Add a specification for how the official DeckLink driver stores interlaced frmaes.

diff --git a/bmusb/bmusb.h b/bmusb/bmusb.h
index 6aabd51fcc39f68c1b0780e24d5451319083b317..b61b66f2e2e9e59b72b3ba42faddcbcb611cfbb8 100644 (file)
--- a/bmusb/bmusb.h
+++ b/bmusb/bmusb.h
@@ -102,8 +102,10 @@ struct VideoMode {
 
 // Represents the format of an actual frame coming in.
 // Note: Frame rate is _frame_ rate, not field rate. So 1080i60 gets 30/1, _not_ 60/1.
-// "second_field_start" is only valid for interlaced modes; it signifies
-// how many lines from the very top of the frame there are before the second field
+// "second_field_start" is only valid for interlaced modes. If it is 1,
+// the two fields are actually stored interlaced (ie., every other line).
+// If not, each field is stored consecutively, and it signifies how many lines
+// from the very top of the frame there are before the second field
 // starts (so it will always be >= height/2 + extra_lines_top).
 struct VideoFormat {
        uint16_t id = 0;  // For debugging/logging only.