scale: make the filter parametric

Make the filter accept parametric expressions for the output video
size.
Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/filters.texi

@@ -772,13 +772,33 @@ can be used to test the monowhite pixel format descriptor definition.
 
 Scale the input video to @var{width}:@var{height} and/or convert the image format.
 
-For example the command:
+The parameters @var{width} and @var{height} are expressions containing
+the following constants:
 
-@example
-./ffmpeg -i in.avi -vf "scale=200:100" out.avi
-@end example
+@table @option
+@item E, PI, PHI
+the corresponding mathematical approximated values for e
+(euler number), pi (greek PI), phi (golden ratio)
+
+@item in_w, in_h
+the input width and heigth
+
+@item iw, ih
+same as @var{in_w} and @var{in_h}
+
+@item out_w, out_h
+the output (cropped) width and heigth
+
+@item ow, oh
+same as @var{out_w} and @var{out_h}
 
-will scale the input video to a size of 200x100.
+@item a
+input display aspect ratio, same as @var{iw} / @var{ih}
+
+@item hsub, vsub
+horizontal and vertical chroma subsample values. For example for the
+pixel format "yuv422p" @var{hsub} is 2 and @var{vsub} is 1.
+@end table
 
 If the input image format is different from the format requested by
 the next filter, the scale filter will convert the input to the
@@ -793,6 +813,36 @@ ratio of the input image.
 
 The default value of @var{width} and @var{height} is 0.
 
+Some examples follow:
+@example
+# scale the input video to a size of 200x100.
+scale=200:100
+
+# scale the input to 2x
+scale=2*iw:2*ih
+# the above is the same as
+scale=2*in_w:2*in_h
+
+# scale the input to half size
+scale=iw/2:ih/2
+
+# increase the width, and set the height to the same size
+scale=3/2*iw:ow
+
+# seek for Greek harmony
+scale=iw:1/PHI*iw
+scale=ih*PHI:ih
+
+# increase the height, and set the width to 3/2 of the height
+scale=3/2*oh:3/5*ih
+
+# increase the size, but make the size a multiple of the chroma
+scale="trunc(3/2*iw/hsub)*hsub:trunc(3/2*ih/vsub)*vsub"
+
+# increase the width to a maximum of 500 pixels, keep the same input aspect ratio
+scale='min(500\, iw*3/2):-1'
+@end example
+
 @anchor{setdar}
 @section setdar
 

libavfilter/vf_scale.c

@@ -24,9 +24,39 @@
  */
 
 #include "avfilter.h"
+#include "libavutil/avstring.h"
+#include "libavutil/eval.h"
 #include "libavutil/pixdesc.h"
 #include "libswscale/swscale.h"
 
+static const char *var_names[] = {
+    "PI",
+    "PHI",
+    "E",
+    "in_w",   "iw",
+    "in_h",   "ih",
+    "out_w",  "ow",
+    "out_h",  "oh",
+    "a",
+    "hsub",
+    "vsub",
+    NULL
+};
+
+enum var_name {
+    VAR_PI,
+    VAR_PHI,
+    VAR_E,
+    VAR_IN_W,   VAR_IW,
+    VAR_IN_H,   VAR_IH,
+    VAR_OUT_W,  VAR_OW,
+    VAR_OUT_H,  VAR_OH,
+    VAR_A,
+    VAR_HSUB,
+    VAR_VSUB,
+    VARS_NB
+};
+
 typedef struct {
     struct SwsContext *sws;     ///< software scaler context
 
@@ -41,6 +71,9 @@ typedef struct {
     int hsub, vsub;             ///< chroma subsampling
     int slice_y;                ///< top of current output slice
     int input_is_pal;           ///< set to 1 if the input format is paletted
+
+    char w_expr[256];           ///< width  expression string
+    char h_expr[256];           ///< height expression string
 } ScaleContext;
 
 static av_cold int init(AVFilterContext *ctx, const char *args, void *opaque)
@@ -48,21 +81,16 @@ static av_cold int init(AVFilterContext *ctx, const char *args, void *opaque)
     ScaleContext *scale = ctx->priv;
     const char *p;
 
