Re: [FFmpeg-devel] [PATCH 2/2] lavfi/setpts: introduce rand() function in expression

[Stefano Sabatini <stefasab@gmail.com>]

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On date Thursday 2023-12-28 22:25:08 +0100, Michael Niedermayer wrote:
> On Thu, Dec 28, 2023 at 07:49:18PM +0100, Stefano Sabatini wrote:
> > On date Thursday 2023-12-28 16:00:57 +0100, Stefano Sabatini wrote:
> > > On date Thursday 2023-12-28 12:02:59 +0100, Andreas Rheinhardt wrote:
> > > > Stefano Sabatini:
> > > > > This is useful to simulate random jitter.
> > > > > ---
> > > > >  Changelog            |  1 +
> > > > >  doc/filters.texi     | 10 +++++++++-
> > > > >  libavfilter/setpts.c | 39 +++++++++++++++++++++++++++++++++------
> > > > >  3 files changed, 43 insertions(+), 7 deletions(-)
> > [...]
> > > > Why is this added here and not in lavu/eval so that it is available with
> > > > all expressions?
> > > 
> > > There is no specific reason. Sometimes you need to have a control over
> > > the seed, in this case you need to provide the PRNG context.
> > > 
> > > For the general case, probably we can place a global LFG in the eval
> > > module and fetch its generated values.
> > 
> > Another possible solution in attachment, leveraging the same affine
> > PRNG used in random(). For other use cases you might need to use a
> > high-quality PRNG and this might not be good enough.
> 
> >  eval.c |   18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> > 94413dcc88e3046b4987715f6c152aeb1c24c703  0001-lavu-eval-add-randomi-function-to-compute-random-val.patch
> > From 4dc5213b0913a2585f75dfd03b497ca2efc093de Mon Sep 17 00:00:00 2001
> > From: Stefano Sabatini <stefasab@gmail.com>
> > Date: Thu, 28 Dec 2023 19:09:22 +0100
> > Subject: [PATCH] lavu/eval: add randomi function to compute random value in
> >  interval
> 
> probably ok if this is usefull
> 
> thx

Will apply the edited patch in a few days if I see no comments.

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From c4c9eef66401f930a6428c6889020ff5e65c60d6 Mon Sep 17 00:00:00 2001
From: Stefano Sabatini <stefasab@gmail.com>
Date: Thu, 28 Dec 2023 19:09:22 +0100
Subject: [PATCH 1/2] lavu/eval: add randomi function to compute random value
 in interval

---
 Changelog        |  1 +
 doc/utils.texi   | 12 +++++++++---
 libavutil/eval.c | 26 +++++++++++++++++++-------
 3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/Changelog b/Changelog
index 424bfc11af..419075dc56 100644
--- a/Changelog
+++ b/Changelog
@@ -15,6 +15,7 @@ version <next>:
 - tiltandshift filter
 - qrencode filter and qrencodesrc source
 - quirc filter
+- lavu/eval: introduce randomi() function in expressions
 
 version 6.1:
 - libaribcaption decoder
diff --git a/doc/utils.texi b/doc/utils.texi
index a0b8d4b62d..0c4f146f4f 100644
--- a/doc/utils.texi
+++ b/doc/utils.texi
@@ -939,9 +939,15 @@ Returns the value of the expression printed.
 
 Prints t with loglevel l
 
-@item random(x)
-Return a pseudo random value between 0.0 and 1.0. @var{x} is the index of the
-internal variable which will be used to save the seed/state.
+@item random(idx)
+Return a pseudo random value between 0.0 and 1.0. @var{idx} is the
+index of the internal variable which will be used to save the
+seed/state.
+
+@item randomi(idx, min, max)
+Return a pseudo random value in the interval between @var{min} and
+@var{max}. @var{idx} is the index of the internal variable which will
+be used to save the seed/state.
 
 @item root(expr, max)
 Find an input value for which the function represented by @var{expr}
diff --git a/libavutil/eval.c b/libavutil/eval.c
index bad9e4ecb8..dc6b3697bc 100644
--- a/libavutil/eval.c
+++ b/libavutil/eval.c
@@ -162,7 +162,7 @@ struct AVExpr {
         e_last, e_st, e_while, e_taylor, e_root, e_floor, e_ceil, e_trunc, e_round,
         e_sqrt, e_not, e_random, e_hypot, e_gcd,
         e_if, e_ifnot, e_print, e_bitand, e_bitor, e_between, e_clip, e_atan2, e_lerp,
-        e_sgn,
+        e_sgn, e_randomi
     } type;
     double value; // is sign in other types
     int const_index;
@@ -228,12 +228,22 @@ static double eval_expr(Parser *p, AVExpr *e)
             av_log(p, level, "%f\n", x);
             return x;
         }
-        case e_random:{
-            int idx= av_clip(eval_expr(p, e->param[0]), 0, VARS-1);
-            uint64_t r= isnan(p->var[idx]) ? 0 : p->var[idx];
-            r= r*1664525+1013904223;
-            p->var[idx]= r;
-            return e->value * (r * (1.0/UINT64_MAX));
+
+#define COMPUTE_NEXT_RANDOM()                                        \
+            int idx = av_clip(eval_expr(p, e->param[0]), 0, VARS-1); \
+            uint64_t r = isnan(p->var[idx]) ? 0 : p->var[idx];       \
+            r = r * 1664525 + 1013904223;                            \
+            p->var[idx] = r;                                         \
+
+        case e_random: {
+            COMPUTE_NEXT_RANDOM();
+            return r * (1.0/UINT64_MAX);
+        }
+        case e_randomi: {
+            double min = eval_expr(p, e->param[1]);
+            double max = eval_expr(p, e->param[2]);
+            COMPUTE_NEXT_RANDOM();
+            return min + (max - min) * r / UINT64_MAX;
         }
         case e_while: {
             double d = NAN;
@@ -461,6 +471,7 @@ static int parse_primary(AVExpr **e, Parser *p)
     else if (strmatch(next, "pow"   )) d->type = e_pow;
     else if (strmatch(next, "print" )) d->type = e_print;
     else if (strmatch(next, "random")) d->type = e_random;
+    else if (strmatch(next, "randomi")) d->type = e_randomi;
     else if (strmatch(next, "hypot" )) d->type = e_hypot;
     else if (strmatch(next, "gcd"   )) d->type = e_gcd;
     else if (strmatch(next, "if"    )) d->type = e_if;
@@ -674,6 +685,7 @@ static int verify_expr(AVExpr *e)
         case e_between:
         case e_clip:
         case e_lerp:
+        case e_randomi:
             return verify_expr(e->param[0]) &&
                    verify_expr(e->param[1]) &&
                    verify_expr(e->param[2]);
-- 
2.34.1


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From 9f7bc9370ad853392622ac3008f14df99ed6210a Mon Sep 17 00:00:00 2001
From: Stefano Sabatini <stefasab@gmail.com>
Date: Fri, 29 Dec 2023 12:48:08 +0100
Subject: [PATCH 2/2] doc/filters/setpts: add random jitter generation example

---
 doc/filters.texi | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/doc/filters.texi b/doc/filters.texi
index d1f95b9781..6a9c7d532c 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -31039,6 +31039,12 @@ Set fixed rate of 25 frames per second:
 setpts=N/(25*TB)
 @end example
 
+@item
+Apply a random jitter effect of +/-100 TB units:
+@example
+setpts=PTS+randomi(0, -100\,100)
+@end example
+
 @item
 Set fixed rate 25 fps with some jitter:
 @example
-- 
2.34.1


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