X-Git-Url: https://git.sesse.net/?p=movit;a=blobdiff_plain;f=padding_effect.cpp;h=f8d30d480886aa364d103f90e3d38b1899e0f32d;hp=3c4919a0c3b1b98a3aa31ebf9aad14ff536bcdcf;hb=7af4d1b54ba141fdb74cd13ddc6110708855d157;hpb=572e7aaa57028d7eda4bc445a6249637134a2b02

diff --git a/padding_effect.cpp b/padding_effect.cpp
index 3c4919a..f8d30d4 100644
--- a/padding_effect.cpp
+++ b/padding_effect.cpp
@@ -89,13 +89,25 @@ bool PaddingEffect::needs_srgb_primaries() const
 	return true;
 }
 
-// If the border color is black, it doesn't matter if we're pre- or postmultiplied
-// (or even blank, as a hack). Otherwise, it does.
 Effect::AlphaHandling PaddingEffect::alpha_handling() const
 {
-	if (border_color.r == 0.0 && border_color.g == 0.0 && border_color.b == 0.0) {
+	// If the border color is black, it doesn't matter if we're pre- or postmultiplied.
+	// Note that for non-solid black (i.e. alpha < 1.0), we're equally fine with
+	// pre- and postmultiplied, but later effects might change this status
+	// (consider e.g. blur), so setting DONT_CARE_ALPHA_TYPE is inappropriate,
+	// as it propagate blank alpha through this effect.
+	if (border_color.r == 0.0 && border_color.g == 0.0 && border_color.b == 0.0 && border_color.a == 1.0) {
 		return DONT_CARE_ALPHA_TYPE;
 	}
+
+	// If the border color is solid, we preserve blank alpha, as we never output any
+	// new non-solid pixels.
+	if (border_color.a == 1.0) {
+		return INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK;
+	}
+
+	// Otherwise, we're going to output our border color in premultiplied alpha,
+	// so the other pixels better be premultiplied as well.
 	return INPUT_AND_OUTPUT_PREMULTIPLIED_ALPHA;
 }