Add CMYK support for pens/fills in the PDF engine

Insofar, painting with a CMYK color (pen/brush) was completely ignored
by QPdfWriter, although the PDF format can faithfully represent CMYK
colors.

This commit adds support for CMYK colors in the PDF engine. The support
is opt-in, in the name of backwards compatibility; an enumeration on
QPdfWriter controls the output.

QPrinter was using a hidden hook in QPdfEngine in order to do grayscale
printing; this hook can now be made public API through the same
enumeration.

This work has been kindly sponsored by the QGIS project
(https://qgis.org/).

[ChangeLog][QtGui][QPdfWriter] QPdfWriter can now use CMYK colors
directly, without converting them into RGB colors.

Change-Id: Ia27c19ec81a58ab68ddc8b9c89c4e57d7d637301
Reviewed-by: Lars Knoll <lars@knoll.priv.no>

src/gui/painting/qpdf.cpp

@@ -44,7 +44,7 @@ QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
 
-inline QPaintEngine::PaintEngineFeatures qt_pdf_decide_features()
+constexpr QPaintEngine::PaintEngineFeatures qt_pdf_decide_features()
 {
     QPaintEngine::PaintEngineFeatures f = QPaintEngine::AllFeatures;
     f &= ~(QPaintEngine::PorterDuff
@@ -1239,17 +1239,8 @@ void QPdfEngine::setPen()
     QBrush b = d->pen.brush();
     Q_ASSERT(b.style() == Qt::SolidPattern && b.isOpaque());
 
-    QColor rgba = b.color();
+    d->writeColor(QPdfEnginePrivate::ColorDomain::Stroking, b.color());
-    if (d->grayscale) {
-        qreal gray = qGray(rgba.rgba())/255.;
-        *d->currentPage << gray << gray << gray;
-    } else {
-        *d->currentPage << rgba.redF()
-                        << rgba.greenF()
-                        << rgba.blueF();
-    }
     *d->currentPage << "SCN\n";
-
     *d->currentPage << d->pen.widthF() << "w ";
 
     int pdfCapStyle = 0;
@@ -1303,18 +1294,9 @@ void QPdfEngine::setBrush()
     if (!patternObject && !specifyColor)
         return;
 
-    *d->currentPage << (patternObject ? "/PCSp cs " : "/CSp cs ");
+    const auto domain = patternObject ? QPdfEnginePrivate::ColorDomain::NonStrokingPattern
-    if (specifyColor) {
+                                      : QPdfEnginePrivate::ColorDomain::NonStroking;
-        QColor rgba = d->brush.color();
+    d->writeColor(domain, specifyColor ? d->brush.color() : QColor());
-        if (d->grayscale) {
-            qreal gray = qGray(rgba.rgba())/255.;
-            *d->currentPage << gray << gray << gray;
-        } else {
-            *d->currentPage << rgba.redF()
-                            << rgba.greenF()
-                            << rgba.blueF();
-        }
-    }
     if (patternObject)
         *d->currentPage << "/Pat" << patternObject;
     *d->currentPage << "scn\n";
@@ -1454,9 +1436,9 @@ int QPdfEngine::metric(QPaintDevice::PaintDeviceMetric metricType) const
 QPdfEnginePrivate::QPdfEnginePrivate()
     : clipEnabled(false), allClipped(false), hasPen(true), hasBrush(false), simplePen(false),
       needsTransform(false), pdfVersion(QPdfEngine::Version_1_4),
+      colorModel(QPdfEngine::ColorModel::RGB),
       outDevice(nullptr), ownsDevice(false),
       embedFonts(true),
-      grayscale(false),
       m_pageLayout(QPageSize(QPageSize::A4), QPageLayout::Portrait, QMarginsF(10, 10, 10, 10))
 {
     initResources();
@@ -1511,7 +1493,9 @@ bool QPdfEngine::begin(QPaintDevice *pdev)
     d->catalog = 0;
     d->info = 0;
     d->graphicsState = 0;
-    d->patternColorSpace = 0;
+    d->patternColorSpaceRGB = 0;
+    d->patternColorSpaceGrayscale = 0;
+    d->patternColorSpaceCMYK = 0;
     d->simplePen = false;
     d->needsTransform = false;
 
