可以在面板中選擇顯示的顏色。

效果如圖：

核心程式碼：

#ifndef QTCOLORTRIANGLE_H
#define QTCOLORTRIANGLE_H
#include <QImage>
#include <QWidget>
#include <QPainter>
class QPointF;
struct Vertex;

#if defined(Q_WS_WIN)
#  if !defined(QT_QTCOLORTRIANGLE_EXPORT) && !defined(QT_QTCOLORTRIANGLE_IMPORT)
#    define QT_QTCOLORTRIANGLE_EXPORT
#  elif defined(QT_QTCOLORTRIANGLE_IMPORT)
#    if defined(QT_QTCOLORTRIANGLE_EXPORT)
#      undef QT_QTCOLORTRIANGLE_EXPORT
#    endif
#    define QT_QTCOLORTRIANGLE_EXPORT __declspec(dllimport)
#  elif defined(QT_QTCOLORTRIANGLE_EXPORT)
#    undef QT_QTCOLORTRIANGLE_EXPORT
#    define QT_QTCOLORTRIANGLE_EXPORT __declspec(dllexport)
#  endif
#else
#  define QT_QTCOLORTRIANGLE_EXPORT
#endif

class QT_QTCOLORTRIANGLE_EXPORT QtColorTriangle : public QWidget
{
    Q_OBJECT

public:
    QtColorTriangle(QWidget *parent = 0);
    ~QtColorTriangle();

    QSize sizeHint() const;
    int heightForWidth(int w) const;

    void polish();
    QColor color() const;

Q_SIGNALS:
    void colorChanged(const QColor &col);

public Q_SLOTS:
    void setColor(const QColor &col);

protected:
    void paintEvent(QPaintEvent *);
    void mouseMoveEvent(QMouseEvent *);
    void mousePressEvent(QMouseEvent *);
    void mouseReleaseEvent(QMouseEvent *);
    void keyPressEvent(QKeyEvent *e);
    void resizeEvent(QResizeEvent *);
    void drawTrigon(QImage *p, const QPointF &a, const QPointF &b,
		    const QPointF &c, const QColor &color);

private:
    double radiusAt(const QPointF &pos, const QRect &rect) const;
    double angleAt(const QPointF &pos, const QRect &rect) const;
    QPointF movePointToTriangle(double x, double y, const Vertex &a,
				    const Vertex &b, const Vertex &c) const;

    QPointF pointFromColor(const QColor &col) const;
    QColor colorFromPoint(const QPointF &p) const;

    void genBackground();

    QImage bg;
    double a, b, c;
    QPointF pa, pb, pc, pd;

    QColor curColor;
    int curHue;

    bool mustGenerateBackground;
    int penWidth;
    int ellipseSize;

    int outerRadius;
    QPointF selectorPos;

    enum SelectionMode {
	Idle,
	SelectingHue,
	SelectingSatValue
    } selMode;
};

#endif
 

#include "qtcolortriangle.h"
#include <QtCore/QEvent>
#include <QtCore/QMap>
#include <QtCore/QVarLengthArray>
#include <QConicalGradient>
#include <QFrame>
#include <QImage>
#include <QKeyEvent>
#include <QLayout>
#include <QMouseEvent>
#include <QPainter>
#include <QPainterPath>
#include <QPixmap>
#include <QResizeEvent>
#include <QToolTip>
#include <QVBoxLayout>

#include <math.h>

/*! \class QtColorTriangle

    \brief The QtColorTriangle class provides a triangular color
    selection widget.

    This widget uses the HSV color model, and is therefore useful for
    selecting colors by eye.

    The triangle in the center of the widget is used for selecting
    saturation and value, and the surrounding circle is used for
    selecting hue.

    Use setColor() and color() to set and get the current color.

