avfilter/vf_libplacebo: introduce fit_sense option

This allows choosing whether the `fit_mode` merely controls the placement
of the image within the output resolution, or whether the output resolution
is also adjusted according to the given `fit_mode`.

doc/filters.texi

@@ -16449,7 +16449,8 @@ will be performed.
 
 @item force_original_aspect_ratio
 @item force_divisible_by
-Work the same as the identical @ref{scale} filter options.
+Work the same as the identical @ref{scale} filter options. Note that
+@option{force_divisible_by} also works with @code{fit_sense=constraint}.
 
 @item reset_sar
 If enabled, output frames will always have a pixel aspect ratio of 1:1.  If
@@ -16503,6 +16504,22 @@ to either @code{contain} or @code{none}, depending on whether the input is
 larger than the output or not.
 @end table
 
+@item fit_sense
+When @option{fit_mode} is in use, this option controls how the fit strategy
+is applied against the specified output resolution. Mutually exclusive with
+@option{force_original_aspect_ratio}. Valid values are:
+
+@table @samp
+@item target
+The computed output resolution is taken as the exact size of the output frame.
+This is the default behavior.
+
+@item constraint
+The computed output resolution is a size reference against which the fit mode
+is applied, enlarging or decreasing the true frame size as needed to fit the
+content.
+@end table
+
 @item fillcolor
 Set the color used to fill the output area not covered by the output image, for
 example as a result of @option{normalize_sar}. For the general syntax of this

libavfilter/vf_libplacebo.c

@@ -161,6 +161,12 @@ enum fit_mode {
     FIT_MODE_NB,
 };
 
+enum fit_sense {
+    FIT_TARGET,
+    FIT_CONSTRAINT,
+    FIT_SENSE_NB,
+};
+
 typedef struct LibplaceboContext {
     /* lavfi vulkan*/
     FFVulkanContext vkctx;
@@ -206,6 +212,7 @@ typedef struct LibplaceboContext {
     int reset_sar;
     int normalize_sar;
     int fit_mode;
+    int fit_sense;
     int apply_filmgrain;
     int apply_dovi;
     int colorspace;
@@ -1452,11 +1459,24 @@ static int libplacebo_config_output(AVFilterLink *outlink)
     double sar_in = inlink->sample_aspect_ratio.num ?
                     av_q2d(inlink->sample_aspect_ratio) : 1.0;
 
+    int force_oar = s->force_original_aspect_ratio;
+    if (!force_oar && s->fit_sense == FIT_CONSTRAINT) {
+        if (s->fit_mode == FIT_CONTAIN || s->fit_mode == FIT_SCALE_DOWN) {
+            force_oar = SCALE_FORCE_OAR_DECREASE;
+        } else if (s->fit_mode == FIT_COVER) {
+            force_oar = SCALE_FORCE_OAR_INCREASE;
+        }
+    }
+
     ff_scale_adjust_dimensions(inlink, &outlink->w, &outlink->h,
-                               s->force_original_aspect_ratio,
+                               force_oar, s->force_divisible_by,
-                               s->force_divisible_by,
                                s->reset_sar ? sar_in : 1.0);
 
+    if (s->fit_mode == FIT_SCALE_DOWN && s->fit_sense == FIT_CONSTRAINT) {
+        int w_adj = s->reset_sar ? sar_in * inlink->w : inlink->w;
+        outlink->w = FFMIN(outlink->w, w_adj);
+        outlink->h = FFMIN(outlink->h, inlink->h);
+    }
 
     if (s->nb_inputs > 1 && !s->disable_fbos) {
         /* Create a separate renderer and composition texture */
@@ -1586,6 +1606,9 @@ static const AVOption libplacebo_options[] = {
         { "none",       "Keep input unscaled, padding and cropping as needed",  0, AV_OPT_TYPE_CONST, {.i64 = FIT_NONE },       0, 0, STATIC, .unit = "fit_mode" },
         { "place",      "Keep input unscaled, padding and cropping as needed",  0, AV_OPT_TYPE_CONST, {.i64 = FIT_NONE },       0, 0, STATIC, .unit = "fit_mode" },
         { "scale_down", "Downscale only if larger, padding to preserve aspect", 0, AV_OPT_TYPE_CONST, {.i64 = FIT_SCALE_DOWN }, 0, 0, STATIC, .unit = "fit_mode" },
+    { "fit_sense", "Output size strategy (for the base layer only)", OFFSET(fit_sense), AV_OPT_TYPE_INT, {.i64 = FIT_TARGET }, 0, FIT_SENSE_NB - 1, STATIC, .unit = "fit_sense" },
+        { "target",     "Computed resolution is the exact output size",         0, AV_OPT_TYPE_CONST, {.i64 = FIT_TARGET     }, 0, 0, STATIC, .unit = "fit_sense" },
+        { "constraint", "Computed resolution constrains the output size",       0, AV_OPT_TYPE_CONST, {.i64 = FIT_CONSTRAINT }, 0, 0, STATIC, .unit = "fit_sense" },
     { "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", "Path to custom LUT file to apply", OFFSET(lut_filename), AV_OPT_TYPE_STRING, { .str = NULL }, .flags = STATIC },