Add PNG extractor

Textual information, such as author information, creator software,
and a description can be stored in a PNG's tEXt, zTXt, and
iTXt chunks, [1].

libexiv2 can extract EXIF information out of PNGs but does not
support PNG Textual Information, see [2], however Qt's PNG handler
makes this information readily available as textKeys.

[1] https://www.w3.org/TR/PNG/#11keywords
[2] Exiv2/exiv2#1343

src/extractors/CMakeLists.txt

@@ -255,3 +255,14 @@ if(libappimage_FOUND AND KF5Config_FOUND AND Qt${QT_MAJOR_VERSION}Gui_FOUND)
     TARGETS kfilemetadata_appimageextractor
     DESTINATION ${KDE_INSTALL_PLUGINDIR}/kf${QT_MAJOR_VERSION}/kfilemetadata)
 endif()
+
+add_library(kfilemetadata_pngextractor MODULE pngextractor.cpp )
+target_link_libraries(kfilemetadata_pngextractor
+    KF5::FileMetaData
+    Qt${QT_MAJOR_VERSION}::Gui
+)
+
+set_target_properties(kfilemetadata_pngextractor PROPERTIES LIBRARY_OUTPUT_DIRECTORY "${CMAKE_BINARY_DIR}/bin/kf${QT_MAJOR_VERSION}/kfilemetadata")
+install(
+TARGETS kfilemetadata_pngextractor
+DESTINATION ${KDE_INSTALL_PLUGINDIR}/kf${QT_MAJOR_VERSION}/kfilemetadata)

src/extractors/pngextractor.cpp

@@ -0,0 +1,86 @@
+/*
+    SPDX-FileCopyrightText: 2022 Kai Uwe Broulik <kde@broulik.de>
+
+    SPDX-License-Identifier: LGPL-2.1-or-later
+*/
+
+#include "pngextractor.h"
+#include "propertyinfo.h"
+
+#include <QImageReader>
+
+using namespace KFileMetaData;
+
+// Keywords specified in https://www.w3.org/TR/PNG/#11keywords
+static const struct {
+    QString key;
+    Property::Property property;
+} s_textMapping[] = {
+    // Short (one line) title or caption for image
+    {QStringLiteral("Title"), Property::Title},
+    // Name of image's creator
+    {QStringLiteral("Author"), Property::Author},
+    // Description of image (possibly long)
+    {QStringLiteral("Description"), Property::Description},
+    // Copyright notice
+    {QStringLiteral("Copyright"), Property::Copyright},
+    // Time of original image creation
+    {QStringLiteral("Creation Time"), Property::CreationDate},
+    // Software used to create the image
+    {QStringLiteral("Software"), Property::Generator},
+    // Disclaimer - Legal disclaimer
+    // Warning - Warning of nature of content
+    // Source - Device used to create the image
+    // Miscellaneous comment
+    {QStringLiteral("Comment"), Property::Comment},
+};
+
+PngExtractor::PngExtractor(QObject* parent)
+    : ExtractorPlugin(parent)
+{
+}
+
+QStringList PngExtractor::mimetypes() const
+{
+    return {
+        QStringLiteral("image/png")
+    };
+}
+
+void PngExtractor::extract(ExtractionResult* result)
+{
+    QImageReader reader(result->inputUrl(), "png");
+    if (!reader.canRead()) {
+        return;
+    }
+
+    result->addType(Type::Image);
+
+    for (const auto &mapping : s_textMapping) {
+        QString text = reader.text(mapping.key);
+        if (text.isEmpty()) {
+            // Spec says, keywords are case-sensitive but of course the real world looks different.
+            text = reader.text(mapping.key.toLower());
+        }
+        if (text.isEmpty()) {
+            continue;
+        }
+
+        const auto propertyInfo = PropertyInfo(mapping.property);
+
+        if (propertyInfo.valueType() == QVariant::DateTime) {
+            // "For the Creation Time keyword, the date format defined in section 5.2.14 of RFC 1123 is suggested"
+            // which in turn references RFC822...
+            const QDateTime dt = QDateTime::fromString(text, Qt::RFC2822Date);
+
+            if (!dt.isValid()) {
+                continue;
+            }
+
+            result->add(mapping.property, dt);
+            continue;
+        }
+
+        result->add(mapping.property, text);
+    }
+}

src/extractors/pngextractor.h

@@ -0,0 +1,31 @@
+/*
+    SPDX-FileCopyrightText: 2022 Kai Uwe Broulik <kde@broulik.de>
+
+    SPDX-License-Identifier: LGPL-2.1-or-later
+*/
+
+#ifndef PNGEXTRACTOR_H
+#define PNGEXTRACTOR_H
+
+#include "extractorplugin.h"
+
+namespace KFileMetaData
+{
+
+class PngExtractor : public ExtractorPlugin
+{
+    Q_OBJECT
+    Q_PLUGIN_METADATA(IID "org.kde.kf5.kfilemetadata.ExtractorPlugin"
+                      FILE "pngextractor.json")
+    Q_INTERFACES(KFileMetaData::ExtractorPlugin)
+
+public:
+    explicit PngExtractor(QObject *parent = nullptr);
+
+    void extract(ExtractionResult *result) override;
+    QStringList mimetypes() const override;
+};
+
+} // namespace KFileMetaData
+
+#endif // PNGEXTRACTOR_H

src/extractors/pngextractor.json

@@ -0,0 +1,7 @@
+{
+    "Name" : "PngExtractor",
+    "Id"   : "org.kde.pngextractor",
+    "MimeTypes" : {
+        "image/png"             : { "version" : "0.0" }
+    }
+}