Add enough padding for \be to avoid clipping

To avoid making bitmaps unnecessarily large, use just
the necessary amount of padding for the given \be value.

3 files changed, 31 insertions, 1 deletions

diff --git a/libass/ass_bitmap.c b/libass/ass_bitmap.c
index 5f0991d..286f904 100644
--- a/libass/ass_bitmap.c
+++ b/libass/ass_bitmap.c
@@ -749,6 +749,35 @@ void be_blur_post(uint8_t *buf, intptr_t w, intptr_t h, intptr_t stride)
     }
 }
 
+/*
+ * To find these values, simulate blur on the border between two
+ * half-planes, one zero-filled (background) and the other filled
+ * with the maximum supported value (foreground). Keep incrementing
+ * the \be argument. The necessary padding is the distance by which
+ * the blurred foreground image extends beyond the original border
+ * and into the background. Initially it increases along with \be,
+ * but very soon it grinds to a halt. At some point, the blurred
+ * image actually reaches a stationary point and stays unchanged
+ * forever after, simply _shifting_ by one pixel for each \be
+ * step--moving in the direction of the non-zero half-plane and
+ * thus decreasing the necessary padding (although the large
+ * padding is still needed for intermediate results). In practice,
+ * images are finite rather than infinite like half-planes, but
+ * this can only decrease the required padding. Half-planes filled
+ * with extreme values are the theoretical limit of the worst case.
+ * Make sure to use the right pixel value range in the simulation!
+ */
+int be_padding(int be)
+{
+    if (be <= 3)
+        return be;
+    if (be <= 7)
+        return 4;
+    if (be <= 123)
+        return 5;
+    return FFMAX(128 - be, 0);
+}
+
 int outline_to_bitmap2(ASS_Renderer *render_priv,
                        ASS_Outline *outline, ASS_Outline *border,
                        Bitmap **bm_g, Bitmap **bm_o)
diff --git a/libass/ass_bitmap.h b/libass/ass_bitmap.h
index 5014f51..901adc7 100644
--- a/libass/ass_bitmap.h
+++ b/libass/ass_bitmap.h
@@ -69,6 +69,7 @@ void ass_free_bitmap(Bitmap *bm);
 void ass_gauss_blur(unsigned char *buffer, unsigned *tmp2,
                     int width, int height, int stride,
                     unsigned *m2, int r, int mwidth);
+int be_padding(int be);
 void be_blur_c(uint8_t *buf, intptr_t w,
                intptr_t h, intptr_t stride,
                uint16_t *tmp);
diff --git a/libass/ass_render.c b/libass/ass_render.c
index 6140788..a02bcab 100644
--- a/libass/ass_render.c
+++ b/libass/ass_render.c
@@ -2257,7 +2257,7 @@ static void render_and_combine_glyphs(ASS_Renderer *render_priv,
             continue;
         }
 
-        int bbord = info->filter.be > 0 ? sqrt(2 * info->filter.be) : 0;
+        int bbord = be_padding(info->filter.be);
         int gbord = info->filter.blur > 0.0 ? FFMIN(info->filter.blur + 1, INT_MAX) : 0;
         int bord = FFMAX(bbord, gbord);