Per-band nodata never gives alpha=0

[warmerdam]

If you do a warp with several (or perhaps even one?) band using per band nodata rather than UNIFIED_SRC_NODATA and you have an output mask or alpha band then the output mask is never set to zero.

This small patch will fix this for at least one mode.

warmerdam@gdal:~/gdal/alg$ svn diff
Index: gdalwarpkernel.cpp
===================================================================
--- gdalwarpkernel.cpp	(revision 26332)
+++ gdalwarpkernel.cpp	(working copy)
@@ -4057,6 +4057,7 @@
 /*      Loop processing each band.                                      */
 /* ==================================================================== */
             int iBand;
+	    int bGotAtLeastOneBand = FALSE;
 
             iDstOffset = iDstX + iDstY * nDstXSize;
 
@@ -4081,11 +4082,13 @@
                             GWKSetPixelValue( poWK, iBand, iDstOffset, 
                                               dfBandDensity, (double) bValue, 
                                               0.0 );
+			    bGotAtLeastOneBand = TRUE;
                         }
                     }
                     else
                     {
                         poWK->papabyDstImage[iBand][iDstOffset] = bValue;
+			bGotAtLeastOneBand = TRUE;
                     }
                 }
             }
@@ -4093,6 +4096,9 @@
 /* -------------------------------------------------------------------- */
 /*      Mark this pixel valid/opaque in the output.                     */
 /* -------------------------------------------------------------------- */
+	    if (!bGotAtLeastOneBand)
+	      dfDensity = 0.0;
+
             GWKOverlayDensity( poWK, iDstOffset, dfDensity );
 
             if( poWK->panDstValid != NULL )

[Jukka Rahkonen]

The patch is not applied to gdal/alg/gdalwarpkernel.cpp yet.

[Even Rouault]

This ticket has been automatically closed because Trac is no longer used for GDAL bug tracking, since the project has migrated to GitHub. If you believe this ticket is still valid, you may file it to ​https://github.com/OSGeo/gdal/issues if it is not already reported there.