Add a new alpha handling method, INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK.

[movit] / effect.h

diff --git a/effect.h b/effect.h
index 43849dfa5cdddd61035a58a528061f7045276ead..771648cde059ec269c514c845ed798694cdf6ba0 100644 (file)
--- a/effect.h
+++ b/effect.h
@@ -99,7 +99,8 @@ public:
        // This is the most natural format for processing, and the default in
        // most of Movit (just like linear light is).
        //
-       // If you set INPUT_AND_OUTPUT_PREMULTIPLIED_ALPHA, all of your inputs
+       // If you set INPUT_AND_OUTPUT_PREMULTIPLIED_ALPHA or
+       // INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK, all of your inputs
        // (if any) are guaranteed to also be in premultiplied alpha.
        // Otherwise, you can get postmultiplied or premultiplied alpha;
        // you won't know. If you have multiple inputs, you will get the same
@@ -121,9 +122,19 @@ public:
                // If you set this, you should also set needs_linear_light().
                INPUT_AND_OUTPUT_PREMULTIPLIED_ALPHA,
 
-               // Keeps the type of alpha unchanged from input to output.
-               // Usually appropriate if you process all color channels
-               // in a linear fashion, and do not change alpha.
+               // Like INPUT_AND_OUTPUT_PREMULTIPLIED_ALPHA, but also guarantees
+               // that if you get blank alpha in, you also keep blank alpha out.
+               // This is a somewhat weaker guarantee than DONT_CARE_ALPHA_TYPE,
+               // but is still useful in many situations, and appropriate when
+               // e.g. you don't touch alpha at all.
+               //
+               // Does not make sense for inputs.
+               INPUT_PREMULTIPLIED_ALPHA_KEEP_BLANK,
+
+               // Keeps the type of alpha (premultiplied, postmultiplied, blank)
+               // unchanged from input to output. Usually appropriate if you
+               // process all color channels in a linear fashion, do not change
+               // alpha, and do not produce any new pixels thare have alpha != 1.0.
                //
                // Does not make sense for inputs.
                DONT_CARE_ALPHA_TYPE,