Fix and simplify computation of painter monochrome mode

The old code in GuiPainter::filterColor did not work. Tricks with
colors should take place in HSV space, not RGB IMO.

Replace the code with a simpler one which maps the grayscale value
of the original color on the blend color. It works nin the case where
original color is red, but might not work as well when blend color is
not black. Time will tell.

Fixes bug #11904.

src/frontends/NullPainter.h

@@ -95,8 +95,8 @@ public:
 	int preeditText(int, int, char_type, FontInfo const &,
 	                preedit_style) { return 0; }
 
-	/// start monochrome painting mode, i.e. map every color into [min,max]
+	/// start monochrome painting mode, i.e. map every color a shade of \c blend.
-	void enterMonochromeMode(Color const &, Color const &) {}
+	void enterMonochromeMode(Color const &) {}
 	/// leave monochrome painting mode
 	void leaveMonochromeMode() {}
 	/// draws a wavy line that can be used for underlining.

src/frontends/Painter.h

@@ -176,9 +176,8 @@ public:
 	virtual int preeditText(int x, int y,
 		char_type c, FontInfo const & f, preedit_style style) = 0;
 
-	/// start monochrome painting mode, i.e. map every color into [min,max]
+	/// start monochrome painting mode, i.e. map every color a shade of \c blend.
-	virtual void enterMonochromeMode(Color const & min,
+	virtual void enterMonochromeMode(Color const & blend) = 0;
-		Color const & max) = 0;
 	/// leave monochrome painting mode
 	virtual void leaveMonochromeMode() = 0;
 	/// draws a wavy line that can be used for underlining.

src/frontends/qt/GuiPainter.cpp

@@ -91,44 +91,25 @@ QColor GuiPainter::computeColor(Color col)
 
 QColor GuiPainter::filterColor(QColor const & col)
 {
-	if (monochrome_min_.empty())
+	if (monochrome_blend_.empty())
 		return col;
 
-	// map into [min,max] interval
+	QColor const blend = monochrome_blend_.top();
-	QColor const & min = monochrome_min_.top();
+	return QColor::fromHsv(blend.hue(), blend.saturation(), qGray(col.rgb()));
-	QColor const & max = monochrome_max_.top();
-
-	qreal v = col.valueF();
-	v *= v; // make it a bit steeper (i.e. darker)
-
-	qreal minr, ming, minb;
-	qreal maxr, maxg, maxb;
-	min.getRgbF(&minr, &ming, &minb);
-	max.getRgbF(&maxr, &maxg, &maxb);
-
-	QColor c;
-	c.setRgbF(
-		v * (minr - maxr) + maxr,
-		v * (ming - maxg) + maxg,
-		v * (minb - maxb) + maxb);
-	return c;
 }
 
 
-void GuiPainter::enterMonochromeMode(Color const & min, Color const & max)
+void GuiPainter::enterMonochromeMode(Color const & blend)
 {
-	QColor qmin = filterColor(guiApp->colorCache().get(min));
+	QColor qblend = filterColor(guiApp->colorCache().get(blend));
-	QColor qmax = filterColor(guiApp->colorCache().get(max));
+	monochrome_blend_.push(qblend);
-	monochrome_min_.push(qmin);
-	monochrome_max_.push(qmax);
 }
 
 
 void GuiPainter::leaveMonochromeMode()
 {
-	LASSERT(!monochrome_min_.empty(), return);
+	LASSERT(!monochrome_blend_.empty(), return);
-	monochrome_min_.pop();
+	monochrome_blend_.pop();
-	monochrome_max_.pop();
 }
 
 

src/frontends/qt/GuiPainter.h

@@ -135,9 +135,8 @@ public:
 	virtual void buttonText(int x, int baseline, docstring const & s,
 		FontInfo const & font, Color back, Color frame, int offset);
 
-	/// start monochrome painting mode, i.e. map every color into [min,max]
+	/// start monochrome painting mode, i.e. map every color a shade of \c blend.
-	virtual void enterMonochromeMode(Color const & min,
+	virtual void enterMonochromeMode(Color const & blend);
-		Color const & max);
 	/// leave monochrome painting mode
 	virtual void leaveMonochromeMode();
 