    \img colortriangle.png
*/

/*! \fn QtColorTriangle::colorChanged(const QColor &color)

    Whenever the color triangles color changes this signal is emitted
    with the new \a color.
*/

const double PI = 3.14159265358979323846264338327950288419717;
const double TWOPI = 2.0*PI;

/*
    Used to store color values in the range 0..255 as doubles.
*/
struct DoubleColor
{
    double r, g, b;

    DoubleColor() : r(0.0), g(0.0), b(0.0) {}
    DoubleColor(double red, double green, double blue) : r(red), g(green), b(blue) {}
    DoubleColor(const DoubleColor &c) : r(c.r), g(c.g), b(c.b) {}
};

/*
    Used to store pairs of DoubleColor and DoublePoint in one structure.
*/
struct Vertex {
    DoubleColor color;
    QPointF point;

    Vertex(const DoubleColor &c, const QPointF &p) : color(c), point(p) {}
    Vertex(const QColor &c, const QPointF &p)
	: color(DoubleColor((double) c.red(), (double) c.green(),
			    (double) c.blue())), point(p) {}
};

/*! \internal

Swaps the Vertex at *a with the one at *b.
 */
static void swap(Vertex **a, Vertex **b)
{
    Vertex *tmp = *a;
    *a = *b;
    *b = tmp;
}

/*!
    Constructs a color triangle widget with the given \a parent.
*/
QtColorTriangle::QtColorTriangle(QWidget *parent)
    : QWidget(parent), bg(sizeHint(), QImage::Format_RGB32), selMode(Idle)
{
    setSizePolicy(QSizePolicy::Minimum, QSizePolicy::Minimum);
    setFocusPolicy(Qt::StrongFocus);

    mustGenerateBackground = true;

    QColor tmp;
    tmp.setHsv(76, 184, 206);
    setColor(tmp);
}

/*!
  Destructs the color triangle.
*/
QtColorTriangle::~QtColorTriangle()
{
}

/*!
    \internal

    Generates the first background image.
*/
void QtColorTriangle::polish()
{
    outerRadius = (contentsRect().width() - 1) / 2;
    if ((contentsRect().height() - 1) / 2 < outerRadius)
	outerRadius = (contentsRect().height() - 1) / 2;

    penWidth = (int) floor(outerRadius / 50.0);
    ellipseSize = (int) floor(outerRadius / 12.5);

    double cx = (double) contentsRect().center().x();
    double cy = (double) contentsRect().center().y();

    pa = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 5.0))),
		     cy - (sin(a) * (outerRadius - (outerRadius / 5.0))));
    pb = QPointF(cx + (cos(b) * (outerRadius - (outerRadius / 5.0))),
		     cy - (sin(b) * (outerRadius - (outerRadius / 5.0))));
    pc = QPointF(cx + (cos(c) * (outerRadius - (outerRadius / 5.0))),
		     cy - (sin(c) * (outerRadius - (outerRadius / 5.0))));
    pd = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 10.0))),
		     cy - (sin(a) * (outerRadius - (outerRadius / 10.0))));

    // Find the current position of the selector
    selectorPos = pointFromColor(curColor);

    update();
}

/*! \reimp
 */
QSize QtColorTriangle::sizeHint() const
{
    return QSize(100, 100);
}

/*!
    Forces the triangle widget to always be square. Returns the value
    \a w.
*/
int QtColorTriangle::heightForWidth(int w) const
{
    return w;
}

/*!
    \internal

    Generates a new static background image. This function is called
    initially, and in resizeEvent.
*/
void QtColorTriangle::genBackground()
{
    // Find the inner radius of the hue donut.
    double innerRadius = outerRadius - outerRadius / 5;

    // Create an image of the same size as the contents rect.
    bg = QImage(contentsRect().size(), QImage::Format_RGB32);
    QPainter p(&bg);
    p.setRenderHint(QPainter::Antialiasing);
    p.fillRect(bg.rect(), palette().mid());

