avfilter/scale*: add option reset_sar

GyanD · GyanD · commit a28dc06869fe · 2025-02-10T16:25:23.000+05:30
For anamorphic videos, enabling this option leads to adjustment of
output dimensions to obtain square pixels when the user requests
proportional scaling through either of the w/h expressions or
force_original_aspect_ratio.

Output SAR is always reset to 1.

Option added to scale, scale_cuda, scale_npp &amp; scale_vaapi.

libplacebo already has a similar option with different semantics,
scale_vt and scale_vulkan don't implement force_oar, so for these
three filters, I've made minimal changes needed to not break building
or change output.
diff --git a/doc/filters.texi b/doc/filters.texi
@@ -21285,6 +21285,14 @@ This option can be handy if you need to have a video fit within or exceed
 a defined resolution using @option{force_original_aspect_ratio} but also have
 encoder restrictions on width or height divisibility.
 
+@item reset_sar
+Enabling this option leads to the output SAR being reset to 1.
+Additionally, if the user requests proportional scaling either
+through the width or height expressions, e.g. @code{w=-4:h=360} or @code{w=iw/2:h=-1}
+or by enabling @code{force_original_aspect_ratio}, then the input DAR is taken into
+account and the output is scaled to produce square pixels.
+Default is false.
+
 @end table
 
 The values of the @option{w} and @option{h} options are expressions
@@ -21445,10 +21453,28 @@ scale='trunc(ih*dar):ih',setsar=1/1
 @end example
 
 @item
-Make pixels square by combining scale and setsar,
+Make pixels square using reset_sar,
 making sure the resulting resolution is even (required by some codecs):
 @example
-scale='trunc(ih*dar/2)*2:trunc(ih/2)*2',setsar=1/1
+scale='-2:ih-mod(ih,2):reset_sar=1'
+@end example
+
+@item
+Scale to target exactly, however reset SAR to 1:
+@example
+scale='400:300:reset_sar=1'
+@end example
+
+@item
+Scale to even dimensions that fit within 400x300, preserving input SAR:
+@example
+scale='400:300:force_original_aspect_ratio=decrease:force_divisible_by=2'
+@end example
+
+@item
+Scale to produce square pixels with even dimensions that fit within 400x300:
+@example
+scale='400:300:force_original_aspect_ratio=decrease:force_divisible_by=2:reset_sar=1'
 @end example
 
 @item
@@ -21538,6 +21564,9 @@ Affects the curves of the bicubic algorithm.
 @item force_divisible_by
 Work the same as the identical @ref{scale} filter options.
 
+@item reset_sar
+Works the same as the identical @ref{scale} filter option.
+
 @end table
 
 @subsection Examples
@@ -21641,6 +21670,9 @@ This option can be handy if you need to have a video fit within or exceed
 a defined resolution using @option{force_original_aspect_ratio} but also have
 encoder restrictions on width or height divisibility.
 
+@item reset_sar
+Works the same as the identical @ref{scale} filter option.
+
 @item eval
 Specify when to evaluate @var{width} and @var{height} expression. It accepts the following values:
 
diff --git a/libavfilter/scale_eval.c b/libavfilter/scale_eval.c
@@ -112,7 +112,8 @@ int ff_scale_eval_dimensions(void *log_ctx,
 
 int ff_scale_adjust_dimensions(AVFilterLink *inlink,
     int *ret_w, int *ret_h,
-    int force_original_aspect_ratio, int force_divisible_by)
+    int force_original_aspect_ratio, int force_divisible_by,
+    double w_adj)
 {
     int64_t w, h;
     int factor_w, factor_h;
@@ -132,26 +133,26 @@ int ff_scale_adjust_dimensions(AVFilterLink *inlink,
     }
 
     if (w < 0 && h < 0) {
-        w = inlink->w;
+        w = inlink->w * w_adj;
         h = inlink->h;
     }
 
     /* Make sure that the result is divisible by the factor we determined
      * earlier. If no factor was set, nothing will happen as the default
      * factor is 1 */
     if (w < 0)
-        w = av_rescale(h, inlink->w, inlink->h * factor_w) * factor_w;
+        w = av_rescale(h, inlink->w * w_adj, inlink->h * factor_w) * factor_w;
     if (h < 0)
-        h = av_rescale(w, inlink->h, inlink->w * factor_h) * factor_h;
+        h = av_rescale(w, inlink->h, inlink->w * w_adj * factor_h) * factor_h;
 
