lavfi/scale: implement clever/insane parsing heuristic, and add a size option

If the first argument can be read as a video size, set that output size
in the scale. This allows to specify in a filtergraph scale=qcif or
scale=320x240.

This is not completely safe, for example in case of a typo in the video
size string the first argument will be read as the input width
expression, giving rise to a confusing comment.

doc/filters.texi

@@ -3017,7 +3017,9 @@ of the input, by changing the output sample aspect ratio.
 This filter accepts a list of named options in the form of
 @var{key}=@var{value} pairs separated by ":". If the key for the first
 two options is not specified, the assumed keys for the first two
-values are @code{w} and @code{h}.
+values are @code{w} and @code{h}. If the first option has no key and
+can be interpreted like a video size specification, it will be used
+to set the video size.
 
 A description of the accepted options follows.
 
@@ -3050,6 +3052,10 @@ Default value is @code{0}.
 @item flags
 Set libswscale scaling flags. If not explictly specified the filter
 applies a bilinear scaling algorithm.
+
+@item size, s
+Set the video size, the value must be a valid abbreviation or in the
+form @var{width}x@var{height}.
 @end table
 
 The values of the @var{w} and @var{h} options are expressions
@@ -3102,12 +3108,27 @@ Scale the input video to a size of 200x100:
 scale=200:100
 @end example
 
-@item
-The above example is the same as:
+This is equivalent to:
 @example
 scale=w=200:h=100
 @end example
 
+or:
+@example
+scale=200x100
+@end example
+
+@item
+Specify a size abbreviation for the output size:
+@example
+scale=qcif
+@end example
+
+which can also be written as:
+@example
+scale=size=qcif
+@end example
+
 @item
 Scale the input to 2x:
 @example

libavfilter/version.h

@@ -30,7 +30,7 @@
 
 #define LIBAVFILTER_VERSION_MAJOR  3
 #define LIBAVFILTER_VERSION_MINOR  20
-#define LIBAVFILTER_VERSION_MICRO 107
+#define LIBAVFILTER_VERSION_MICRO 108
 
 #define LIBAVFILTER_VERSION_INT AV_VERSION_INT(LIBAVFILTER_VERSION_MAJOR, \
                                                LIBAVFILTER_VERSION_MINOR, \

libavfilter/vf_scale.c

@@ -35,6 +35,7 @@
 #include "libavutil/internal.h"
 #include "libavutil/mathematics.h"
 #include "libavutil/opt.h"
+#include "libavutil/parseutils.h"
 #include "libavutil/pixdesc.h"
 #include "libavutil/imgutils.h"
 #include "libavutil/avassert.h"
@@ -78,6 +79,7 @@ typedef struct {
      */
     int w, h;
     char *flags_str;            ///sws flags string
+    char *size_str;
     unsigned int flags;         ///sws flags
 
     int hsub, vsub;             ///< chroma subsampling
@@ -94,12 +96,14 @@ typedef struct {
 #define FLAGS AV_OPT_FLAG_VIDEO_PARAM|AV_OPT_FLAG_FILTERING_PARAM
 
 static const AVOption scale_options[] = {
-    { "w",      "set width expression",    OFFSET(w_expr), AV_OPT_TYPE_STRING, {.str = "iw"}, 0, 0, FLAGS },
-    { "width",  "set width expression",    OFFSET(w_expr), AV_OPT_TYPE_STRING, {.str = "iw"}, 0, 0, FLAGS },
-    { "h",      "set height expression",   OFFSET(h_expr), AV_OPT_TYPE_STRING, {.str = "ih"}, 0, 0, FLAGS },
-    { "height", "set height expression",   OFFSET(h_expr), AV_OPT_TYPE_STRING, {.str = "ih"}, 0, 0, FLAGS },
+    { "w",      "set width expression",    OFFSET(w_expr), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, FLAGS },
+    { "width",  "set width expression",    OFFSET(w_expr), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, FLAGS },
+    { "h",      "set height expression",   OFFSET(h_expr), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, FLAGS },
+    { "height", "set height expression",   OFFSET(h_expr), AV_OPT_TYPE_STRING, {.str = NULL}, 0, 0, FLAGS },
     { "flags",  "set libswscale flags",    OFFSET(flags_str), AV_OPT_TYPE_STRING, {.str = NULL}, 0, INT_MAX, FLAGS },
     { "interl", "set interlacing", OFFSET(interlaced), AV_OPT_TYPE_INT, {.i64 = 0 }, -1, 1, FLAGS },
+    { "size",   "set video size",          OFFSET(size_str), AV_OPT_TYPE_STRING, {.str = NULL}, 0, FLAGS },
+    { "s",      "set video size",          OFFSET(size_str), AV_OPT_TYPE_STRING, {.str = NULL}, 0, FLAGS },
     { NULL },
 };
 
@@ -110,13 +114,45 @@ static av_cold int init(AVFilterContext *ctx, const char *args)
     ScaleContext *scale = ctx->priv;
     static const char *shorthand[] = { "w", "h", NULL };
     int ret;
+    const char *args0 = args;
 
     scale->class = &scale_class;
     av_opt_set_defaults(scale);
 
+    if (args && (scale->size_str = av_get_token(&args, ":"))) {
+        if (av_parse_video_size(&scale->w, &scale->h, scale->size_str) < 0) {
+            av_freep(&scale->size_str);
+            args = args0;
+        } else if (*args)
+            args++;
+    }
+
     if ((ret = av_opt_set_from_string(scale, args, shorthand, "=", ":")) < 0)
         return ret;
 
+    if (scale->size_str && (scale->w_expr || scale->h_expr)) {
+        av_log(ctx, AV_LOG_ERROR,
+               "Size and width/height expressions cannot be set at the same time.\n");
+            return AVERROR(EINVAL);
+    }
+
+    if (scale->size_str) {
+        char buf[32];
+        if ((ret = av_parse_video_size(&scale->w, &scale->h, scale->size_str)) < 0) {
+            av_log(ctx, AV_LOG_ERROR,
+                   "Invalid size '%s'\n", scale->size_str);
+            return ret;
+        }
+        snprintf(buf, sizeof(buf)-1, "%d", scale->w);
+        av_opt_set(scale, "w", buf, 0);
+        snprintf(buf, sizeof(buf)-1, "%d", scale->h);
+        av_opt_set(scale, "h", buf, 0);
+    }
+    if (!scale->w_expr)
+        av_opt_set(scale, "w", "iw", 0);
+    if (!scale->h_expr)
+        av_opt_set(scale, "h", "ih", 0);
+
     av_log(ctx, AV_LOG_VERBOSE, "w:%s h:%s flags:%s interl:%d\n",
            scale->w_expr, scale->h_expr, scale->flags_str, scale->interlaced);