avfilter/vf_crop: make possible to do exact cropping for subsampled videos

doc/filters.texi

@@ -5633,6 +5633,11 @@ This expression is evaluated per-frame.
 If set to 1 will force the output display aspect ratio
 to be the same of the input, by changing the output sample aspect
 ratio. It defaults to 0.
+
+@item exact
+Enable exact cropping. If enabled, subsampled videos will be cropped at exact
+width/height/x/y as specified and will not be rounded to nearest smaller value.
+It defaults to 0.
 @end table
 
 The @var{out_w}, @var{out_h}, @var{x}, @var{y} parameters are

libavfilter/vf_crop.c

@@ -82,6 +82,7 @@ typedef struct CropContext {
 
     AVRational out_sar; ///< output sample aspect ratio
     int keep_aspect;    ///< keep display aspect ratio when cropping
+    int exact;          ///< exact cropping, for subsampled formats
 
     int max_step[4];    ///< max pixel step for each plane, expressed as a number of bytes
     int hsub, vsub;     ///< chroma subsampling
@@ -184,8 +185,11 @@ static int config_input(AVFilterLink *link)
                s->w_expr, s->h_expr);
         return AVERROR(EINVAL);
     }
+
+    if (!s->exact) {
         s->w &= ~((1 << s->hsub) - 1);
         s->h &= ~((1 << s->vsub) - 1);
+    }
 
     av_expr_free(s->x_pexpr);
     av_expr_free(s->y_pexpr);
@@ -219,8 +223,10 @@ static int config_input(AVFilterLink *link)
     /* set default, required in the case the first computed value for x/y is NAN */
     s->x = (link->w - s->w) / 2;
     s->y = (link->h - s->h) / 2;
+    if (!s->exact) {
         s->x &= ~((1 << s->hsub) - 1);
         s->y &= ~((1 << s->vsub) - 1);
+    }
     return 0;
 
 fail_expr:
@@ -269,8 +275,10 @@ static int filter_frame(AVFilterLink *link, AVFrame *frame)
         s->x = link->w - s->w;
     if ((unsigned)s->y + (unsigned)s->h > link->h)
         s->y = link->h - s->h;
+    if (!s->exact) {
         s->x &= ~((1 << s->hsub) - 1);
         s->y &= ~((1 << s->vsub) - 1);
+    }
 
     av_log(ctx, AV_LOG_TRACE, "n:%d t:%f pos:%f x:%d y:%d x+w:%d y+h:%d\n",
             (int)s->var_values[VAR_N], s->var_values[VAR_T], s->var_values[VAR_POS],
@@ -344,6 +352,7 @@ static const AVOption crop_options[] = {
     { "x",           "set the x crop area expression",       OFFSET(x_expr), AV_OPT_TYPE_STRING, {.str = "(in_w-out_w)/2"}, CHAR_MIN, CHAR_MAX, FLAGS },
     { "y",           "set the y crop area expression",       OFFSET(y_expr), AV_OPT_TYPE_STRING, {.str = "(in_h-out_h)/2"}, CHAR_MIN, CHAR_MAX, FLAGS },
     { "keep_aspect", "keep aspect ratio",                    OFFSET(keep_aspect), AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS },
+    { "exact",       "do exact cropping",                    OFFSET(exact),  AV_OPT_TYPE_BOOL, {.i64=0}, 0, 1, FLAGS },
     { NULL }
 };