@@ -193,9 +192,7 @@ private:
 	Painter::line_style current_ls_;
 	int current_lw_;
 	///
-	std::stack<QColor> monochrome_min_;
+	std::stack<QColor> monochrome_blend_;
-	///
-	std::stack<QColor> monochrome_max_;
 	/// convert into Qt color, possibly applying the monochrome mode
 	QColor computeColor(Color col);
 	/// possibly apply monochrome mode

src/mathed/InsetMathMacro.cpp

@@ -142,7 +142,7 @@ public:
 	void afterDraw(PainterInfo const & pi) const
 	{
 		if (mathMacro_->editMetrics(pi.base.bv))
-			pi.pain.enterMonochromeMode(Color_mathbg, Color_mathmacroblend);
+			pi.pain.enterMonochromeMode(Color_mathmacroblend);
 	}
 	///
 	void metrics(MetricsInfo &, Dimension &) const {
@@ -386,7 +386,7 @@ void InsetMathMacro::afterMetrics() const
 void InsetMathMacro::beforeDraw(PainterInfo const & pi) const
 {
 	if (d->editing_[pi.base.bv])
-		pi.pain.enterMonochromeMode(Color_mathbg, Color_mathmacroblend);
+		pi.pain.enterMonochromeMode(Color_mathmacroblend);
 }
 
 

src/mathed/InsetMathMacroTemplate.cpp

@@ -286,9 +286,9 @@ void InsetMathWrapper::draw(PainterInfo & pi, int x, int y) const
 class InsetColoredCell : public InsetMathNest {
 public:
 	///
-	InsetColoredCell(Buffer * buf, ColorCode min, ColorCode max);
+	InsetColoredCell(Buffer * buf, ColorCode blend);
 	///
-	InsetColoredCell(Buffer * buf, ColorCode min, ColorCode max, MathAtom const & atom);
+	InsetColoredCell(Buffer * buf, ColorCode blend, MathAtom const & atom);
 	///
 	void draw(PainterInfo &, int x, int y) const;
 	///
@@ -298,20 +298,18 @@ protected:
 	///
 	Inset * clone() const;
 	///
-	ColorCode min_;
+	ColorCode blend_;
-	///
-	ColorCode max_;
 };
 
 
-InsetColoredCell::InsetColoredCell(Buffer * buf, ColorCode min, ColorCode max)
+InsetColoredCell::InsetColoredCell(Buffer * buf, ColorCode blend)
-	: InsetMathNest(buf, 1), min_(min), max_(max)
+	: InsetMathNest(buf, 1), blend_(blend)
 {
 }
 
 
-InsetColoredCell::InsetColoredCell(Buffer * buf, ColorCode min, ColorCode max, MathAtom const & atom)
+InsetColoredCell::InsetColoredCell(Buffer * buf, ColorCode blend, MathAtom const & atom)
-	: InsetMathNest(buf, 1), min_(min), max_(max)
+	: InsetMathNest(buf, 1), blend_(blend)
 {
 	cell(0).insert(0, atom);
 }
@@ -331,7 +329,7 @@ void InsetColoredCell::metrics(MetricsInfo & mi, Dimension & dim) const
 
 void InsetColoredCell::draw(PainterInfo & pi, int x, int y) const
 {
-	pi.pain.enterMonochromeMode(min_, max_);
+	pi.pain.enterMonochromeMode(blend_);
 	cell(0).draw(pi, x, y);
 	pi.pain.leaveMonochromeMode();
 }
@@ -497,7 +495,7 @@ void InsetMathMacroTemplate::createLook(int args) const
 			// color it light grey, if it is to be removed when the cursor leaves
 			if (i == argsInLook_) {
 				optData->push_back(MathAtom(
-					new InsetColoredCell(buffer_, Color_mathbg, Color_mathmacrooldarg)));
+					new InsetColoredCell(buffer_, Color_mathmacrooldarg)));
 				optData = &(*optData)[optData->size() - 1].nucleus()->cell(0);
 			}
 
@@ -513,7 +511,7 @@ void InsetMathMacroTemplate::createLook(int args) const
 		arg.push_back(MathAtom(new InsetMathMacroArgument(i + 1)));
 		if (i >= argsInLook_) {
 			look_.push_back(MathAtom(new InsetColoredCell(buffer_,
-				Color_mathbg, Color_mathmacrooldarg,
+				Color_mathmacrooldarg,
 				MathAtom(new InsetMathBrace(arg)))));
 		} else
 			look_.push_back(MathAtom(new InsetMathBrace(arg)));
@@ -522,7 +520,7 @@ void InsetMathMacroTemplate::createLook(int args) const
 		MathData arg;
 		arg.push_back(MathAtom(new InsetMathMacroArgument(i + 1)));
 		look_.push_back(MathAtom(new InsetColoredCell(buffer_,
-			Color_mathbg, Color_mathmacronewarg,
+			Color_mathmacronewarg,
 			MathAtom(new InsetMathBrace(arg)))));
 	}