GTiff/gdalwarp/COG: preserve pre-multiplied alpha information from so…

…urce TIFF Fixes #11377
OSGeo · Nov 28, 2024 · b24644d · b24644d
1 parent 4cfbcf8
commit b24644d
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Showing 5 changed files with 89 additions and 5 deletions.
diff --git a/apps/gdalwarp_lib.cpp b/apps/gdalwarp_lib.cpp
@@ -4712,6 +4712,19 @@ static GDALDatasetH GDALWarpCreateOutput(
             aosCreateOptions.FetchNameValue("PHOTOMETRIC") == nullptr)
         {
             aosCreateOptions.SetNameValue("PHOTOMETRIC", "RGB");
+
+            // Preserve potential ALPHA=PREMULTIPLIED from source alpha band
+            const char *pszAlpha;
+            if (aosCreateOptions.FetchNameValue("ALPHA") == nullptr &&
+                apeColorInterpretations.size() == 4 &&
+                apeColorInterpretations[3] == GCI_AlphaBand &&
+                GDALGetRasterCount(pahSrcDS[0]) == 4 &&
+                (pszAlpha =
+                     GDALGetMetadataItem(GDALGetRasterBand(pahSrcDS[0], 4),
+                                         "ALPHA", "IMAGE_STRUCTURE")))
+            {
+                aosCreateOptions.SetNameValue("ALPHA", pszAlpha);
+            }
         }
 
         /* The GTiff driver now supports writing band color interpretation */

diff --git a/autotest/gcore/cog.py b/autotest/gcore/cog.py
@@ -1961,3 +1961,32 @@ def test_cog_write_check_golden_file(tmp_path, src_filename, creation_options):
             )
     assert os.stat(src_filename).st_size == os.stat(out_filename).st_size
     assert open(src_filename, "rb").read() == open(out_filename, "rb").read()
+
+
+###############################################################################
+
+
+def test_cog_preserve_ALPHA_PREMULTIPLIED_on_copy(tmp_vsimem):
+
+    src_filename = str(tmp_vsimem / "src.tif")
+    src_ds = gdal.GetDriverByName("GTiff").Create(
+        src_filename, 1, 1, 4, options=["ALPHA=PREMULTIPLIED", "PROFILE=BASELINE"]
+    )
+    src_ds.SetGeoTransform([500000, 1, 0, 4500000, 0, -1])
+    srs = osr.SpatialReference()
+    srs.ImportFromEPSG(32631)
+    src_ds.SetProjection(srs.ExportToWkt())
+
+    out_filename = str(tmp_vsimem / "out.tif")
+    gdal.GetDriverByName("COG").CreateCopy(
+        out_filename,
+        src_ds,
+        options=[
+            "TILING_SCHEME=GoogleMapsCompatible",
+        ],
+    )
+    with gdal.Open(out_filename) as ds:
+        assert (
+            ds.GetRasterBand(4).GetMetadataItem("ALPHA", "IMAGE_STRUCTURE")
+            == "PREMULTIPLIED"
+        )
diff --git a/autotest/gcore/tiff_write.py b/autotest/gcore/tiff_write.py
@@ -948,7 +948,7 @@ def test_tiff_write_20():
 
 
 ###############################################################################
-# Test RGBA images with TIFFTAG_EXTRASAMPLES=EXTRASAMPLE_ASSOCALPHA
+# Test RGBA images with TIFFTAG_EXTRASAMPLES=EXTRASAMPLE_UNASSOCALPHA
 