@@ -1629,10 +1613,99 @@ void QPdfEnginePrivate::writeHeader()
             ">>\n"
             "endobj\n");
 
-    // color space for pattern
+    // color spaces for pattern
-    patternColorSpace = addXrefEntry(-1);
+    patternColorSpaceRGB = addXrefEntry(-1);
     xprintf("[/Pattern /DeviceRGB]\n"
             "endobj\n");
+    patternColorSpaceGrayscale = addXrefEntry(-1);
+    xprintf("[/Pattern /DeviceGray]\n"
+            "endobj\n");
+    patternColorSpaceCMYK = addXrefEntry(-1);
+    xprintf("[/Pattern /DeviceCMYK]\n"
+            "endobj\n");
+}
+
+QPdfEngine::ColorModel QPdfEnginePrivate::colorModelForColor(const QColor &color) const
+{
+    switch (colorModel) {
+    case QPdfEngine::ColorModel::RGB:
+    case QPdfEngine::ColorModel::Grayscale:
+    case QPdfEngine::ColorModel::CMYK:
+        return colorModel;
+    case QPdfEngine::ColorModel::Auto:
+        switch (color.spec()) {
+        case QColor::Invalid:
+        case QColor::Rgb:
+        case QColor::Hsv:
+        case QColor::Hsl:
+        case QColor::ExtendedRgb:
+            return QPdfEngine::ColorModel::RGB;
+        case QColor::Cmyk:
+            return QPdfEngine::ColorModel::CMYK;
+        }
+
+        break;
+    }
+
+    Q_UNREACHABLE_RETURN(QPdfEngine::ColorModel::RGB);
+}
+
+void QPdfEnginePrivate::writeColor(ColorDomain domain, const QColor &color)
+{
+    // Switch to the right colorspace.
+    // For simplicity: do it even if it redundant (= already in that colorspace)
+    const QPdfEngine::ColorModel actualColorModel = colorModelForColor(color);
+
+    switch (actualColorModel) {
+    case QPdfEngine::ColorModel::RGB:
+    case QPdfEngine::ColorModel::Grayscale:
+        switch (domain) {
+        case ColorDomain::Stroking:
+            *currentPage << "/CSp CS\n"; break;
+        case ColorDomain::NonStroking:
+            *currentPage << "/CSp cs\n"; break;
+        case ColorDomain::NonStrokingPattern:
+            *currentPage << "/PCSp cs\n"; break;
+        }
+        break;
+    case QPdfEngine::ColorModel::CMYK:
+        switch (domain) {
+        case ColorDomain::Stroking:
+            *currentPage << "/CSpcmyk CS\n"; break;
+        case ColorDomain::NonStroking:
+            *currentPage << "/CSpcmyk cs\n"; break;
+        case ColorDomain::NonStrokingPattern:
+            *currentPage << "/PCSpcmyk cs\n"; break;
+        }
+        break;
+    case QPdfEngine::ColorModel::Auto:
+        Q_UNREACHABLE_RETURN();
+    }
+
+    // If we also have a color specified, write it out.
+    if (!color.isValid())
+        return;
+
+    switch (actualColorModel) {
+    case QPdfEngine::ColorModel::RGB:
+        *currentPage << color.redF()
+                     << color.greenF()
+                     << color.blueF();
+        break;
+    case QPdfEngine::ColorModel::Grayscale: {
+        const qreal gray = qGray(color.rgba()) / 255.;
+        *currentPage << gray << gray << gray;
+        break;
+    }
+    case QPdfEngine::ColorModel::CMYK:
+        *currentPage << color.cyanF()
+                     << color.magentaF()
+                     << color.yellowF()
+                     << color.blackF();
+        break;
+    case QPdfEngine::ColorModel::Auto:
+        Q_UNREACHABLE_RETURN();
+    }
 }
 
 void QPdfEnginePrivate::writeInfo()
@@ -2078,12 +2151,18 @@ void QPdfEnginePrivate::writePage()
     xprintf("<<\n"
             "/ColorSpace <<\n"
             "/PCSp %d 0 R\n"
+            "/PCSpg %d 0 R\n"
+            "/PCSpcmyk %d 0 R\n"
             "/CSp /DeviceRGB\n"
             "/CSpg /DeviceGray\n"
+            "/CSpcmyk /DeviceCMYK\n"
             ">>\n"
             "/ExtGState <<\n"
             "/GSa %d 0 R\n",
-            patternColorSpace, graphicsState);
+            patternColorSpaceRGB,
+            patternColorSpaceGrayscale,
+            patternColorSpaceCMYK,
+            graphicsState);
 
