From 17a7ac42d581c537cfc1da4324e53047ad82b13e Mon Sep 17 00:00:00 2001
From: Even Rouault <even.rouault@spatialys.com>
Date: Mon, 21 Aug 2017 12:25:38 +0200
Subject: [PATCH] Add comments for filter_width values

---
 src/lib/openjp2/dwt.c | 5 +++++
 src/lib/openjp2/tcd.c | 8 ++++++++
 2 files changed, 13 insertions(+)

diff --git a/src/lib/openjp2/dwt.c b/src/lib/openjp2/dwt.c
index 96d54df3..b8bbe20a 100644
--- a/src/lib/openjp2/dwt.c
+++ b/src/lib/openjp2/dwt.c
@@ -1676,6 +1676,8 @@ static OPJ_BOOL opj_dwt_decode_partial_tile(opj_tcd_t *tcd,
     opj_dwt_t h;
     opj_dwt_t v;
     OPJ_UINT32 resno;
+    /* This value matches the maximum left/right extension given in tables */
+    /* F.2 and F.3 of the standard. */
     const OPJ_UINT32 filter_width = 2U;
 
     opj_tcd_resolution_t* tr = tilec->resolutions;
@@ -2255,6 +2257,9 @@ OPJ_BOOL opj_dwt_decode_partial_97(opj_tcd_t *tcd,
     opj_v4dwt_t h;
     opj_v4dwt_t v;
     OPJ_UINT32 resno;
+    /* This value matches the maximum left/right extension given in tables */
+    /* F.2 and F.3 of the standard. Note: in opj_tcd_is_subband_area_of_interest() */
+    /* we currently use 3. */
     const OPJ_UINT32 filter_width = 4U;
 
     opj_tcd_resolution_t* tr = tilec->resolutions;
diff --git a/src/lib/openjp2/tcd.c b/src/lib/openjp2/tcd.c
index 5e4681c4..7a5e2b44 100644
--- a/src/lib/openjp2/tcd.c
+++ b/src/lib/openjp2/tcd.c
@@ -2381,6 +2381,14 @@ OPJ_BOOL opj_tcd_is_subband_area_of_interest(opj_tcd_t *tcd,
         OPJ_UINT32 band_x1,
         OPJ_UINT32 band_y1)
 {
+    /* Note: those values for filter_margin are in part the result of */
+    /* experimentation. The value 2 for QMFBID=1 (5x3 filter) can be linked */
+    /* to the maximum left/right extension given in tables F.2 and F.3 of the */
+    /* standard. The value 3 for QMFBID=0 (9x7 filter) is more suspicious, */
+    /* since F.2 and F.3 would lead to 4 instead, so the current 3 might be */
+    /* needed to be bumped to 4, in case inconsistencies are found while */
+    /* decoding parts of irreversible coded images. */
+    /* See opj_dwt_decode_partial_53 and opj_dwt_decode_partial_97 as well */
     OPJ_UINT32 filter_margin = (tcd->tcp->tccps[compno].qmfbid == 1) ? 2 : 3;
     opj_tcd_tilecomp_t *tilec = &(tcd->tcd_image->tiles->comps[compno]);
     opj_image_comp_t* image_comp = &(tcd->image->comps[compno]);