Explain why the code in get_txb_ctx is correct.
get_txb_ctx() calculates top and left differently from the spec, using
bitwise OR instead of the Max() function. Add a comment to justify this
method.
Background info:
This code was added in commit 4ab9a5dcd443fd5cc347fcea03e685b3644364cf
(reverted because of a bug in the dc_sign_contexts table):
https://aomedia-review.googlesource.com/c/aom/+/39524
and commit ed629f8370e3a5bd9c3c1c157ca36c0f0cfa495e (second attempt):
https://aomedia-review.googlesource.com/c/aom/+/39902
But the use of bitwise OR was not explained. I am not sure if this is
a well-known trick in certain circles, but it was not obvious to me
why it is equivalent to the spec.
Change-Id: Ia911065e2efd31157bbcabdf9b9d335a5cbec531
diff --git a/av1/common/txb_common.h b/av1/common/txb_common.h
index 0b6ab38..54a2778 100644
--- a/av1/common/txb_common.h
+++ b/av1/common/txb_common.h
@@ -404,6 +404,11 @@
{ 2, 4, 4, 4, 5 },
{ 2, 4, 4, 4, 5 },
{ 3, 5, 5, 5, 6 } };
+ // For top and left, we only care about which of the following three
+ // categories they belong to: { 0 }, { 1, 2, 3 }, or { 4, 5, ... }. The
+ // spec calculates top and left with the Max() function. We can calculate
+ // an approximate max with bitwise OR because the real max and the
+ // approximate max belong to the same category.
int top = 0;
int left = 0;
