Support SSA's AlphaLevel style field

Procesing of the actual style field emulates VSFilter's behaviour
by parsing the inputs the same way and always using 0x80 for shadows.
For user supplied style overrides the field is accepted for all
script versions and the provided alpha value is applied to all colours,
continuing the existing practice of override tags not caring for
version-dependent quirks and limitations.

Fixes: https://github.com/libass/libass/issues/40

1 files changed, 20 insertions, 0 deletions

diff --git a/libass/ass.c b/libass/ass.c
index 7c2ccde..e08a76a 100644
--- a/libass/ass.c
+++ b/libass/ass.c
@@ -380,6 +380,16 @@ static int process_event_tail(ASS_Track *track, ASS_Event *event,
     return 1;
 }
 
+static void set_style_alpha(ASS_Style *style, int32_t front_alpha, int32_t back_alpha)
+{
+        front_alpha  = FFMAX(FFMIN(front_alpha, 0xFF), 0);
+        back_alpha = FFMAX(FFMIN(back_alpha, 0xFF), 0);
+        style->PrimaryColour   = (style->PrimaryColour   & 0xFFFFFF00) | front_alpha;
+        style->SecondaryColour = (style->SecondaryColour & 0xFFFFFF00) | front_alpha;
+        style->OutlineColour   = (style->OutlineColour   & 0xFFFFFF00) | front_alpha;
+        style->BackColour      = (style->BackColour      & 0xFFFFFF00) | back_alpha;
+}
+
 /**
  * \brief Parse command line style overrides (--ass-force-style option)
  * \param track track to apply overrides to
@@ -436,6 +446,9 @@ void ass_process_force_style(ASS_Track *track)
                     COLORVAL(SecondaryColour)
                     COLORVAL(OutlineColour)
                     COLORVAL(BackColour)
+                    } else if (ass_strcasecmp(tname, "AlphaLevel") == 0) {
+                        int32_t alpha = parse_int_header(token);
+                        set_style_alpha(target, alpha, alpha);
                     FPVAL(FontSize)
                     INTVAL(Bold)
                     INTVAL(Italic)
@@ -512,6 +525,8 @@ static int process_style(ASS_Track *track, char *str)
     style->ScaleX = 100.;
     style->ScaleY = 100.;
 
+    int32_t ssa_alpha = 0;
+
     while (1) {
         NEXT(q, tname);
         NEXT(p, token);
@@ -527,6 +542,8 @@ static int process_style(ASS_Track *track, char *str)
                 // this will destroy SSA's TertiaryColour, but i'm not going to use it anyway
                 if (track->track_type == TRACK_TYPE_SSA)
                     target->OutlineColour = target->BackColour;
+            } else if (ass_strcasecmp(tname, "AlphaLevel") == 0) {
+                ssa_alpha = parse_int_header(token);
             FPVAL(FontSize)
             INTVAL(Bold)
             INTVAL(Italic)
@@ -554,6 +571,9 @@ static int process_style(ASS_Track *track, char *str)
         PARSE_END
     }
     free(format);
+    // VSF compat: always set BackColour Alpha to 0x80 in SSA
+    if (track->track_type == TRACK_TYPE_SSA)
+        set_style_alpha(style, ssa_alpha, 0x80);
     style->ScaleX = FFMAX(style->ScaleX, 0.) / 100.;
     style->ScaleY = FFMAX(style->ScaleY, 0.) / 100.;
     style->Spacing = FFMAX(style->Spacing, 0.);