(expr) |
| Compute the square root of @var{expr}. This is equivalent to |
| "(@var{expr})^.5". |
| |
| @item not(expr) |
| Return 1.0 if @var{expr} is zero, 0.0 otherwise. |
| |
| @item pow(x, y) |
| Compute the power of @var{x} elevated @var{y}, it is equivalent to |
| "(@var{x})^(@var{y})". |
| @end table |
| |
| Note that: |
| |
| @code{*} works like AND |
| |
| @code{+} works like OR |
| |
| thus |
| @example |
| if A then B else C |
| @end example |
| is equivalent to |
| @example |
| A*B + not(A)*C |
| @end example |
| |
| 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. |
| @table @option |
| @item y |
| -24 / -80 |
| @item z |
| -21 / -70 |
| @item a |
| -18 / -60 |
| @item f |
| -15 / -50 |
| @item p |
| -12 / -40 |
| @item n |
| -9 / -30 |
| @item u |
| -6 / -20 |
| @item m |
| -3 / -10 |
| @item c |
| -2 |
| @item d |
| -1 |
| @item h |
| 2 |
| @item k |
| 3 / 10 |
| @item K |
| 3 / 10 |
| @item M |
| 6 / 20 |
| @item G |
| 9 / 30 |
| @item T |
| 12 / 40 |
| @item P |
| 15 / 40 |
| @item E |
| 18 / 50 |
| @item Z |
| 21 / 60 |
| @item Y |
| 24 / 70 |
| @end table |
| |
| @c man end |