+    av_strlcpy(scale->w_expr, "iw", sizeof(scale->w_expr));
+    av_strlcpy(scale->h_expr, "ih", sizeof(scale->h_expr));
+
     scale->flags = SWS_BILINEAR;
     if (args) {
-        sscanf(args, "%d:%d", &scale->w, &scale->h);
+        sscanf(args, "%255[^:]:%255[^:]", scale->w_expr, scale->h_expr);
         p = strstr(args,"flags=");
         if (p) scale->flags = strtoul(p+6, NULL, 0);
     }
 
-    /* sanity check params */
-    if (scale->w <  -1 || scale->h <  -1) {
-        av_log(ctx, AV_LOG_ERROR, "Size values less than -1 are not acceptable.\n");
-        return AVERROR(EINVAL);
-    }
-    if (scale->w == -1 && scale->h == -1)
-        scale->w = scale->h = 0;
-
     return 0;
 }
 
@@ -109,6 +137,48 @@ static int config_props(AVFilterLink *outlink)
     AVFilterLink *inlink = outlink->src->inputs[0];
     ScaleContext *scale = ctx->priv;
     int64_t w, h;
+    double var_values[VARS_NB], res;
+    char *expr;
+    int ret;
+
+    var_values[VAR_PI]    = M_PI;
+    var_values[VAR_PHI]   = M_PHI;
+    var_values[VAR_E]     = M_E;
+    var_values[VAR_IN_W]  = var_values[VAR_IW] = inlink->w;
+    var_values[VAR_IN_H]  = var_values[VAR_IH] = inlink->h;
+    var_values[VAR_OUT_W] = var_values[VAR_OW] = NAN;
+    var_values[VAR_OUT_H] = var_values[VAR_OH] = NAN;
+    var_values[VAR_A]     = (float) inlink->w / inlink->h;
+    var_values[VAR_HSUB]  = 1<<av_pix_fmt_descriptors[inlink->format].log2_chroma_w;
+    var_values[VAR_VSUB]  = 1<<av_pix_fmt_descriptors[inlink->format].log2_chroma_h;
+
+    /* evaluate width and height */
+    av_expr_parse_and_eval(&res, (expr = scale->w_expr),
+                           var_names, var_values,
+                           NULL, NULL, NULL, NULL, NULL, 0, ctx);
+    scale->w = var_values[VAR_OUT_W] = var_values[VAR_OW] = res;
+    if ((ret = av_expr_parse_and_eval(&res, (expr = scale->h_expr),
+                                      var_names, var_values,
+                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
+        goto fail;
+    scale->h = var_values[VAR_OUT_H] = var_values[VAR_OH] = res;
+    /* evaluate again the width, as it may depend on the output height */
+    if ((ret = av_expr_parse_and_eval(&res, (expr = scale->w_expr),
+                                      var_names, var_values,
+                                      NULL, NULL, NULL, NULL, NULL, 0, ctx)) < 0)
+        goto fail;
+    scale->w = res;
+
+    w = scale->w;
+    h = scale->h;
+
+    /* sanity check params */
+    if (w <  -1 || h <  -1) {
+        av_log(ctx, AV_LOG_ERROR, "Size values less than -1 are not acceptable.\n");
+        return AVERROR(EINVAL);
+    }
+    if (w == -1 && h == -1)
+        scale->w = scale->h = 0;
 
     if (!(w = scale->w))
         w = inlink->w;
@@ -142,6 +212,11 @@ static int config_props(AVFilterLink *outlink)
         return AVERROR(EINVAL);
 
     return 0;
+
+fail:
+    av_log(NULL, AV_LOG_ERROR,
+           "Error when evaluating the expression '%s'\n", expr);
+    return ret;
 }
 
 static void start_frame(AVFilterLink *link, AVFilterBufferRef *picref)