 
 def test_tiff_write_21():
@@ -11927,3 +11927,30 @@ def test_tiff_write_check_golden_file(tmp_path, src_filename, creation_options):
         )
     assert os.stat(src_filename).st_size == os.stat(out_filename).st_size
     assert open(src_filename, "rb").read() == open(out_filename, "rb").read()
+
+
+###############################################################################
+# Test preserving ALPHA=PREMULTIPLIED on copy
+
+
+def test_tiff_write_preserve_ALPHA_PREMULTIPLIED_on_copy(tmp_path):
+
+    src_filename = str(tmp_path / "src.tif")
+    out_filename = str(tmp_path / "out.tif")
+    gdal.GetDriverByName("GTiff").Create(
+        src_filename, 1, 1, 4, options=["ALPHA=PREMULTIPLIED", "PROFILE=BASELINE"]
+    )
+    assert gdal.VSIStatL(src_filename + ".aux.xml") is None
+    with gdal.Open(src_filename) as src_ds:
+        assert (
+            src_ds.GetRasterBand(4).GetMetadataItem("ALPHA", "IMAGE_STRUCTURE")
+            == "PREMULTIPLIED"
+        )
+        gdal.GetDriverByName("GTiff").CreateCopy(
+            out_filename, src_ds, options=["PROFILE=BASELINE"]
+        )
+        with gdal.Open(out_filename) as out_ds:
+            assert (
+                out_ds.GetRasterBand(4).GetMetadataItem("ALPHA", "IMAGE_STRUCTURE")
+                == "PREMULTIPLIED"
+            )
diff --git a/frmts/gtiff/gtiffdataset_write.cpp b/frmts/gtiff/gtiffdataset_write.cpp
@@ -7055,10 +7055,10 @@ GDALDataset *GTiffDataset::CreateCopy(const char *pszFilename,
             CPLMalloc(nExtraSamples * sizeof(uint16_t)));
         memcpy(pasNewExtraSamples, extraSamples,
                nExtraSamples * sizeof(uint16_t));
-        uint16_t nAlpha = GTiffGetAlphaValue(
-            CPLGetConfigOption("GTIFF_ALPHA",
-                               CSLFetchNameValue(papszOptions, "ALPHA")),
-            DEFAULT_ALPHA_TYPE);
+        const char *pszAlpha = CPLGetConfigOption(
+            "GTIFF_ALPHA", CSLFetchNameValue(papszOptions, "ALPHA"));
+        const uint16_t nAlpha =
+            GTiffGetAlphaValue(pszAlpha, DEFAULT_ALPHA_TYPE);
         const int nBaseSamples = l_nBands - nExtraSamples;
         for (int iExtraBand = nBaseSamples + 1; iExtraBand <= l_nBands;
              iExtraBand++)
@@ -7067,6 +7067,16 @@ GDALDataset *GTiffDataset::CreateCopy(const char *pszFilename,
                 GCI_AlphaBand)
             {
                 pasNewExtraSamples[iExtraBand - nBaseSamples - 1] = nAlpha;
+                if (!pszAlpha)
+                {
+                    // Use the ALPHA metadata item from the source band, when
+                    // present, if no explicit ALPHA creation option
+                    pasNewExtraSamples[iExtraBand - nBaseSamples - 1] =
+                        GTiffGetAlphaValue(
+                            poSrcDS->GetRasterBand(iExtraBand)
+                                ->GetMetadataItem("ALPHA", "IMAGE_STRUCTURE"),
+                            nAlpha);
+                }
             }
         }
         TIFFSetField(l_hTIFF, TIFFTAG_EXTRASAMPLES, nExtraSamples,

diff --git a/frmts/gtiff/gtiffrasterband.cpp b/frmts/gtiff/gtiffrasterband.cpp
@@ -166,7 +166,12 @@ GTiffRasterBand::GTiffRasterBand(GTiffDataset *poDSIn, int nBandIn)
             if (nBand > nBaseSamples && nBand - nBaseSamples - 1 < count &&
                 (v[nBand - nBaseSamples - 1] == EXTRASAMPLE_ASSOCALPHA ||
                  v[nBand - nBaseSamples - 1] == EXTRASAMPLE_UNASSALPHA))
+            {
+                if (v[nBand - nBaseSamples - 1] == EXTRASAMPLE_ASSOCALPHA)
+                    m_oGTiffMDMD.SetMetadataItem("ALPHA", "PREMULTIPLIED",
+                                                 "IMAGE_STRUCTURE");
                 m_eBandInterp = GCI_AlphaBand;
+            }
             else
                 m_eBandInterp = GCI_Undefined;
         }