Export gamma setting and metadata in QImageReader

The PNG image plugin has the capability of applying gamma correction
to read PNGs, but doesn't do so by default, and we export no way of
accessing this through the qimagereader API.

This patch adds the ability to read the PNG file gamma, and overriding
decoded gamma to get gamma correction.

Change-Id: Iaf9eaac0827516db6bc02247c58026270cd1f610
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

3 files changed, 49 insertions, 9 deletions

diff --git a/src/gui/image/qimagereader.cpp b/src/gui/image/qimagereader.cpp
index ba79bf40e5..83ec91bb40 100644
--- a/src/gui/image/qimagereader.cpp
+++ b/src/gui/image/qimagereader.cpp
@@ -1203,6 +1203,39 @@ bool QImageReader::autoTransform() const
 }
 
 /*!
+    \since 5.6
+
+    This is an image format specific function that forces images with
+    gamma information to be gamma corrected to \a gamma. For image formats
+    that do not support gamma correction, this value is ignored.
+
+    To gamma correct to a standard PC color-space, set gamma to \c 1/2.2.
+
+    \sa gamma()
+*/
+void QImageReader::setGamma(float gamma)
+{
+    if (d->initHandler() && d->handler->supportsOption(QImageIOHandler::Gamma))
+        d->handler->setOption(QImageIOHandler::Gamma, gamma);
+}
+
+/*!
+    \since 5.6
+
+    Returns the gamma level of the decoded image. If setGamma() has been
+    called and gamma correction is supported it will return the gamma set.
+    If gamma level is not supported by the image format, \c 0.0 is returned.
+
+    \sa setGamma()
+*/
+float QImageReader::gamma() const
+{
+    if (d->initHandler() && d->handler->supportsOption(QImageIOHandler::Gamma))
+        return d->handler->option(QImageIOHandler::Gamma).toFloat();
+    return 0.0;
+}
+
+/*!
     Returns \c true if an image can be read for the device (i.e., the
     image format is supported, and the device seems to contain valid
     data); otherwise returns \c false.
diff --git a/src/gui/image/qimagereader.h b/src/gui/image/qimagereader.h
index 27a29bed49..6745c55b97 100644
--- a/src/gui/image/qimagereader.h
+++ b/src/gui/image/qimagereader.h
@@ -110,6 +110,9 @@ public:
     void setAutoTransform(bool enabled);
     bool autoTransform() const;
 
+    void setGamma(float gamma);
+    float gamma() const;
+
     QByteArray subType() const;
     QList<QByteArray> supportedSubTypes() const;
 
diff --git a/src/gui/image/qpnghandler.cpp b/src/gui/image/qpnghandler.cpp
index 7fbd24787e..776a61d8fe 100644
--- a/src/gui/image/qpnghandler.cpp
+++ b/src/gui/image/qpnghandler.cpp
@@ -108,10 +108,11 @@ public:
     };
 
     QPngHandlerPrivate(QPngHandler *qq)
-        : gamma(0.0), quality(2), png_ptr(0), info_ptr(0), end_info(0), state(Ready), q(qq)
+        : gamma(0.0), fileGamma(0.0), quality(2), png_ptr(0), info_ptr(0), end_info(0), state(Ready), q(qq)
     { }
 
     float gamma;
+    float fileGamma;
     int quality;
     QString description;
     QSize scaledSize;
@@ -234,13 +235,10 @@ void CALLBACK_CALL_TYPE qpiw_flush_fn(png_structp /* png_ptr */)
 }
 
 static
-void setup_qt(QImage& image, png_structp png_ptr, png_infop info_ptr, QSize scaledSize, bool *doScaledRead, float screen_gamma=0.0)
+void setup_qt(QImage& image, png_structp png_ptr, png_infop info_ptr, QSize scaledSize, bool *doScaledRead, float screen_gamma=0.0, float file_gamma=0.0)
 {
-    if (screen_gamma != 0.0 && png_get_valid(png_ptr, info_ptr, PNG_INFO_gAMA)) {
-        double file_gamma;
-        png_get_gAMA(png_ptr, info_ptr, &file_gamma);
-        png_set_gamma(png_ptr, screen_gamma, file_gamma);
-    }
+    if (screen_gamma != 0.0 && file_gamma != 0.0)
+        png_set_gamma(png_ptr, 1.0f / screen_gamma, file_gamma);
 
     png_uint_32 width;
     png_uint_32 height;
@@ -557,6 +555,12 @@ bool Q_INTERNAL_WIN_NO_THROW QPngHandlerPrivate::readPngHeader()
 
     readPngTexts(info_ptr);
 
+    if (png_get_valid(png_ptr, info_ptr, PNG_INFO_gAMA)) {
+        double file_gamma = 0.0;
+        png_get_gAMA(png_ptr, info_ptr, &file_gamma);
+        fileGamma = file_gamma;
+    }
+
     state = ReadHeader;
     return true;
 }
@@ -580,7 +584,7 @@ bool Q_INTERNAL_WIN_NO_THROW QPngHandlerPrivate::readPngImage(QImage *outImage)
     }
 
     bool doScaledRead = false;
-    setup_qt(*outImage, png_ptr, info_ptr, scaledSize, &doScaledRead, gamma);
+    setup_qt(*outImage, png_ptr, info_ptr, scaledSize, &doScaledRead, gamma, fileGamma);
 
     if (outImage->isNull()) {
         png_destroy_read_struct(&png_ptr, &info_ptr, &end_info);
@@ -1063,7 +1067,7 @@ QVariant QPngHandler::option(ImageOption option) const
         return QVariant();
 
     if (option == Gamma)
-        return d->gamma;
+        return d->gamma == 0.0 ? d->fileGamma : d->gamma;
     else if (option == Quality)
         return d->quality;
     else if (option == Description)