Update basis_spec.txt
diff --git a/spec/basis_spec.txt b/spec/basis_spec.txt
index 024660a..f731b34 100644
--- a/spec/basis_spec.txt
+++ b/spec/basis_spec.txt
@@ -464,8 +464,8 @@
using by the decoder, and section 10.3 describes how the array of ETC1S blocks
is actually decoded.
-10.1 Approximate Move to Front Routines
----------------------------------------
+10.1 ETC1S Approximate Move to Front Routines
+---------------------------------------------
An approximate Move to Front (MTF) approach is used to efficiently encode the
selector codebook references. Here is the C++ example class for approximate MTF
@@ -513,8 +513,8 @@
uint32_t m_rover;
};
-10.2 VLC Decoding Procedure
----------------------------
+10.2 ETC1S VLC Decoding Procedure
+---------------------------------
ETC1S slice decoding utilizes a simple Variable Length Coding (VLC) scheme that
sends raw bits using variable-size chunks. Here is the VLC decoding procedure:
@@ -838,18 +838,22 @@
ETC1S .basis files can have optional alpha channels, stored in odd slices. If any slice needs an alpha channel,
all slices must have alpha channels. basis_file_header::m_flags will be logically OR'd with
cBASISHeaderFlagHasAlphaSlices. Alpha channel ETC1S files will contain two slices for each mipmap level
-(or face, or video frame, etc.). The even slices will contain the RGB data, and the odd slices will contain
-the alpha data, both stored in ETC1S format. A decoder can first decode the RGB data slice, then the next alpha
-channel slice, or it can decode them in parallel using multithreading. The ETC1S green channel (on the odd slices)
-contains the alpha values.
+(or face, or video frame, etc.). The basis_slice_desc::m_flags field will be logically OR'd with
+cSliceDescFlagsHasAlpha for all odd alpha slices.
+
+The even slices will contain the RGB data, and the odd slices will contain the alpha data, both stored in ETC1S
+format. Alpha channel ETC1S files must always have an even total number of slices. A decoder can first decode
+the RGB data slice, then the next alpha channel slice, or it can decode them in parallel using multithreading.
+The ETC1S green channel (on the odd slices) contains the alpha values.
12.0 Texture Video
------------------
-Both ETC1S and UASTC format files support texture video. Texture video files can be optionally mipmapped, and
-can contain optional alpha channels (stored as separate slices in ETC1S format files). Currently, the first frame
-is always an i-frame, and all subsequent frames are p-frames, but the file format and transcoder supports any
-frame being an i-frame (and the encoder will be enhanced to support this feature).
+Both ETC1S and UASTC format files support texture video. Texture video files can be optionally mipmapped, and can
+contain optional alpha channels (stored as separate slices in ETC1S format files). Currently, the first frame is
+always an i-frame, and all subsequent frames are p-frames, but the file format and transcoder supports any
+frame being an i-frame (and the encoder will be enhanced to support this feature). Decoders must track the previously
+decoded frame's endpoints/selectors for all mipmap levels (if any), not just the top level's.
Skip blocks always refer to the previous frame. i-frames cannot use skip blocks (encoded as endpoint predictor index 2).
