Re: [FFmpeg-devel] lavfi: add perlin noise generator

[Andrew Sayers <ffmpeg-devel@pileofstuff.org>]

Some documentation nitpicks.  Nothing jumped out about the code, but I don't
know the algorithm well enough to spot anything deep.

> From 9932cfc19500acbd0685eb2cc8fd88e9af3f5dbd Mon Sep 17 00:00:00 2001
> From: Stefano Sabatini <stefasab@gmail.com>
> Date: Mon, 27 May 2024 11:19:08 +0200
> Subject: [PATCH] lavfi: add Perlin noise generator
> 
> ---
>  Changelog                 |   1 +
>  doc/filters.texi          | 100 +++++++++++++++++
>  libavfilter/Makefile      |   1 +
>  libavfilter/allfilters.c  |   1 +
>  libavfilter/perlin.c      | 224 ++++++++++++++++++++++++++++++++++++++
>  libavfilter/perlin.h      | 101 +++++++++++++++++
>  libavfilter/vsrc_perlin.c | 169 ++++++++++++++++++++++++++++
>  7 files changed, 597 insertions(+)
>  create mode 100644 libavfilter/perlin.c
>  create mode 100644 libavfilter/perlin.h
>  create mode 100644 libavfilter/vsrc_perlin.c
> 
> diff --git a/Changelog b/Changelog
> index 03d6b29ad8..b8dcf452ac 100644
> --- a/Changelog
> +++ b/Changelog
> @@ -12,6 +12,7 @@ version <next>:
>  - qsv_params option added for QSV encoders
>  - VVC decoder compatible with DVB test content
>  - xHE-AAC decoder
> +- perlin source
>  
>  
>  version 7.0:
> diff --git a/doc/filters.texi b/doc/filters.texi
> index 347103c04f..7af299b2a2 100644
> --- a/doc/filters.texi
> +++ b/doc/filters.texi
> @@ -17290,6 +17290,9 @@ The command accepts the same syntax of the corresponding option.
>  If the specified expression is not valid, it is kept at its current
>  value.
>  
> +@anchor{lutrgb}
> +@anchor{lutyuv}
> +@anchor{lut}
>  @section lut, lutrgb, lutyuv
>  
>  Compute a look-up table for binding each pixel component input value
> @@ -29281,6 +29284,103 @@ ffplay -f lavfi life=s=300x200:mold=10:r=60:ratio=0.1:death_color=#C83232:life_c
>  @end example
>  @end itemize
>  
> +@section perlin
> +Generate Perlin noise.
> +
> +Perlin noise is a kind of noise with local continuity in space. This
> +can be used to generate patterns with continuity in space and time,
> +e.g. to simulate smoke, fluids, or terrain.
> +
> +In case more than one octave is specified through the @option{octaves}
> +option, Perlin noise is generated as a sum of components, each one
> +with doubled frequency. In this case the @option{persistence} option
> +specify the ratio of the amplitude with respect to the previous
> +component. More octave components enable to specify more high
> +frequency details in the generated noise (e.g. small size variations
> +due to bolders in a generated terrain).

Typo: s/bolders/boulders/

> +
> +@subsection Options
> +@table @option
> +
> +@item size, s
> +Specify the size (width and height) of the buffered video frames. For the
> +syntax of this option, check the
> +@ref{video size syntax,,"Video size" section in the ffmpeg-utils manual,ffmpeg-utils}.
> +
> +@item rate, r
> +Specify the frame rate expected for the video stream, expressed as a
> +number of frames per second.
> +
> +@item octaves
> +Specify the total number of components making up the noise, each one
> +with doubled frequency.
> +
> +@item persistence
> +Set the ratio used to compute the amplitude of the next octave
> +component with respect to the previous component amplitude.
> +
> +@item xscale
> +@item yscale
> +Define a scale factor used to multiple the x, y coordinates. This can
> +be useful to define an effect with a pattern stretched along the x or
> +y axis.
> +
> +@item tscale
> +Define a scale factor used to multiple the time coordinate. This can
> +be useful to change the time variation speed.
> +
> +@item random_mode
> +Set random mode used to compute initial pattern.
> +
> +Supported values are:
> +@table @option
> +@item random
> +Compute and use random seed.
> +
> +@item ken
> +Use the predefined initial pattern defined by Ken Perlin in the
> +original article, can be useful to compare the output with other
> +sources.
> +
> +@item seed
> +Use the value specified by @option{random_seed} option.
> +@end table

Nit: "Define a...", "Use the..." etc. is redundant - remove them to
optimise for reading time.

