Re: [FFmpeg-devel] [PATCH 1/7] vf_libplacebo: add support for specifying a LUT for the input

[Niklas Haas <ffmpeg@haasn.xyz>]

On Fri, 11 Jul 2025 00:13:29 +0900 Lynne <dev@lynne.ee> wrote:
> This makes it possible to apply Adobe .cube files to inputs.
> ---
>  libavfilter/vf_libplacebo.c | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
>
> diff --git a/libavfilter/vf_libplacebo.c b/libavfilter/vf_libplacebo.c
> index 475030c80d..d74a6e1bf7 100644
> --- a/libavfilter/vf_libplacebo.c
> +++ b/libavfilter/vf_libplacebo.c
> @@ -159,6 +159,7 @@ typedef struct LibplaceboContext {
>      pl_vulkan vulkan;
>      pl_gpu gpu;
>      pl_tex tex[4];
> +    struct pl_custom_lut *lut;
>
>      /* input state */
>      LibplaceboInput *inputs;
> @@ -184,6 +185,7 @@ typedef struct LibplaceboContext {
>      AVExpr *pos_x_pexpr, *pos_y_pexpr, *pos_w_pexpr, *pos_h_pexpr;
>      float pad_crop_ratio;
>      float corner_rounding;
> +    char *lut_filename;
>      int force_original_aspect_ratio;
>      int force_divisible_by;
>      int reset_sar;
> @@ -371,6 +373,28 @@ static int find_scaler(AVFilterContext *avctx,
>      return AVERROR(EINVAL);
>  }
>
> +static int update_lut(LibplaceboContext *s)
> +{
> +    int ret;
> +    uint8_t *lutbuf;
> +    size_t lutbuf_size;
> +
> +    pl_lut_free(&s->lut);
> +
> +    if ((ret = av_file_map(s->lut_filename, &lutbuf, &lutbuf_size, 0, s)) < 0) {
> +        av_log(s, AV_LOG_ERROR,
> +               "The LUT file '%s' could not be read: %s\n",
> +               s->lut_filename, av_err2str(ret));
> +        return ret;
> +    }
> +
> +    s->lut = pl_lut_parse_cube(s->log, lutbuf, lutbuf_size);

It would be a good idea to at least warn or ideally error out if parsing
the file fails (in which case, this function returns NULL).

> +
> +    av_file_unmap(lutbuf, lutbuf_size);
> +
> +    return 0;
> +}
> +
>  static int update_settings(AVFilterContext *ctx)
>  {
>      int err = 0;
> @@ -468,6 +492,7 @@ static int update_settings(AVFilterContext *ctx)
>      RET(find_scaler(ctx, &opts->params.upscaler, s->upscaler, 0));
>      RET(find_scaler(ctx, &opts->params.downscaler, s->downscaler, 0));
>      RET(find_scaler(ctx, &opts->params.frame_mixer, s->frame_mixer, 1));
> +    RET(update_lut(s));

Putting this function here will cause it to get called every time any of
the settings change, which could in principle be very frequent.

I would either embed a lightweight check to see if the filename matches the
currently loaded filename, or else move this to the init() section and change
it from DYNAMIC to STATIC, like how the custom shaders are handled.

>
>  #if PL_API_VER >= 309
>      while ((e = av_dict_get(s->extra_opts, "", e, AV_DICT_IGNORE_SUFFIX))) {
> @@ -757,6 +782,7 @@ static void libplacebo_uninit(AVFilterContext *avctx)
>          av_freep(&s->inputs);
>      }
>
> +    pl_lut_free(&s->lut);
>  #if PL_API_VER >= 351
>      pl_cache_destroy(&s->cache);
>  #endif
> @@ -1005,6 +1031,7 @@ static bool map_frame(pl_gpu gpu, pl_tex *tex,
>          .tex        = tex,
>          .map_dovi   = s->apply_dovi,
>      ));
> +    out->lut = s->lut;
>
>      if (!s->apply_filmgrain)
>          out->film_grain.type = PL_FILM_GRAIN_NONE;
> @@ -1406,6 +1433,7 @@ static const AVOption libplacebo_options[] = {
>      { "pad_crop_ratio", "ratio between padding and cropping when normalizing SAR (0=pad, 1=crop)", OFFSET(pad_crop_ratio), AV_OPT_TYPE_FLOAT, {.dbl=0.0}, 0.0, 1.0, DYNAMIC },
>      { "fillcolor", "Background fill color", OFFSET(fillcolor), AV_OPT_TYPE_COLOR, {.str = "black@0"}, .flags = DYNAMIC },
>      { "corner_rounding", "Corner rounding radius", OFFSET(corner_rounding), AV_OPT_TYPE_FLOAT, {.dbl = 0.0}, 0.0, 1.0, .flags = DYNAMIC },
> +    { "lut", "Apply a look-up table", OFFSET(lut_filename), AV_OPT_TYPE_STRING, { .str = NULL }, .flags = DYNAMIC },

Nit: Change it to "Path to custom LUT file to apply", since it takes a filename

New option is missing an entry in doc/filters.texi

>      { "extra_opts", "Pass extra libplacebo-specific options using a :-separated list of key=value pairs", OFFSET(extra_opts), AV_OPT_TYPE_DICT, .flags = DYNAMIC },
>  #if PL_API_VER >= 351
>      { "shader_cache",  "Set shader cache path", OFFSET(shader_cache), AV_OPT_TYPE_STRING, {.str = NULL}, .flags = STATIC },
> --
> 2.49.0.395.g12beb8f557c
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