From b28a386c946269d310789cc916af43ed1bcfcb84 Mon Sep 17 00:00:00 2001
From: Even Rouault <even.rouault@spatialys.com>
Date: Thu, 23 Nov 2023 17:44:33 +0100
Subject: [PATCH] gdal_footprint: fix -ovr on RGBA datasets (fixes #8792)

---
 apps/gdal_footprint_lib.cpp                   |  3 ++-
 autotest/utilities/test_gdal_footprint_lib.py | 25 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/apps/gdal_footprint_lib.cpp b/apps/gdal_footprint_lib.cpp
index dd02f0b911e8..aa09b6d378a8 100644
--- a/apps/gdal_footprint_lib.cpp
+++ b/apps/gdal_footprint_lib.cpp
@@ -703,7 +703,8 @@ static bool GDALFootprintProcess(GDALDataset *poSrcDS, OGRLayer *poDstLayer,
         {
             GDALRasterBand *poMaskBand;
             const int nMaskFlags = poBand->GetMaskFlags();
-            if ((nMaskFlags & GMF_ALPHA) != 0)
+            if ((nMaskFlags & GMF_ALPHA) != 0 ||
+                poBand->GetColorInterpretation() == GCI_AlphaBand)
             {
                 poMaskBand = poBand;
             }
diff --git a/autotest/utilities/test_gdal_footprint_lib.py b/autotest/utilities/test_gdal_footprint_lib.py
index fdfbed3bb350..8accaa983330 100755
--- a/autotest/utilities/test_gdal_footprint_lib.py
+++ b/autotest/utilities/test_gdal_footprint_lib.py
@@ -427,3 +427,28 @@ def test_gdaldem_footprint_dict_arguments():
         "-lco",
         "STRING_DEFAULT_WIDTH=10",
     ]
+
+
+###############################################################################
+# Test footprint, RGBA and overviews
+
+
+def test_gdaldem_footprint_rgba_overviews():
+
+    src_ds = gdal.GetDriverByName("MEM").Create("", 6, 6, 4)
+    for i in range(4):
+        src_ds.GetRasterBand(i + 1).SetColorInterpretation(gdal.GCI_RedBand + i)
+    src_ds.BuildOverviews("NONE", [2])
+    for i in range(4):
+        src_ds.GetRasterBand(i + 1).GetOverview(0).WriteRaster(1, 1, 1, 1, b"\xFF")
+    out_ds = gdal.Footprint(
+        "",
+        src_ds,
+        format="Memory",
+        targetCoordinateSystem="pixel",
+        ovr=0,
+    )
+    assert out_ds is not None
+    lyr = out_ds.GetLayer(0)
+    f = lyr.GetNextFeature()
+    ogrtest.check_feature_geometry(f, "MULTIPOLYGON (((2 2,2 4,4 4,4 2,2 2)))")