     for (int i = 0; i < currentPage->graphicStates.size(); ++i)
         xprintf("/GState%d %d 0 R\n", currentPage->graphicStates.at(i), currentPage->graphicStates.at(i));
@@ -2396,7 +2475,22 @@ struct QGradientBound {
 };
 Q_DECLARE_TYPEINFO(QGradientBound, Q_PRIMITIVE_TYPE);
 
-int QPdfEnginePrivate::createShadingFunction(const QGradient *gradient, int from, int to, bool reflect, bool alpha)
+void QPdfEnginePrivate::ShadingFunctionResult::writeColorSpace(QPdf::ByteStream *stream) const
+{
+    *stream << "/ColorSpace ";
+    switch (colorModel) {
+    case QPdfEngine::ColorModel::RGB:
+    case QPdfEngine::ColorModel::Grayscale:
+        *stream << "/DeviceRGB\n"; break;
+    case QPdfEngine::ColorModel::CMYK:
+        *stream << "/DeviceCMYK\n"; break;
+    case QPdfEngine::ColorModel::Auto:
+        Q_UNREACHABLE(); break;
+    }
+}
+
+QPdfEnginePrivate::ShadingFunctionResult
+QPdfEnginePrivate::createShadingFunction(const QGradient *gradient, int from, int to, bool reflect, bool alpha)
 {
     QGradientStops stops = gradient->stops();
     if (stops.isEmpty()) {
@@ -2408,6 +2502,35 @@ int QPdfEnginePrivate::createShadingFunction(const QGradient *gradient, int from
     if (stops.at(stops.size() - 1).first < 1)
         stops.append(QGradientStop(1, stops.at(stops.size() - 1).second));
 
+    // Color to use which colorspace to use
+    const QColor referenceColor = stops.constFirst().second;
+
+    switch (colorModel) {
+    case QPdfEngine::ColorModel::RGB:
+    case QPdfEngine::ColorModel::Grayscale:
+    case QPdfEngine::ColorModel::CMYK:
+        break;
+    case QPdfEngine::ColorModel::Auto: {
+        // Make sure that all the stops have the same color spec
+        // (we don't support anything else)
+        const QColor::Spec referenceSpec = referenceColor.spec();
+        bool warned = false;
+        for (QGradientStop &stop : stops) {
+            if (stop.second.spec() != referenceSpec) {
+                if (!warned) {
+                    qWarning("QPdfEngine: unable to create a gradient between colors of different spec");
+                    warned = true;
+                }
+                stop.second = stop.second.convertTo(referenceSpec);
+            }
+        }
+        break;
+    }
+    }
+
+    ShadingFunctionResult result;
+    result.colorModel = colorModelForColor(referenceColor);
+
     QList<int> functions;
     const int numStops = stops.size();
     functions.reserve(numStops - 1);
@@ -2423,8 +2546,30 @@ int QPdfEnginePrivate::createShadingFunction(const QGradient *gradient, int from
             s << "/C0 [" << stops.at(i).second.alphaF() << "]\n"
                  "/C1 [" << stops.at(i + 1).second.alphaF() << "]\n";
         } else {
+            switch (result.colorModel) {
+            case QPdfEngine::ColorModel::RGB:
+            case QPdfEngine::ColorModel::Grayscale:
+                // For backwards compatibility, Grayscale emits RGB colors
                 s << "/C0 [" << stops.at(i).second.redF() << stops.at(i).second.greenF() <<  stops.at(i).second.blueF() << "]\n"
                      "/C1 [" << stops.at(i + 1).second.redF() << stops.at(i + 1).second.greenF() <<  stops.at(i + 1).second.blueF() << "]\n";
+                break;
+
+            case QPdfEngine::ColorModel::CMYK:
+                s << "/C0 [" << stops.at(i).second.cyanF()
+                             << stops.at(i).second.magentaF()
+                             << stops.at(i).second.yellowF()
+                             << stops.at(i).second.blackF() << "]\n"
+                     "/C1 [" << stops.at(i + 1).second.cyanF()
+                             << stops.at(i + 1).second.magentaF()
+                             << stops.at(i + 1).second.yellowF()
+                             << stops.at(i + 1).second.blackF() << "]\n";
+                break;
+
+            case QPdfEngine::ColorModel::Auto:
+                Q_UNREACHABLE();
+                break;
+            }
+
         }
         s << ">>\n"
              "endobj\n";
@@ -2492,7 +2637,8 @@ int QPdfEnginePrivate::createShadingFunction(const QGradient *gradient, int from
     } else {
         function = functions.at(0);
     }
-    return function;
+    result.function = function;
+    return result;
 }
 