> +
> +@item random_seed, seed
> +Use this value to compute the initial pattern, it is only considered
> +when @option{random_mode} is set to @var{random_seed}.
> +@end table

Nit: the reader needs to read the second clause before the first makes sense.
Consider something like:

When @option{random_mode} is set to @var{random_seed},
this value computes the initial pattern.

> +
> +@subsection Examples
> +@itemize
> +@item
> +Generate single component:
> +@example
> +perlin
> +@end example
> +
> +@item
> +Use Perlin noise with 7 components, each one with a halved contribute

s/contribute/contribution/

> +to total amplitude:
> +@example
> +perlin=octaves=7:persistence=0.5
> +@end example
> +
> +@item
> +Chain Perlin noise with the @ref{lutyuv} to generate a black&white
> +effect:
> +@example
> +perlin:octaves=7:tscale=0.3,lutyuv=y='if(lt(val\,128)\,255\,0)'
> +@end example
> +
> +@item
> +Stretch noise along the y axis, and convert gray level to red-only

I initially thought this was a typo for "read-only" - maybe s/red/blue/?

> +signal:
> +@example
> +perlin=octaves=7:tscale=0.4:yscale=0.3,lutrgb=r=val:b=0:g=0
> +@end example
> +@end itemize
> +
>  @section qrencodesrc
>  
>  Generate a QR code using the libqrencode library (see
> diff --git a/libavfilter/Makefile b/libavfilter/Makefile
> index 5992fd161f..63088e9286 100644
> --- a/libavfilter/Makefile
> +++ b/libavfilter/Makefile
> @@ -603,6 +603,7 @@ OBJS-$(CONFIG_NULLSRC_FILTER)                += vsrc_testsrc.o
>  OBJS-$(CONFIG_OPENCLSRC_FILTER)              += vf_program_opencl.o opencl.o
>  OBJS-$(CONFIG_PAL75BARS_FILTER)              += vsrc_testsrc.o
>  OBJS-$(CONFIG_PAL100BARS_FILTER)             += vsrc_testsrc.o
> +OBJS-$(CONFIG_PERLIN_FILTER)                 += vsrc_perlin.o perlin.o
>  OBJS-$(CONFIG_QRENCODE_FILTER)               += qrencode.o textutils.o
>  OBJS-$(CONFIG_QRENCODESRC_FILTER)            += qrencode.o textutils.o
>  OBJS-$(CONFIG_RGBTESTSRC_FILTER)             += vsrc_testsrc.o
> diff --git a/libavfilter/allfilters.c b/libavfilter/allfilters.c
> index c532682fc2..63600e9b58 100644
> --- a/libavfilter/allfilters.c
> +++ b/libavfilter/allfilters.c
> @@ -569,6 +569,7 @@ extern const AVFilter ff_vsrc_openclsrc;
>  extern const AVFilter ff_vsrc_qrencodesrc;
>  extern const AVFilter ff_vsrc_pal75bars;
>  extern const AVFilter ff_vsrc_pal100bars;
> +extern const AVFilter ff_vsrc_perlin;
>  extern const AVFilter ff_vsrc_rgbtestsrc;
>  extern const AVFilter ff_vsrc_sierpinski;
>  extern const AVFilter ff_vsrc_smptebars;
> diff --git a/libavfilter/perlin.c b/libavfilter/perlin.c
> new file mode 100644
> index 0000000000..f09020bf8f
> --- /dev/null
> +++ b/libavfilter/perlin.c
> @@ -0,0 +1,224 @@
> +/*
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License along
> + * with FFmpeg; if not, write to the Free Software Foundation, Inc.,
> + * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
> + */
> +
> +/**
> + * @file
> + * Perlin Noise generator, based on code from:
> + * https://adrianb.io/2014/08/09/perlinnoise.html
> + *
> + * Original article from Ken Perlin:
> + * http://mrl.nyu.edu/~perlin/paper445.pdf
> + */
> +
> +#include <math.h>
> +
> +#include "libavutil/lfg.h"
> +#include "libavutil/random_seed.h"
> +#include "perlin.h"
> +
> +static inline int inc(int num, int period)
> +{
> +    num++;
> +    if (period > 0)
> +        num %= period;
> +    return num;
> +}
> +
> +static inline double grad(int hash, double x, double y, double z)
> +{
> +    // Take the hashed value and take the first 4 bits of it (15 == 0b1111)
> +    int h = hash & 15;
> +    // If the most significant bit (MSB) of the hash is 0 then set u = x.  Otherwise y.
> +    double u = h < 8 /* 0b1000 */ ? x : y;
> +    double v;
> +
> +    // In Ken Perlin's original implementation this was another
> +    // conditional operator (?:), then expanded for readability.
> +    if (h < 4 /* 0b0100 */)
> +        // If the first and second significant bits are 0 set v = y
> +        v = y;
> +    // If the first and second significant bits are 1 set v = x
> +    else if (h == 12 /* 0b1100 */ || h == 14 /* 0b1110*/)

