Improve interpretation of undefined alpha-channels

Libtiff does not process the alpha-channel if its definition is not
specified. However to match how Qt used to save TIFF images and how
tested image viewers interpret them, we need to treat unspecified alpha
channels as unpremultiplied.

Task-number: QTBUG-50902
Change-Id: Id72218ed5bf702b54ffa3b5b47d6230facbfa0c4
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/src/plugins/imageformats/tiff/qtiffhandler.cpp b/src/plugins/imageformats/tiff/qtiffhandler.cpp
index a6b87af..875ab16 100644
--- a/src/plugins/imageformats/tiff/qtiffhandler.cpp
+++ b/src/plugins/imageformats/tiff/qtiffhandler.cpp
@@ -251,8 +251,19 @@ bool QTiffHandlerPrivate::openForRead(QIODevice *device)
         format = QImage::Format_Indexed8;
     else if (samplesPerPixel < 4)
         format = QImage::Format_RGB32;
-    else
-        format = QImage::Format_ARGB32_Premultiplied;
+    else {
+        uint16 count;
+        uint16 *extrasamples;
+        // If there is any definition of the alpha-channel, libtiff will return premultiplied
+        // data to us. If there is none, libtiff will not touch it and  we assume it to be
+        // non-premultiplied, matching behavior of tested image editors, and how older Qt
+        // versions used to save it.
+        bool gotField = TIFFGetField(tiff, TIFFTAG_EXTRASAMPLES, &count, &extrasamples);
+        if (!gotField || !count || extrasamples[0] == EXTRASAMPLE_UNSPECIFIED)
+            format = QImage::Format_ARGB32;
+        else
+            format = QImage::Format_ARGB32_Premultiplied;
+    }
 
     headersRead = true;
     return true;