 int QPdfEnginePrivate::generateLinearGradientShader(const QLinearGradient *gradient, const QTransform &matrix, bool alpha)
@@ -2538,17 +2684,22 @@ int QPdfEnginePrivate::generateLinearGradientShader(const QLinearGradient *gradi
     }
     }
 
-    int function = createShadingFunction(gradient, from, to, reflect, alpha);
+    const auto shadingFunctionResult = createShadingFunction(gradient, from, to, reflect, alpha);
 
     QByteArray shader;
     QPdf::ByteStream s(&shader);
     s << "<<\n"
-        "/ShadingType 2\n"
+         "/ShadingType 2\n";
-        "/ColorSpace " << (alpha ? "/DeviceGray\n" : "/DeviceRGB\n") <<
+
-        "/AntiAlias true\n"
+    if (alpha)
+        s << "/ColorSpace /DeviceGray\n";
+    else
+        shadingFunctionResult.writeColorSpace(&s);
+
+    s << "/AntiAlias true\n"
         "/Coords [" << start.x() << start.y() << stop.x() << stop.y() << "]\n"
         "/Extend [true true]\n"
-        "/Function " << function << "0 R\n"
+        "/Function " << shadingFunctionResult.function << "0 R\n"
         ">>\n"
         "endobj\n";
     int shaderObject = addXrefEntry(-1);
@@ -2606,18 +2757,23 @@ int QPdfEnginePrivate::generateRadialGradientShader(const QRadialGradient *gradi
     }
     }
 
-    int function = createShadingFunction(gradient, from, to, reflect, alpha);
+    const auto shadingFunctionResult = createShadingFunction(gradient, from, to, reflect, alpha);
 
     QByteArray shader;
     QPdf::ByteStream s(&shader);
     s << "<<\n"
-        "/ShadingType 3\n"
+         "/ShadingType 3\n";
-        "/ColorSpace " << (alpha ? "/DeviceGray\n" : "/DeviceRGB\n") <<
+
-        "/AntiAlias true\n"
+    if (alpha)
+        s << "/ColorSpace /DeviceGray\n";
+    else
+        shadingFunctionResult.writeColorSpace(&s);
+
+    s << "/AntiAlias true\n"
         "/Domain [0 1]\n"
         "/Coords [" << p0.x() << p0.y() << r0 << p1.x() << p1.y() << r1 << "]\n"
         "/Extend [true true]\n"
-        "/Function " << function << "0 R\n"
+        "/Function " << shadingFunctionResult.function << "0 R\n"
         ">>\n"
         "endobj\n";
     int shaderObject = addXrefEntry(-1);
@@ -2856,6 +3012,7 @@ int QPdfEnginePrivate::addImage(const QImage &img, bool *bitmap, bool lossless,
 
     QImage image = img;
     QImage::Format format = image.format();
+    const bool grayscale = (colorModel == QPdfEngine::ColorModel::Grayscale);
 
     if (pdfVersion == QPdfEngine::Version_A1b) {
         if (image.hasAlphaChannel()) {

src/gui/painting/qpdf_p.h

@@ -142,7 +142,7 @@ public:
     };
 
     QPdfEngine();
-    QPdfEngine(QPdfEnginePrivate &d);
+    explicit QPdfEngine(QPdfEnginePrivate &d);
     ~QPdfEngine() {}
 
     void setOutputFilename(const QString &filename);
@@ -157,6 +157,18 @@ public:
 
     void addFileAttachment(const QString &fileName, const QByteArray &data, const QString &mimeType);
 