     /* Note that force_original_aspect_ratio may overwrite the previous set
      * dimensions so that it is not divisible by the set factors anymore
      * unless force_divisible_by is defined as well */
     if (force_original_aspect_ratio) {
         // Including force_divisible_by here rounds to the nearest multiple of it.
-        int64_t tmp_w = av_rescale(h, inlink->w, inlink->h * (int64_t)force_divisible_by)
+        int64_t tmp_w = av_rescale(h, inlink->w * w_adj, inlink->h * (int64_t)force_divisible_by)
                     * force_divisible_by;
-        int64_t tmp_h = av_rescale(w, inlink->h, inlink->w * (int64_t)force_divisible_by)
+        int64_t tmp_h = av_rescale(w, inlink->h, inlink->w * w_adj * (int64_t)force_divisible_by)
                     * force_divisible_by;
 
         if (force_original_aspect_ratio == 1) {
diff --git a/libavfilter/scale_eval.h b/libavfilter/scale_eval.h
@@ -40,10 +40,13 @@ int ff_scale_eval_dimensions(void *ctx,
  * or both of the evaluated values are of the form '-n' or if
  * force_original_aspect_ratio is set. force_divisible_by is used only when
  * force_original_aspect_ratio is set and must be at least 1.
+ * w_adj is the input SAR when the output dimensions are intended to be square
+ * pixels, else should be 1.
  *
  * Returns negative error code on error or non negative on success
  */
 int ff_scale_adjust_dimensions(AVFilterLink *inlink,
     int *ret_w, int *ret_h,
-    int force_original_aspect_ratio, int force_divisible_by);
+    int force_original_aspect_ratio, int force_divisible_by,
+    double w_adj);
 #endif
diff --git a/libavfilter/vf_libplacebo.c b/libavfilter/vf_libplacebo.c
@@ -1201,7 +1201,7 @@ static int libplacebo_config_output(AVFilterLink *outlink)
 
     ff_scale_adjust_dimensions(inlink, &outlink->w, &outlink->h,
                                s->force_original_aspect_ratio,
-                               s->force_divisible_by);
+                               s->force_divisible_by, 1.f);
 
     if (s->normalize_sar || s->nb_inputs > 1) {
         /* SAR is normalized, or we have multiple inputs, set out to 1:1 */
diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
@@ -176,6 +176,7 @@ typedef struct ScaleContext {
 
     int force_original_aspect_ratio;
     int force_divisible_by;
+    int reset_sar;
 
     int eval_mode;              ///< expression evaluation mode
 
@@ -645,6 +646,7 @@ static int config_props(AVFilterLink *outlink)
                             outlink->src->inputs[0];
     ScaleContext *scale = ctx->priv;
     uint8_t *flags_val = NULL;
+    double w_adj = 1.0;
     int ret;
 
     if ((ret = scale_eval_dimensions(ctx)) < 0)
@@ -653,9 +655,13 @@ static int config_props(AVFilterLink *outlink)
     outlink->w = scale->w;
     outlink->h = scale->h;
 
+    if (scale->reset_sar)
+        w_adj = IS_SCALE2REF(ctx) ? scale->var_values[VAR_S2R_MAIN_SAR] :
+                                    scale->var_values[VAR_SAR];
+
     ret = ff_scale_adjust_dimensions(inlink, &outlink->w, &outlink->h,
                                scale->force_original_aspect_ratio,
-                               scale->force_divisible_by);
+                               scale->force_divisible_by, w_adj);
 
     if (ret < 0)
         goto fail;
@@ -668,7 +674,9 @@ static int config_props(AVFilterLink *outlink)
 
