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Add CMYK support for pens/fills in the PDF engine
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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>
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@dangelog
dangelog committed Feb 2, 2024
1 parent efc5791 commit 0092f06
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Showing 8 changed files with 324 additions and 51 deletions.
241 changes: 199 additions & 42 deletions src/gui/painting/qpdf.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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
Expand Down Expand Up @@ -1239,17 +1239,8 @@ void QPdfEngine::setPen()
QBrush b = d->pen.brush();
Q_ASSERT(b.style() == Qt::SolidPattern && b.isOpaque());

QColor rgba = 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->writeColor(QPdfEnginePrivate::ColorDomain::Stroking, b.color());
*d->currentPage << "SCN\n";

*d->currentPage << d->pen.widthF() << "w ";

int pdfCapStyle = 0;
Expand Down Expand Up @@ -1303,18 +1294,9 @@ void QPdfEngine::setBrush()
if (!patternObject && !specifyColor)
return;

*d->currentPage << (patternObject ? "/PCSp cs " : "/CSp cs ");
if (specifyColor) {
QColor rgba = d->brush.color();
if (d->grayscale) {
qreal gray = qGray(rgba.rgba())/255.;
*d->currentPage << gray << gray << gray;
} else {
*d->currentPage << rgba.redF()
<< rgba.greenF()
<< rgba.blueF();
}
}
const auto domain = patternObject ? QPdfEnginePrivate::ColorDomain::NonStrokingPattern
: QPdfEnginePrivate::ColorDomain::NonStroking;
d->writeColor(domain, specifyColor ? d->brush.color() : QColor());
if (patternObject)
*d->currentPage << "/Pat" << patternObject;
*d->currentPage << "scn\n";
Expand Down Expand Up @@ -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();
Expand Down Expand Up @@ -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;

Expand Down Expand Up @@ -1629,10 +1613,99 @@ void QPdfEnginePrivate::writeHeader()
">>\n"
"endobj\n");

// color space for pattern
patternColorSpace = addXrefEntry(-1);
// color spaces for pattern
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()
Expand Down Expand Up @@ -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));
Expand Down Expand Up @@ -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()) {
Expand All @@ -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);
Expand All @@ -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 {
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";
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";
Expand Down Expand Up @@ -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)
Expand Down Expand Up @@ -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"
"/ColorSpace " << (alpha ? "/DeviceGray\n" : "/DeviceRGB\n") <<
"/AntiAlias true\n"
"/ShadingType 2\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);
Expand Down Expand Up @@ -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"
"/ColorSpace " << (alpha ? "/DeviceGray\n" : "/DeviceRGB\n") <<
"/AntiAlias true\n"
"/ShadingType 3\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);
Expand Down Expand Up @@ -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()) {
Expand Down
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