+    // keep in sync with QPdfWriter
+    enum class ColorModel
+    {
+        RGB,
+        Grayscale,
+        CMYK,
+        Auto,
+    };
+
+    ColorModel colorModel() const;
+    void setColorModel(ColorModel model);
+
     // reimplementations QPaintEngine
     bool begin(QPaintDevice *pdev) override;
     bool end() override;
@@ -240,6 +252,7 @@ public:
     bool needsTransform;
     qreal opacity;
     QPdfEngine::PdfVersion pdfVersion;
+    QPdfEngine::ColorModel colorModel;
 
     QHash<QFontEngine::FaceId, QFontSubset *> fonts;
 
@@ -255,7 +268,6 @@ public:
     QString creator;
     bool embedFonts;
     int resolution;
-    bool grayscale;
 
     // Page layout: size, orientation and margins
     QPageLayout m_pageLayout;
@@ -265,8 +277,22 @@ private:
     int generateGradientShader(const QGradient *gradient, const QTransform &matrix, bool alpha = false);
     int generateLinearGradientShader(const QLinearGradient *lg, const QTransform &matrix, bool alpha);
     int generateRadialGradientShader(const QRadialGradient *gradient, const QTransform &matrix, bool alpha);
-    int createShadingFunction(const QGradient *gradient, int from, int to, bool reflect, bool alpha);
+    struct ShadingFunctionResult
+    {
+        int function;
+        QPdfEngine::ColorModel colorModel;
+        void writeColorSpace(QPdf::ByteStream *stream) const;
+    };
+    ShadingFunctionResult createShadingFunction(const QGradient *gradient, int from, int to, bool reflect, bool alpha);
 
+    enum class ColorDomain {
+        Stroking,
+        NonStroking,
+        NonStrokingPattern,
+    };
+
+    QPdfEngine::ColorModel colorModelForColor(const QColor &color) const;
+    void writeColor(ColorDomain domain, const QColor &color);
     void writeInfo();
     int writeXmpDcumentMetaData();
     int writeOutputIntent();
@@ -316,7 +342,10 @@ private:
 
     // various PDF objects
     int pageRoot, namesRoot, destsRoot, attachmentsRoot, catalog, info;
-    int graphicsState, patternColorSpace;
+    int graphicsState;
+    int patternColorSpaceRGB;
+    int patternColorSpaceGrayscale;
+    int patternColorSpaceCMYK;
     QList<uint> pages;
     QHash<qint64, uint> imageCache;
     QHash<QPair<uint, uint>, uint > alphaCache;

src/gui/painting/qpdfwriter.cpp

@@ -295,6 +295,52 @@ bool QPdfWriter::newPage()
     return d->engine->newPage();
 }
 
+/*!
+  \enum QPdfWriter::ColorModel
+  \since 6.8
+
+  This enumeration describes the way in which the PDF engine interprets
+  stroking and filling colors, set as a QPainter's pen or brush (via
+  QPen and QBrush).
+
+  \value RGB All colors are converted to RGB and saved as such in the
+  PDF. This is the default.
+
+  \value Grayscale All colors are converted to grayscale. For backwards
+  compatibility, they are emitted in the PDF output as RGB colors, with
+  identical quantities of red, green and blue.
+
+  \value CMYK All colors are converted to CMYK and saved as such.
+
+  \value Auto RGB colors are emitted as RGB; CMYK colors are emitted as
+  CMYK. Colors of any other color spec are converted to RGB.
+
+  \sa QColor, QGradient
+*/
+
+/*!
+  \since 6.8
+
+  Returns the color model used by this PDF writer.
+  The default is QPdfWriter::ColorModel::RGB.
+*/
+QPdfWriter::ColorModel QPdfWriter::colorModel() const
+{
+    Q_D(const QPdfWriter);
+    return static_cast<ColorModel>(d->engine->d_func()->colorModel);
+}
+
+/*!
+  \since 6.8
+
+  Sets the color model used by this PDF writer to \a model.
+*/
+void QPdfWriter::setColorModel(ColorModel model)
+{
+    Q_D(QPdfWriter);
+    d->engine->d_func()->colorModel = static_cast<QPdfEngine::ColorModel>(model);
+}
+
 QT_END_NAMESPACE
 