Nit: add space before final *

> +        v = x;
> +    else
> +        // If the first and second significant bits are not equal (0/1, 1/0) set v = z
> +        v = z;
> +
> +    // Use the last 2 bits to decide if u and v are positive or negative.  Then return their addition.
> +    return ((h&1) == 0 ? u : -u)+((h&2) == 0 ? v : -v);
> +}
> +
> +static inline double fade(double t)
> +{
> +    // Fade function as defined by Ken Perlin. This eases coordinate values
> +    // so that they will "ease" towards integral values. This ends up smoothing
> +    // the final output.

Given the verbosity of the comments so far, do you want to explicitly mention
how this is using Horner's method?

https://en.wikipedia.org/wiki/Horner%27s_method

> +    return t * t * t * (t * (t * 6 - 15) + 10);                     // 6t^5 - 15t^4 + 10t^3
> +}
> +
> +static double lerp(double a, double b, double x)
> +{
> +    return a + x * (b - a);
> +}
> +
> +// Hash lookup table as defined by Ken Perlin.  This is a randomly
> +// arranged array of all numbers from 0-255 inclusive.
> +static uint8_t ken_permutations[] = {
> +    151, 160, 137,  91,  90,  15, 131,  13, 201,  95,  96,  53, 194, 233,   7, 225,
> +    140,  36, 103,  30,  69, 142,   8,  99,  37, 240,  21,  10,  23, 190,   6, 148,
> +    247, 120, 234,  75,   0,  26, 197,  62,  94, 252, 219, 203, 117,  35,  11,  32,
> +     57, 177,  33,  88, 237, 149,  56,  87, 174,  20, 125, 136, 171, 168,  68, 175,
> +     74, 165,  71, 134, 139,  48,  27, 166,  77, 146, 158, 231,  83, 111, 229, 122,
> +     60, 211, 133, 230, 220, 105,  92,  41,  55,  46, 245,  40, 244, 102, 143,  54,
> +     65,  25,  63, 161,   1, 216,  80,  73, 209,  76, 132, 187, 208,  89,  18, 169,
> +    200, 196, 135, 130, 116, 188, 159,  86, 164, 100, 109, 198, 173, 186,   3,  64,
> +     52, 217, 226, 250, 124, 123,   5, 202,  38, 147, 118, 126, 255,  82,  85, 212,
> +    207, 206,  59, 227,  47,  16,  58,  17, 182, 189,  28,  42, 223, 183, 170, 213,
> +    119, 248, 152,   2,  44, 154, 163,  70, 221, 153, 101, 155, 167,  43, 172,   9,
> +    129,  22,  39, 253,  19,  98, 108, 110,  79, 113, 224, 232, 178, 185, 112, 104,
> +    218, 246,  97, 228, 251,  34, 242, 193, 238, 210, 144,  12, 191, 179, 162, 241,
> +     81,  51, 145, 235, 249,  14, 239, 107,  49, 192, 214,  31, 181, 199, 106, 157,
> +    184,  84, 204, 176, 115, 121,  50,  45, 127,   4, 150, 254, 138, 236, 205,  93,
> +    222, 114,  67,  29,  24,  72, 243, 141, 128, 195,  78,  66, 215,  61, 156, 180
> +};
> +
> +int ff_perlin_init(FFPerlin *perlin, double period, int octaves, double persistence,
> +                   enum FFPerlinRandomMode random_mode, unsigned int random_seed)
> +{
> +    int i;
> +
> +    /* todo: perform validation? */

Should this line be removed?

