Correctly interpret RGBA tiff images as premultiplied

The TIFFReadRGBAImageOriented method turns out to return colors with
alpha premultiplied by default. The only reason we pass our own tests
is because we also save the colors incorrectly unpremultiplied.

The patch fixes the format type of the returned images, and explictily
writes the how alpha should be interpreted in the saved files.

Change-Id: Ie1c3881acfe07eae25ca735adf243c1636f656a0
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

1 files changed, 12 insertions, 4 deletions

diff --git a/src/plugins/imageformats/tiff/qtiffhandler.cpp b/src/plugins/imageformats/tiff/qtiffhandler.cpp
index 93fd1ed..3fc1680 100644
--- a/src/plugins/imageformats/tiff/qtiffhandler.cpp
+++ b/src/plugins/imageformats/tiff/qtiffhandler.cpp
@@ -232,7 +232,7 @@ bool QTiffHandlerPrivate::openForRead(QIODevice *device)
     else if (samplesPerPixel < 4)
         format = QImage::Format_RGB32;
     else
-        format = QImage::Format_ARGB32;
+        format = QImage::Format_ARGB32_Premultiplied;
 
     headersRead = true;
     return true;
@@ -275,7 +275,9 @@ bool QTiffHandler::read(QImage *image)
         return false;
 
     QImage::Format format = d->format;
-    if (format == QImage::Format_RGB32 && image->format() == QImage::Format_ARGB32)
+    if (format == QImage::Format_RGB32 &&
+            (image->format() == QImage::Format_ARGB32 ||
+             image->format() == QImage::Format_ARGB32_Premultiplied))
         format = image->format();
 
     if (image->size() != d->size || image->format() != format)
@@ -647,10 +649,14 @@ bool QTiffHandler::write(const QImage &image)
         }
         TIFFClose(tiff);
     } else {
+        const bool premultiplied = image.format() != QImage::Format_ARGB32
+                                && image.format() != QImage::Format_RGBA8888;
+        const uint16 extrasamples = premultiplied ? EXTRASAMPLE_ASSOCALPHA : EXTRASAMPLE_UNASSALPHA;
         if (!TIFFSetField(tiff, TIFFTAG_PHOTOMETRIC, PHOTOMETRIC_RGB)
             || !TIFFSetField(tiff, TIFFTAG_COMPRESSION, compression == NoCompression ? COMPRESSION_NONE : COMPRESSION_LZW)
             || !TIFFSetField(tiff, TIFFTAG_SAMPLESPERPIXEL, 4)
-            || !TIFFSetField(tiff, TIFFTAG_BITSPERSAMPLE, 8)) {
+            || !TIFFSetField(tiff, TIFFTAG_BITSPERSAMPLE, 8)
+            || !TIFFSetField(tiff, TIFFTAG_EXTRASAMPLES, 1, &extrasamples)) {
             TIFFClose(tiff);
             return false;
         }
@@ -658,9 +664,11 @@ bool QTiffHandler::write(const QImage &image)
         const int chunks = (width * height * 4 / (1024 * 1024 * 16)) + 1;
         const int chunkHeight = qMax(height / chunks, 1);
 
+        const QImage::Format format = premultiplied ? QImage::Format_RGBA8888_Premultiplied
+                                                    : QImage::Format_RGBA8888;
         int y = 0;
         while (y < height) {
-            const QImage chunk = image.copy(0, y, width, qMin(chunkHeight, height - y)).convertToFormat(QImage::Format_RGBA8888);
+            const QImage chunk = image.copy(0, y, width, qMin(chunkHeight, height - y)).convertToFormat(format);
 
             int chunkStart = y;
             int chunkEnd = y + chunk.height();