 #include "moc_qpdfwriter.cpp"

src/gui/painting/qpdfwriter.h

@@ -44,6 +44,18 @@ public:
 
     void addFileAttachment(const QString &fileName, const QByteArray &data, const QString &mimeType = QString());
 
+    enum class ColorModel
+    {
+        RGB,
+        Grayscale,
+        CMYK,
+        Auto,
+    };
+    Q_ENUM(ColorModel)
+
+    ColorModel colorModel() const;
+    void setColorModel(ColorModel model);
+
 protected:
     QPaintEngine *paintEngine() const override;
     int metric(PaintDeviceMetric id) const override;

src/plugins/printsupport/cups/qcupsprintengine.cpp

@@ -249,9 +249,12 @@ void QCupsPrintEnginePrivate::changePrinter(const QString &newPrinter)
         duplex = m_printDevice.defaultDuplexMode();
         duplexRequestedExplicitly = false;
     }
-    QPrint::ColorMode colorMode = grayscale ? QPrint::GrayScale : QPrint::Color;
+    QPrint::ColorMode colorMode = static_cast<QPrint::ColorMode>(printerColorMode());
-    if (!m_printDevice.supportedColorModes().contains(colorMode))
+    if (!m_printDevice.supportedColorModes().contains(colorMode)) {
-        grayscale = m_printDevice.defaultColorMode() == QPrint::GrayScale;
+        colorModel = (m_printDevice.defaultColorMode() == QPrint::GrayScale)
+                         ? QPdfEngine::ColorModel::Grayscale
+                         : QPdfEngine::ColorModel::RGB;
+    }
 
     // Get the equivalent page size for this printer as supported names may be different
     if (m_printDevice.supportedPageSize(m_pageLayout.pageSize()).isValid())

src/printsupport/kernel/qprint_p.h

@@ -68,6 +68,7 @@ namespace QPrint {
         DuplexShortSide
     };
 
+    // Note: Keep in sync with QPrinter::ColorMode
     enum ColorMode {
         GrayScale,
         Color

src/printsupport/kernel/qprintengine_pdf.cpp

@@ -104,7 +104,14 @@ void QPdfPrintEngine::setProperty(PrintEnginePropertyKey key, const QVariant &va
         d->collate = value.toBool();
         break;
     case PPK_ColorMode:
-        d->grayscale = (QPrinter::ColorMode(value.toInt()) == QPrinter::GrayScale);
+        switch (QPrinter::ColorMode(value.toInt())) {
+        case QPrinter::GrayScale:
+            d->colorModel = QPdfEngine::ColorModel::Grayscale;
+            break;
+        case QPrinter::Color:
+            d->colorModel = QPdfEngine::ColorModel::RGB;
+            break;
+        }
         break;
     case PPK_Creator:
         d->creator = value.toString();
@@ -221,7 +228,7 @@ QVariant QPdfPrintEngine::property(PrintEnginePropertyKey key) const
         ret = d->collate;
         break;
     case PPK_ColorMode:
-        ret = d->grayscale ? QPrinter::GrayScale : QPrinter::Color;
+        ret = d->printerColorMode();
         break;
     case PPK_Creator:
         ret = d->creator;
@@ -367,6 +374,22 @@ QPdfPrintEnginePrivate::~QPdfPrintEnginePrivate()
 {
 }
 
+QPrinter::ColorMode QPdfPrintEnginePrivate::printerColorMode() const
+{
+    switch (colorModel) {
+    case QPdfEngine::ColorModel::RGB:
+    case QPdfEngine::ColorModel::CMYK:
+    case QPdfEngine::ColorModel::Auto:
+        return QPrinter::Color;
+    case QPdfEngine::ColorModel::Grayscale:
+        return QPrinter::GrayScale;
+    }
+
+    Q_UNREACHABLE();
+    return QPrinter::Color;
+}
+
+
 QT_END_NAMESPACE
 
 #endif // QT_NO_PRINTER

src/printsupport/kernel/qprintengine_pdf_p.h

@@ -79,6 +79,8 @@ public:
     QPdfPrintEnginePrivate(QPrinter::PrinterMode m);
     ~QPdfPrintEnginePrivate();
 
+    QPrinter::ColorMode printerColorMode() const;
+
     virtual bool openPrintDevice();
     virtual void closePrintDevice();