> +    perlin->period = period;
> +    perlin->octaves = octaves;
> +    perlin->persistence = persistence;
> +    perlin->random_mode = random_mode;
> +    perlin->random_seed = random_seed;
> +
> +    if (perlin->random_mode == FF_PERLIN_RANDOM_MODE_KEN) {
> +        for (i = 0; i < 512; i++) {
> +            perlin->permutations[i] = ken_permutations[i % 256];
> +        }
> +    } else {
> +        AVLFG lfg;
> +        uint8_t random_permutations[256];
> +
> +        if (perlin->random_mode == FF_PERLIN_RANDOM_MODE_RANDOM)
> +            perlin->random_seed = av_get_random_seed();
> +
> +        av_lfg_init(&lfg, perlin->random_seed);
> +
> +        for (i = 0; i < 256; i++) {
> +            random_permutations[i] = i;
> +        }
> +
> +        for (i = 0; i < 256; i++) {
> +            unsigned int random_idx = av_lfg_get(&lfg) % (256-i);
> +            uint8_t random_val = random_permutations[random_idx];
> +            random_permutations[random_idx] = random_permutations[256-i];
> +
> +            perlin->permutations[i] = perlin->permutations[i+256] = random_val;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
> +static double perlin_get(FFPerlin *perlin, double x, double y, double z)
> +{
> +    int xi, yi, zi;
> +    double xf, yf, zf;
> +    double u, v, w;
> +    const uint8_t *p = perlin->permutations;
> +    double period = perlin->period;
> +    int aaa, aba, aab, abb, baa, bba, bab, bbb;
> +    double x1, x2, y1, y2;
> +
> +    if (perlin->period > 0) {
> +        // If we have any period on, change the coordinates to their "local" repetitions
> +        x = fmod(x, perlin->period);
> +        y = fmod(y, perlin->period);
> +        z = fmod(z, perlin->period);
> +    }
> +
> +    // Calculate the "unit cube" that the point asked will be located in
> +    // The left bound is ( |_x_|,|_y_|,|_z_| ) and the right bound is that
> +    // plus 1.  Next we calculate the location (from 0.0 to 1.0) in that cube.
> +    xi = (int)x & 255;
> +    yi = (int)y & 255;
> +    zi = (int)z & 255;
> +
> +    xf = x - (int)x;
> +    yf = y - (int)y;
> +    zf = z - (int)z;
> +
> +    // We also fade the location to smooth the result.
> +    u = fade(xf);
> +    v = fade(yf);
> +    w = fade(zf);
> +
> +    aaa = p[p[p[    xi         ] +     yi         ] +     zi         ];
> +    aba = p[p[p[    xi         ] + inc(yi, period)] +     zi         ];
> +    aab = p[p[p[    xi         ] +     yi         ] + inc(zi, period)];
> +    abb = p[p[p[    xi         ] + inc(yi, period)] + inc(zi, period)];
> +    baa = p[p[p[inc(xi, period)] +     yi         ] +     zi         ];
> +    bba = p[p[p[inc(xi, period)] + inc(yi, period)] +     zi         ];
> +    bab = p[p[p[inc(xi, period)] +     yi         ] + inc(zi, period)];
> +    bbb = p[p[p[inc(xi, period)] + inc(yi, period)] + inc(zi, period)];
> +
> +    // The gradient function calculates the dot product between a pseudorandom
> +    // gradient vector and the vector from the input coordinate to the 8