    QConicalGradient gradient(bg.rect().center(), 90);
    QColor color;
    for (double i = 0; i <= 1.0; i += 0.1) {
#if QT_VERSION < 0x040100
        color.setHsv(int(i * 360.0), 255, 255);
#else
        color.setHsv(int(360.0 - (i * 360.0)), 255, 255);
#endif
        gradient.setColorAt(i, color);
    }

    QRectF innerRadiusRect(bg.rect().center().x() - innerRadius, bg.rect().center().y() - innerRadius,
                           innerRadius * 2 + 1, innerRadius * 2 + 1);
    QRectF outerRadiusRect(bg.rect().center().x() - outerRadius, bg.rect().center().y() - outerRadius,
                           outerRadius * 2 + 1, outerRadius * 2 + 1);
    QPainterPath path;
    path.addEllipse(innerRadiusRect);
    path.addEllipse(outerRadiusRect);

    p.save();
    p.setClipPath(path);
    p.fillRect(bg.rect(), gradient);
    p.restore();

    double penThickness = bg.width() / 400.0;
    for (int f = 0; f <= 5760; f += 20) {
        int value = int((0.5 + cos(((f - 1800) / 5760.0) * TWOPI) / 2) * 255.0);

        color.setHsv(int((f / 5760.0) * 360.0), 128 + (255 - value)/2, 255 - (255 - value)/4);
        p.setPen(QPen(color, penThickness));
        p.drawArc(innerRadiusRect, 1440 - f, 20);
        
        color.setHsv(int((f / 5760.0) * 360.0), 128 + value/2, 255 - value/4);
        p.setPen(QPen(color, penThickness));
        p.drawArc(outerRadiusRect, 2880 - 1440 - f, 20);
    }
    return;
}

/*!
    \internal

    Selects new hue or saturation/value values, depending on where the
    mouse button was pressed initially.
*/
void QtColorTriangle::mouseMoveEvent(QMouseEvent *e)
{
    if ((e->buttons() & Qt::LeftButton) == 0)
        return;

    QPointF depos((double) e->pos().x(), (double) e->pos().y());
    bool newColor = false;

    if (selMode == SelectingHue) {
	// If selecting hue, find the new angles for the points a,b,c
	// of the triangle. The following update() will then redraw
	// the triangle.
	a = angleAt(depos, contentsRect());
	b = a + TWOPI / 3.0;
	c = b + TWOPI / 3.0;
	if (b > TWOPI) b -= TWOPI;
	if (c > TWOPI) c -= TWOPI;

	double am = a - PI/2;
	if (am < 0) am += TWOPI;

	curHue = 360 - (int) (((am) * 360.0) / TWOPI);
	int h,s,v;
	curColor.getHsv(&h, &s, &v);

	if (curHue != h) {
	    newColor = true;
	    curColor.setHsv(curHue, s, v);
	}

	double cx = (double) contentsRect().center().x();
	double cy = (double) contentsRect().center().y();

	pa = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 5.0))),
                     cy - (sin(a) * (outerRadius - (outerRadius / 5.0))));
	pb = QPointF(cx + (cos(b) * (outerRadius - (outerRadius / 5.0))),
                     cy - (sin(b) * (outerRadius - (outerRadius / 5.0))));
	pc = QPointF(cx + (cos(c) * (outerRadius - (outerRadius / 5.0))),
                     cy - (sin(c) * (outerRadius - (outerRadius / 5.0))));
	pd = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 10.0))),
                     cy - (sin(a) * (outerRadius - (outerRadius / 10.0))));

	selectorPos = pointFromColor(curColor);
    } else {
	Vertex aa(Qt::black, pa);
	Vertex bb(Qt::black, pb);
	Vertex cc(Qt::black, pc);