     /* TODO: make algorithm configurable */
 
-    if (inlink0->sample_aspect_ratio.num){
+    if (scale->reset_sar)
+        outlink->sample_aspect_ratio = (AVRational){1, 1};
+    else if (inlink0->sample_aspect_ratio.num){
         outlink->sample_aspect_ratio = av_mul_q((AVRational){outlink->h * inlink0->w, outlink->w * inlink0->h}, inlink0->sample_aspect_ratio);
     } else
         outlink->sample_aspect_ratio = inlink0->sample_aspect_ratio;
@@ -1185,6 +1193,7 @@ static const AVOption scale_options[] = {
     { "decrease", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 1 }, 0, 0, FLAGS, .unit = "force_oar" },
     { "increase", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 2 }, 0, 0, FLAGS, .unit = "force_oar" },
     { "force_divisible_by", "enforce that the output resolution is divisible by a defined integer when force_original_aspect_ratio is used", OFFSET(force_divisible_by), AV_OPT_TYPE_INT, { .i64 = 1}, 1, 256, FLAGS },
+    { "reset_sar", "reset SAR to 1 and scale to square pixels if scaling proportionally", OFFSET(reset_sar), AV_OPT_TYPE_BOOL, { .i64 = 0}, 0, 1, FLAGS },
     { "param0", "Scaler param 0",             OFFSET(param[0]),  AV_OPT_TYPE_DOUBLE, { .dbl = DBL_MAX  }, -DBL_MAX, DBL_MAX, FLAGS },
     { "param1", "Scaler param 1",             OFFSET(param[1]),  AV_OPT_TYPE_DOUBLE, { .dbl = DBL_MAX  }, -DBL_MAX, DBL_MAX, FLAGS },
     { "eval", "specify when to evaluate expressions", OFFSET(eval_mode), AV_OPT_TYPE_INT, {.i64 = EVAL_MODE_INIT}, 0, EVAL_MODE_NB-1, FLAGS, .unit = "eval" },
diff --git a/libavfilter/vf_scale_cuda.c b/libavfilter/vf_scale_cuda.c
@@ -96,6 +96,7 @@ typedef struct CUDAScaleContext {
 
     int force_original_aspect_ratio;
     int force_divisible_by;
+    int reset_sar;
 
     CUcontext   cu_ctx;
     CUmodule    cu_module;
@@ -355,6 +356,7 @@ static av_cold int cudascale_config_props(AVFilterLink *outlink)
     AVHWFramesContext     *frames_ctx;
     AVCUDADeviceContext *device_hwctx;
     int w, h;
+    double w_adj = 1.0;
     int ret;
 
     if ((ret = ff_scale_eval_dimensions(s,
@@ -363,8 +365,12 @@ static av_cold int cudascale_config_props(AVFilterLink *outlink)
                                         &w, &h)) < 0)
         goto fail;
 
+    if (s->reset_sar)
+        w_adj = inlink->sample_aspect_ratio.num ?
+        (double)inlink->sample_aspect_ratio.num / inlink->sample_aspect_ratio.den : 1;
+
     ff_scale_adjust_dimensions(inlink, &w, &h,
-                               s->force_original_aspect_ratio, s->force_divisible_by);
+                               s->force_original_aspect_ratio, s->force_divisible_by, w_adj);
 
     if (((int64_t)h * inlink->w) > INT_MAX  ||
         ((int64_t)w * inlink->h) > INT_MAX)
@@ -383,7 +389,9 @@ static av_cold int cudascale_config_props(AVFilterLink *outlink)
     s->hwctx = device_hwctx;
     s->cu_stream = s->hwctx->stream;
 
-    if (inlink->sample_aspect_ratio.num) {
+    if (s->reset_sar)
+        outlink->sample_aspect_ratio = (AVRational){1, 1};
+    else if (inlink->sample_aspect_ratio.num) {
         outlink->sample_aspect_ratio = av_mul_q((AVRational){outlink->h*inlink->w,
                                                              outlink->w*inlink->h},
                                                 inlink->sample_aspect_ratio);
@@ -574,10 +582,14 @@ static int cudascale_filter_frame(AVFilterLink *link, AVFrame *in)
     if (ret < 0)
         goto fail;
 