> +    // surrounding points in its unit cube.
> +    // This is all then lerped together as a sort of weighted average based on the faded (u,v,w)
> +    // values we made earlier.
> +    x1 = lerp(grad(aaa, xf  , yf  , zf),
> +              grad(baa, xf-1, yf  , zf),
> +              u);
> +    x2 = lerp(grad(aba, xf  , yf-1, zf),
> +              grad(bba, xf-1, yf-1, zf),
> +              u);
> +    y1 = lerp(x1, x2, v);
> +
> +    x1 = lerp(grad(aab, xf  , yf  , zf-1),
> +              grad(bab, xf-1, yf  , zf-1),
> +              u);
> +    x2 = lerp(grad(abb, xf  , yf-1, zf-1),
> +              grad(bbb, xf-1, yf-1, zf-1),
> +                    u);
> +    y2 = lerp(x1, x2, v);
> +
> +    // For convenience we bound it to 0 - 1 (theoretical min/max before is -1 - 1)
> +    return (lerp(y1, y2, w) + 1) / 2;
> +}
> +
> +double ff_perlin_get(FFPerlin *perlin, double x, double y, double z)
> +{
> +    double total = 0;
> +    double frequency = 1;
> +    double amplitude = 1;
> +    double max_value = 0;                   // Used for normalizing result to 0.0 - 1.0
> +
> +    for (int i = 0; i < perlin->octaves; i++) {
> +        total += perlin_get(perlin, x * frequency, y * frequency, z * frequency) * amplitude;
> +        max_value += amplitude;
> +        amplitude *= perlin->persistence;
> +        frequency *= 2;
> +    }
> +
> +    return total / max_value;
> +}
> +
> diff --git a/libavfilter/perlin.h b/libavfilter/perlin.h
> new file mode 100644
> index 0000000000..1d2922a587
> --- /dev/null
> +++ b/libavfilter/perlin.h
> @@ -0,0 +1,101 @@
> +/*
> + * Perlin noise generator
> + *
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +/**
> + * @file
> + * Perlin Noise generator
> + */
> +
> +#ifndef AVFILTER_PERLIN_H
> +#define AVFILTER_PERLIN_H
> +
> +#include <stdint.h>
> +
> +enum FFPerlinRandomMode {
> +    FF_PERLIN_RANDOM_MODE_RANDOM,
> +    FF_PERLIN_RANDOM_MODE_KEN,
> +    FF_PERLIN_RANDOM_MODE_SEED,
> +    FF_PERLIN_RANDOM_MODE_NB
> +};
> +
> +/**
> + * Perlin generator context. This needs to be initialized with the
> + * parameters used to generate the Perlin noise.
> + */
> +typedef struct FFPerlin {
> +    /**
> +     * spatial repeat period, if negative it is ignored
> +     */
> +    double period;
> +
> +    /**
> +     * total number of components making up the noise, each one with
> +     * doubled frequency
> +     */
> +    int octaves;
> +
> +    /**
> +     * ratio used to compute the amplitude of the next octave
> +     * component with respect to the previous component
> +     */
> +    double persistence;
> +
> +    /**
> +     * permutations array used to compute the Perlin noise hash
> +     */
> +    uint8_t permutations[512];
> +
> +    /**
> +     * define how to compute the permutations array
> +     */
> +    enum FFPerlinRandomMode random_mode;
> +
> +    /**
> +     * random seed used to compute the permutations array when random_mode is set to
> +     * FF_PERLIN_RANDOM_MODE_RANDOM,

