avfilter:vf_drawtext: add new line space size set parameter

add line_spacing parameter to set the space between two lines

Based on an idea by: Leandro Santiago <leandrosansilva@gmail.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Steven Liu <lq@chinaffmpeg.org>

doc/filters.texi

@@ -6750,6 +6750,10 @@ option, check the "Color" section in the ffmpeg-utils manual.
 
 The default value of @var{boxcolor} is "white".
 
+@item line_spacing
+Set the line spacing in pixels of the border to be drawn around the box using @var{box}.
+The default value of @var{line_spacing} is 0.
+
 @item borderw
 Set the width of the border to be drawn around the text using @var{bordercolor}.
 The default value of @var{borderw} is 0.

libavfilter/vf_drawtext.c

@@ -158,6 +158,7 @@ typedef struct DrawTextContext {
     int borderw;                    ///< border width
     unsigned int fontsize;          ///< font size to use
 
+    int line_spacing;               ///< lines spacing in pixels
     short int draw_box;             ///< draw box around text - true or false
     int boxborderw;                 ///< box border width
     int use_kerning;                ///< font kerning is used - true/false
@@ -209,6 +210,7 @@ static const AVOption drawtext_options[]= {
     {"shadowcolor", "set shadow color",     OFFSET(shadowcolor.rgba),   AV_OPT_TYPE_COLOR,  {.str="black"}, CHAR_MIN, CHAR_MAX, FLAGS},
     {"box",         "set box",              OFFSET(draw_box),           AV_OPT_TYPE_BOOL,   {.i64=0},     0,        1       , FLAGS},
     {"boxborderw",  "set box border width", OFFSET(boxborderw),         AV_OPT_TYPE_INT,    {.i64=0},     INT_MIN,  INT_MAX , FLAGS},
+    {"line_spacing",  "set line spacing in pixels", OFFSET(line_spacing),   AV_OPT_TYPE_INT,    {.i64=0},     INT_MIN,  INT_MAX,FLAGS},
     {"fontsize",    "set font size",        OFFSET(fontsize),           AV_OPT_TYPE_INT,    {.i64=0},     0,        INT_MAX , FLAGS},
     {"x",           "set x expression",     OFFSET(x_expr),             AV_OPT_TYPE_STRING, {.str="0"},   CHAR_MIN, CHAR_MAX, FLAGS},
     {"y",           "set y expression",     OFFSET(y_expr),             AV_OPT_TYPE_STRING, {.str="0"},   CHAR_MIN, CHAR_MAX, FLAGS},
@@ -1250,7 +1252,7 @@ static int draw_text(AVFilterContext *ctx, AVFrame *frame,
         if (is_newline(code)) {
 
             max_text_line_w = FFMAX(max_text_line_w, x);
-            y += s->max_glyph_h;
+            y += s->max_glyph_h + s->line_spacing;
             x = 0;
             continue;
         }