-    av_reduce(&out->sample_aspect_ratio.num, &out->sample_aspect_ratio.den,
-              (int64_t)in->sample_aspect_ratio.num * outlink->h * link->w,
-              (int64_t)in->sample_aspect_ratio.den * outlink->w * link->h,
-              INT_MAX);
+    if (s->reset_sar) {
+        out->sample_aspect_ratio = (AVRational){1, 1};
+    } else {
+        av_reduce(&out->sample_aspect_ratio.num, &out->sample_aspect_ratio.den,
+                  (int64_t)in->sample_aspect_ratio.num * outlink->h * link->w,
+                  (int64_t)in->sample_aspect_ratio.den * outlink->w * link->h,
+                  INT_MAX);
+    }
 
     av_frame_free(&in);
     return ff_filter_frame(outlink, out);
@@ -614,6 +626,7 @@ static const AVOption options[] = {
         { "decrease", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 1 }, 0, 0, FLAGS, .unit = "force_oar" },
         { "increase", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 2 }, 0, 0, FLAGS, .unit = "force_oar" },
     { "force_divisible_by", "enforce that the output resolution is divisible by a defined integer when force_original_aspect_ratio is used", OFFSET(force_divisible_by), AV_OPT_TYPE_INT, { .i64 = 1 }, 1, 256, FLAGS },
+    { "reset_sar", "reset SAR to 1 and scale to square pixels if scaling proportionally", OFFSET(reset_sar), AV_OPT_TYPE_BOOL, { .i64 = 0}, 0, 1, FLAGS },
     { NULL },
 };
 
diff --git a/libavfilter/vf_scale_npp.c b/libavfilter/vf_scale_npp.c
@@ -160,6 +160,7 @@ typedef struct NPPScaleContext {
 
     int force_original_aspect_ratio;
     int force_divisible_by;
+    int reset_sar;
 
     int interp_algo;
 
@@ -650,14 +651,19 @@ static int config_props(AVFilterLink *outlink)
                             outlink->src->inputs[1] :
                             outlink->src->inputs[0];
     NPPScaleContext *s = ctx->priv;
+    double w_adj = 1.0;
     int ret;
 
     if ((ret = nppscale_eval_dimensions(ctx)) < 0)
         goto fail;
 
+    if (s->reset_sar)
+        w_adj = IS_SCALE2REF(ctx) ? s->var_values[VAR_S2R_MAIN_SAR] :
+                                    s->var_values[VAR_SAR];
+
     ff_scale_adjust_dimensions(inlink, &s->w, &s->h,
                                s->force_original_aspect_ratio,
-                               s->force_divisible_by);
+                               s->force_divisible_by, w_adj);
 
     if (s->w > INT_MAX || s->h > INT_MAX ||
         (s->h * inlink->w) > INT_MAX ||
@@ -674,7 +680,9 @@ static int config_props(AVFilterLink *outlink)
     av_log(ctx, AV_LOG_VERBOSE, "w:%d h:%d -> w:%d h:%d\n",
            inlink->w, inlink->h, outlink->w, outlink->h);
 
-    if (inlink->sample_aspect_ratio.num)
+    if (s->reset_sar)
+        outlink->sample_aspect_ratio = (AVRational){1, 1};
+    else if (inlink->sample_aspect_ratio.num)
         outlink->sample_aspect_ratio = av_mul_q((AVRational){outlink->h*inlink->w,
                                                              outlink->w*inlink->h},
                                                 inlink->sample_aspect_ratio);
@@ -1019,6 +1027,7 @@ static const AVOption options[] = {
     { "decrease", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 1 }, 0, 0, FLAGS, .unit = "force_oar" },
     { "increase", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 2 }, 0, 0, FLAGS, .unit = "force_oar" },
     { "force_divisible_by", "enforce that the output resolution is divisible by a defined integer when force_original_aspect_ratio is used", OFFSET(force_divisible_by), AV_OPT_TYPE_INT, { .i64 = 1 }, 1, 256, FLAGS },
+    { "reset_sar", "reset SAR to 1 and scale to square pixels if scaling proportionally", OFFSET(reset_sar), AV_OPT_TYPE_BOOL, { .i64 = 0}, 0, 1, FLAGS },
     { "eval", "specify when to evaluate expressions", OFFSET(eval_mode), AV_OPT_TYPE_INT, { .i64 = EVAL_MODE_INIT }, 0, EVAL_MODE_NB-1, FLAGS, .unit = "eval" },
          { "init",  "eval expressions once during initialization",          0, AV_OPT_TYPE_CONST, { .i64 = EVAL_MODE_INIT  }, 0, 0, FLAGS, .unit = "eval" },
          { "frame", "eval expressions during initialization and per-frame", 0, AV_OPT_TYPE_CONST, { .i64 = EVAL_MODE_FRAME }, 0, 0, FLAGS, .unit = "eval" },
diff --git a/libavfilter/vf_scale_vaapi.c b/libavfilter/vf_scale_vaapi.c
@@ -39,6 +39,7 @@ typedef struct ScaleVAAPIContext {
 
