Add a new alpha handling method, INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK.

[movit] / padding_effect.cpp

diff --git a/padding_effect.cpp b/padding_effect.cpp
index 3c4919a0c3b1b98a3aa31ebf9aad14ff536bcdcf..f8d30d480886aa364d103f90e3d38b1899e0f32d 100644 (file)
--- 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;
 }