Nit: remove trailing comma

> +     */
> +    unsigned int random_seed;
> +} FFPerlin;
> +
> +/**
> + * Initialize the Perlin noise generator with parameters.
> + *
> + * @param perlin Perlin noise generator context
> + * @param period spatial repeat period, if negative it is ignored
> + * @param octaves total number of components making up the noise, each one with doubled frequency
> + * @param persistence define how to ratio used to compute the amplitude of the next
> + *                    octave component with respect to the previous component

I think "how to ratio used" should be "how the ratio is used"?

> + * @param random_mode define how to compute the permutations array
> + * @param random_seed random seed used to compute the permutations array when random_mode is set to
> + *                    FF_PERLIN_RANDOM_MODE_RANDOM,

Nit: trailing comma again

> + * @return a negative AVERROR code in case of error, a non negative value otherwise
> + */
> +int ff_perlin_init(FFPerlin *perlin, double period, int octaves, double persistence,
> +                   enum FFPerlinRandomMode random_mode, unsigned int random_seed);
> +
> +/**
> + * Compute Perlin noise for the 3D coordinates given the x, y, z coordinates.

Nit: "for the 3D coordinates" and "given the x, y, z coordinates" is redundant -
either is fine, but not both.

> + *
> + * @param perlin Perlin noise generator context
> + * @return normalized value for the perlin noise, in the range [0, 1]
> + */
> +double ff_perlin_get(FFPerlin *perlin, double x, double y, double z);
> +
> +#endif  /* AVFILTER_PERLIN_H */
> diff --git a/libavfilter/vsrc_perlin.c b/libavfilter/vsrc_perlin.c
> new file mode 100644
> index 0000000000..27493a1b3b
> --- /dev/null
> +++ b/libavfilter/vsrc_perlin.c
> @@ -0,0 +1,169 @@
> +/*
> + * This file is part of FFmpeg.
> + *
> + * FFmpeg is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2.1 of the License, or (at your option) any later version.
> + *
> + * FFmpeg is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with FFmpeg; if not, write to the Free Software
> + * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
> + */
> +
> +/**
> + * @file
> + * Perlin noise generator
> + */
> +
> +#include <float.h>
> +
> +#include "perlin.h"
> +#include "libavutil/lfg.h"
> +#include "libavutil/opt.h"
> +#include "avfilter.h"
> +#include "internal.h"
> +#include "formats.h"
> +#include "video.h"
> +
> +typedef struct PerlinContext {
> +    const AVClass *class;
> +
> +    int w, h;
> +    AVRational frame_rate;
> +
> +    FFPerlin perlin;
> +    int octaves;
> +    double persistence;
> +    unsigned int random_seed;
> +    enum FFPerlinRandomMode random_mode;
> +
> +    double xscale, yscale, tscale;
> +    uint64_t pts;
> +} PerlinContext;
> +
> +#define OFFSET(x) offsetof(PerlinContext, x)
> +#define FLAGS AV_OPT_FLAG_VIDEO_PARAM|AV_OPT_FLAG_FILTERING_PARAM
> +
> +static const AVOption perlin_options[] = {
> +    { "size",     "set video size",   OFFSET(w),        AV_OPT_TYPE_IMAGE_SIZE, {.str = "320x240"}, 0, 0, FLAGS },
> +    { "s",        "set video size",   OFFSET(w),        AV_OPT_TYPE_IMAGE_SIZE, {.str = "320x240"}, 0, 0, FLAGS },
> +    { "rate",     "set video rate",   OFFSET(frame_rate), AV_OPT_TYPE_VIDEO_RATE, {.str = "25"}, 0, INT_MAX, FLAGS },
> +    { "r",        "set video rate",   OFFSET(frame_rate), AV_OPT_TYPE_VIDEO_RATE, {.str = "25"}, 0, INT_MAX, FLAGS },
> +    { "octaves", "set the number of components to use to generate the noise", OFFSET(octaves), AV_OPT_TYPE_INT, {.i64=1}, 1, INT_MAX, FLAGS },
> +    { "persistence", "set the octaves persistence", OFFSET(persistence), AV_OPT_TYPE_DOUBLE, {.dbl=1}, 0.0, DBL_MAX, FLAGS },
> +
> +    { "xscale", "set x-scale factor", OFFSET(xscale), AV_OPT_TYPE_DOUBLE, {.dbl=1}, 0.0, DBL_MAX, FLAGS },
> +    { "yscale", "set y-scale factor", OFFSET(yscale), AV_OPT_TYPE_DOUBLE, {.dbl=1}, 0.0, DBL_MAX, FLAGS },
> +    { "tscale", "set t-scale factor", OFFSET(tscale), AV_OPT_TYPE_DOUBLE, {.dbl=1}, 0.0, DBL_MAX, FLAGS },
> +
> +    { "random_mode", "set random mode used to compute initial pattern", OFFSET(random_mode), AV_OPT_TYPE_INT, {.i64=FF_PERLIN_RANDOM_MODE_RANDOM}, 0, FF_PERLIN_RANDOM_MODE_NB-1, FLAGS, .unit = "random_mode" },
> +    { "random", "compute and use random seed", 0, AV_OPT_TYPE_CONST, {.i64=FF_PERLIN_RANDOM_MODE_RANDOM},   0, 0, FLAGS, .unit = "random_mode" },
> +    { "ken", "use the predefined initial pattern defined by Ken Perlin in the original article", 0, AV_OPT_TYPE_CONST, {.i64=FF_PERLIN_RANDOM_MODE_KEN}, 0, 0, FLAGS, .unit = "random_mode" },
> +    { "seed", "use the value specified by random_seed", 0, AV_OPT_TYPE_CONST, {.i64=FF_PERLIN_RANDOM_MODE_SEED}, 0, 0, FLAGS, .unit = "random_mode" },
> +
> +    { "random_seed", "set the seed for filling the initial grid randomly", OFFSET(random_seed), AV_OPT_TYPE_UINT, {.i64=0}, 0, UINT_MAX, FLAGS },
> +    { "seed",        "set the seed for filling the initial grid randomly", OFFSET(random_seed), AV_OPT_TYPE_UINT, {.i64=0}, 0, UINT_MAX, FLAGS },

Nit: "set the seed for filling the initial grid randomly" is ambiguous.
Do you mean:

a) Set the seed for the RNG to a specified value (value is the seed number)
b) Pick a random number to fill the grid with (value is a boolean yes/no)

I settled on the former after several reads, but would appreciate clearer wording.