     int force_original_aspect_ratio;
     int force_divisible_by;
+    int reset_sar;
 
     char *colour_primaries_string;
     char *colour_transfer_string;
@@ -73,6 +74,7 @@ static int scale_vaapi_config_output(AVFilterLink *outlink)
     AVFilterContext *avctx   = outlink->src;
     VAAPIVPPContext *vpp_ctx = avctx->priv;
     ScaleVAAPIContext *ctx   = avctx->priv;
+    double w_adj = 1.0;
     int err;
 
     if ((err = ff_scale_eval_dimensions(ctx,
@@ -81,8 +83,12 @@ static int scale_vaapi_config_output(AVFilterLink *outlink)
                                         &vpp_ctx->output_width, &vpp_ctx->output_height)) < 0)
         return err;
 
+    if (ctx->reset_sar)
+        w_adj = inlink->sample_aspect_ratio.num ?
+        (double)inlink->sample_aspect_ratio.num / inlink->sample_aspect_ratio.den : 1;
+
     ff_scale_adjust_dimensions(inlink, &vpp_ctx->output_width, &vpp_ctx->output_height,
-                               ctx->force_original_aspect_ratio, ctx->force_divisible_by);
+                               ctx->force_original_aspect_ratio, ctx->force_divisible_by, w_adj);
 
     if (inlink->w == vpp_ctx->output_width && inlink->h == vpp_ctx->output_height &&
         (vpp_ctx->input_frames->sw_format == vpp_ctx->output_format ||
@@ -98,7 +104,9 @@ static int scale_vaapi_config_output(AVFilterLink *outlink)
     if (err < 0)
         return err;
 
-    if (inlink->sample_aspect_ratio.num)
+    if (ctx->reset_sar)
+        outlink->sample_aspect_ratio = (AVRational){1, 1};
+    else if (inlink->sample_aspect_ratio.num)
         outlink->sample_aspect_ratio = av_mul_q((AVRational){outlink->h * inlink->w, outlink->w * inlink->h}, inlink->sample_aspect_ratio);
     else
         outlink->sample_aspect_ratio = inlink->sample_aspect_ratio;
@@ -274,6 +282,7 @@ static const AVOption scale_vaapi_options[] = {
     { "decrease", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 1 }, 0, 0, FLAGS, .unit = "force_oar" },
     { "increase", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 2 }, 0, 0, FLAGS, .unit = "force_oar" },
     { "force_divisible_by", "enforce that the output resolution is divisible by a defined integer when force_original_aspect_ratio is used", OFFSET(force_divisible_by), AV_OPT_TYPE_INT, { .i64 = 1}, 1, 256, FLAGS },
+    { "reset_sar", "reset SAR to 1 and scale to square pixels if scaling proportionally", OFFSET(reset_sar), AV_OPT_TYPE_BOOL, { .i64 = 0}, 0, 1, FLAGS },
 
     { NULL },
 };
diff --git a/libavfilter/vf_scale_vt.c b/libavfilter/vf_scale_vt.c
@@ -248,7 +248,7 @@ static int scale_vt_config_output(AVFilterLink *outlink)
     if (err < 0)
         return err;
 
-    ff_scale_adjust_dimensions(inlink, &s->output_width, &s->output_height, 0, 1);
+    ff_scale_adjust_dimensions(inlink, &s->output_width, &s->output_height, 0, 1, 1.f);
 
     outlink->w = s->output_width;
     outlink->h = s->output_height;
diff --git a/libavfilter/vf_scale_vulkan.c b/libavfilter/vf_scale_vulkan.c
@@ -336,7 +336,7 @@ static int scale_vulkan_config_output(AVFilterLink *outlink)
     if (err < 0)
         return err;
 
-    ff_scale_adjust_dimensions(inlink, &vkctx->output_width, &vkctx->output_height, 0, 1);
+    ff_scale_adjust_dimensions(inlink, &vkctx->output_width, &vkctx->output_height, 0, 1, 1.f);
 
     outlink->w = vkctx->output_width;
     outlink->h = vkctx->output_height;
