PAR correction for rendering at non-video resolution

The most prominent ASS/SSA renderer (VSFilter) conveniently ignores the
real aspect ratio, and everyone seems to rely on that.  This is fine
when the subtitles are rendered before anamorphic video is stretched
to its native aspect ratio, but results in wrongly stretched text with
native renderers (EOSD).  It can be fixed by making libass aware of
the pixel ratio of the video.  ass_set_aspect_ratio now requires an
extra argument that specifies the pixel ratio, which is just width /
height of the video after decoding.

Glyphs are stretched in x direction before transformation (rotation,
shearing), so there are still issues with transformed glyphs to be
fixed.

1 files changed, 7 insertions, 3 deletions

diff --git a/libass/ass_render.c b/libass/ass_render.c
index d3d53e4..79722e2 100644
--- a/libass/ass_render.c
+++ b/libass/ass_render.c
@@ -69,6 +69,7 @@ typedef struct ass_settings_s {
     int use_margins;            // 0 - place all subtitles inside original frame
     // 1 - use margins for placing toptitles and subtitles
     double aspect;              // frame aspect ratio, d_width / d_height.
+    double pixel_ratio;         // pixel ratio of the source image
     ass_hinting_t hinting;
 
     char *default_font;
@@ -2742,10 +2743,11 @@ void ass_set_use_margins(ass_renderer_t *priv, int use)
     priv->settings.use_margins = use;
 }
 
-void ass_set_aspect_ratio(ass_renderer_t *priv, double ar)
+void ass_set_aspect_ratio(ass_renderer_t *priv, double ar, double par)
 {
-    if (priv->settings.aspect != ar) {
+    if (priv->settings.aspect != ar || priv->settings.pixel_ratio != par) {
         priv->settings.aspect = ar;
+        priv->settings.pixel_ratio = par;
         ass_reconfigure(priv);
     }
 }
@@ -2840,7 +2842,9 @@ ass_start_frame(ass_renderer_t *render_priv, ass_track_t *track,
     else
         render_priv->border_scale = 1.;
 
-    render_priv->font_scale_x = 1.;
+    // PAR correction
+    render_priv->font_scale_x = render_priv->settings.aspect /
+                                render_priv->settings.pixel_ratio;
 
     render_priv->prev_images_root = render_priv->images_root;
     render_priv->images_root = 0;