> +    { NULL }
> +};
> +
> +AVFILTER_DEFINE_CLASS(perlin);
> +
> +static av_cold int init(AVFilterContext *ctx)
> +{
> +    PerlinContext *perlin = ctx->priv;
> +    int ret;
> +
> +    if (ret = ff_perlin_init(&perlin->perlin, -1, perlin->octaves, perlin->persistence,
> +                             perlin->random_mode, perlin->random_seed)) {
> +        return ret;
> +    }
> +
> +    av_log(ctx, AV_LOG_VERBOSE,
> +           "s:%dx%d r:%d/%d octaves:%d persistence:%f xscale:%f yscale:%f tscale:%f\n",
> +           perlin->w, perlin->h, perlin->frame_rate.num, perlin->frame_rate.den,
> +           perlin->octaves, perlin->persistence,
> +           perlin->xscale, perlin->yscale, perlin->tscale);
> +    return 0;
> +}
> +
> +static int config_props(AVFilterLink *outlink)
> +{
> +    PerlinContext *perlin = outlink->src->priv;
> +
> +    outlink->w = perlin->w;
> +    outlink->h = perlin->h;
> +    outlink->time_base = av_inv_q(perlin->frame_rate);
> +    outlink->frame_rate = perlin->frame_rate;
> +
> +    return 0;
> +}
> +
> +static int request_frame(AVFilterLink *outlink)
> +{
> +    AVFilterContext *ctx = outlink->src;
> +    PerlinContext *perlin = ctx->priv;
> +    AVFrame *picref = ff_get_video_buffer(outlink, perlin->w, perlin->h);
> +    int i, j;
> +    uint8_t *data0, *data;
> +    double x, y, t;
> +
> +    if (!picref)
> +        return AVERROR(ENOMEM);
> +
> +    picref->sample_aspect_ratio = (AVRational) {1, 1};
> +    picref->pts = perlin->pts++;
> +    picref->duration = 1;
> +
> +    t = perlin->tscale * (perlin->pts * av_q2d(outlink->time_base));
> +    data0 = picref->data[0];
> +
> +    for (i = 0; i < perlin->h; i++) {
> +        y = perlin->yscale * (double)i / perlin->h;
> +
> +        data = data0;
> +
> +        for (j = 0; j < perlin->w; j++) {
> +            double res;
> +            x = perlin->xscale * (double)j / perlin->w;
> +            res = ff_perlin_get(&perlin->perlin, x, y, t);
> +            av_log(ctx, AV_LOG_DEBUG, "x:%f y:%f t:%f => %f\n", x, y, t, res);
> +            *data++ = res * 255;
> +        }
> +        data0 += picref->linesize[0];
> +    }
> +
> +    return ff_filter_frame(outlink, picref);
> +}
> +
> +static int query_formats(AVFilterContext *ctx)
> +{
> +    enum AVPixelFormat pix_fmts[] = { AV_PIX_FMT_GRAY8, AV_PIX_FMT_NONE };
> +
> +    return ff_set_common_formats_from_list(ctx, pix_fmts);
> +}
> +
> +static const AVFilterPad perlin_outputs[] = {
> +    {
> +        .name          = "default",
> +        .type          = AVMEDIA_TYPE_VIDEO,
> +        .request_frame = request_frame,
> +        .config_props  = config_props,
> +    },
> +};
> +
> +const AVFilter ff_vsrc_perlin = {
> +    .name          = "perlin",
> +    .description   = NULL_IF_CONFIG_SMALL("Generate Perlin noise"),
> +    .priv_size     = sizeof(PerlinContext),
> +    .priv_class    = &perlin_class,
> +    .init          = init,
> +    .inputs        = NULL,
> +    FILTER_OUTPUTS(perlin_outputs),
> +    FILTER_QUERY_FUNC(query_formats),
> +};
> -- 
> 2.34.1
> 

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