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Originally committed as revision 25665 to svn://svn.ffmpeg.org/ffmpeg/trunkoldabi
Stefano Sabatini
14 years ago
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@chapter Expression Evaluation |
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@c man begin EXPRESSION EVALUATION |
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When evaluating an arithemetic expression, FFmpeg uses an internal |
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formula evaluator, implemented through the @file{libavutil/eval.h} |
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interface. |
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An expression may contain unary, binary operators, constants, and |
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functions. |
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Two expressions @var{expr1} and @var{expr2} can be combined to form |
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another expression "@var{expr1};@var{expr2}". |
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@var{expr1} and @var{expr2} are evaluated in turn, and the new |
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expression evaluates to the value of @var{expr2}. |
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The following binary operators are available: @code{+}, @code{-}, |
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@code{*}, @code{/}, @code{^}. |
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The following unary operators are available: @code{+}, @code{-}. |
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The following functions are available: |
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@table @option |
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@item sinh(x) |
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@item cosh(x) |
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@item tanh(x) |
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@item sin(x) |
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@item cos(x) |
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@item tan(x) |
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@item atan(x) |
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@item asin(x) |
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@item acos(x) |
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@item exp(x) |
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@item log(x) |
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@item abs(x) |
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@item squish(x) |
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@item gauss(x) |
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@item mod(x, y) |
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@item max(x, y) |
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@item min(x, y) |
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@item eq(x, y) |
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@item gte(x, y) |
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@item gt(x, y) |
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@item lte(x, y) |
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@item lt(x, y) |
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@item st(var, expr) |
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Allow to store the value of the expression @var{expr} in an internal |
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variable. @var{var} specifies the number of the variable where to |
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store the value, and it is a value ranging from 0 to 9. The function |
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returns the value stored in the internal variable. |
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@item ld(var) |
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Allow to load the value of the internal variable with number |
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@var{var}, which was previosly stored with st(@var{var}, @var{expr}). |
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The function returns the loaded value. |
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@item while(cond, expr) |
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Evaluate expression @var{expr} while the expression @var{cond} is |
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non-zero, and returns the value of the last @var{expr} evaluation, or |
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NAN if @var{cond} was always false. |
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@end table |
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Note that: |
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@code{*} works like AND |
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@code{+} works like OR |
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thus |
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@example |
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if A then B else C |
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@end example |
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is equivalent to |
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@example |
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A*B + not(A)*C |
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@end example |
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When A evaluates to either 1 or 0, that is the same as |
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@example |
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A*B + eq(A,0)*C |
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@end example |
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In your C code, you can extend the list of unary and binary functions, |
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and define recognized constants, so that they are available for your |
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expressions. |
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The evaluator also recognizes the International System number |
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postfixes. If 'i' is appended after the postfix, powers of 2 are used |
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instead of powers of 10. The 'B' postfix multiplies the value for 8, |
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and can be appended after another postfix or used alone. This allows |
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using for example 'KB', 'MiB', 'G' and 'B' as postfix. |
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Follows the list of available International System postfixes, with |
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indication of the corresponding powers of 10 and of 2. |
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@table @option |
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@item y |
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-24 / -80 |
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@item z |
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-21 / -70 |
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@item a |
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-18 / -60 |
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@item f |
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-15 / -50 |
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@item p |
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-12 / -40 |
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@item n |
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-9 / -30 |
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@item u |
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-6 / -20 |
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@item m |
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-3 / -10 |
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@item c |
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-2 |
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@item d |
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-1 |
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@item h |
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2 |
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@item k |
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3 / 10 |
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@item K |
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3 / 10 |
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@item M |
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6 / 20 |
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@item G |
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9 / 30 |
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@item T |
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12 / 40 |
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@item P |
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15 / 40 |
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@item E |
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18 / 50 |
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@item Z |
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21 / 60 |
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@item Y |
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24 / 70 |
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@end table |
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@c man end |
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