	Vertex *p1 = &aa;
	Vertex *p2 = &bb;
	Vertex *p3 = &cc;
	if (p1->point.y() > p2->point.y()) swap(&p1, &p2);
	if (p1->point.y() > p3->point.y()) swap(&p1, &p3);
	if (p2->point.y() > p3->point.y()) swap(&p2, &p3);

	selectorPos = movePointToTriangle(depos.x(), depos.y(), aa, bb, cc);
	QColor col = colorFromPoint(selectorPos);
	if (col != curColor) {
	    // Ensure that hue does not change when selecting
	    // saturation and value.
	    int h,s,v;
	    col.getHsv(&h, &s, &v);
	    curColor.setHsv(curHue, s, v);
	    newColor = true;
	}
    }

    if (newColor)
	emit colorChanged(curColor);

    update();
}

/*!
    \internal

    When the left mouse button is pressed, this function determines
    what part of the color triangle the cursor is, and from that it
    initiates either selecting the hue (outside the triangle's area)
    or the saturation/value (inside the triangle's area).
*/
void QtColorTriangle::mousePressEvent(QMouseEvent *e)
{
    // Only respond to the left mouse button.
    if (e->button() != Qt::LeftButton)
	return;

    QPointF depos((double) e->pos().x(), (double) e->pos().y());
    double rad = radiusAt(depos, contentsRect());
    bool newColor = false;

    // As in mouseMoveEvent, either find the a,b,c angles or the
    // radian position of the selector, then order an update.
    if (rad > (outerRadius - (outerRadius / 5))) {
	selMode = SelectingHue;

	a = angleAt(depos, contentsRect());
	b = a + TWOPI / 3.0;
	c = b + TWOPI / 3.0;
	if (b > TWOPI) b -= TWOPI;
	if (c > TWOPI) c -= TWOPI;

	double am = a - PI/2;
	if (am < 0) am += TWOPI;

	curHue = 360 - (int) ((am * 360.0) / TWOPI);
	int h,s,v;
	curColor.getHsv(&h, &s, &v);

	if (h != curHue) {
	    newColor = true;
	    curColor.setHsv(curHue, s, v);
	}

	double cx = (double) contentsRect().center().x();
	double cy = (double) contentsRect().center().y();

	pa = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 5.0))),
			 cy - (sin(a) * (outerRadius - (outerRadius / 5.0))));
	pb = QPointF(cx + (cos(b) * (outerRadius - (outerRadius / 5.0))),
			 cy - (sin(b) * (outerRadius - (outerRadius / 5.0))));
	pc = QPointF(cx + (cos(c) * (outerRadius - (outerRadius / 5.0))),
			 cy - (sin(c) * (outerRadius - (outerRadius / 5.0))));
	pd = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 10.0))),
			 cy - (sin(a) * (outerRadius - (outerRadius / 10.0))));

	selectorPos = pointFromColor(curColor);
	emit colorChanged(curColor);
    } else {
	selMode = SelectingSatValue;

	Vertex aa(Qt::black, pa);
	Vertex bb(Qt::black, pb);
	Vertex cc(Qt::black, pc);

	Vertex *p1 = &aa;
	Vertex *p2 = &bb;
	Vertex *p3 = &cc;
	if (p1->point.y() > p2->point.y()) swap(&p1, &p2);
	if (p1->point.y() > p3->point.y()) swap(&p1, &p3);
	if (p2->point.y() > p3->point.y()) swap(&p2, &p3);

	selectorPos = movePointToTriangle(depos.x(), depos.y(), aa, bb, cc);
	QColor col = colorFromPoint(selectorPos);
	if (col != curColor) {
	    curColor = col;
	    newColor = true;
	}
    }

    if (newColor)
	emit colorChanged(curColor);

    update();
}

/*!
    \internal

    Stops selecting of colors with the mouse.
*/
void QtColorTriangle::mouseReleaseEvent(QMouseEvent *e)
{
    if (e->button() == Qt::LeftButton)
	selMode = Idle;
}

/*!
    \internal
*/
void QtColorTriangle::keyPressEvent(QKeyEvent *e)
{
    switch (e->key()) {
	case Qt::Key_Left: {
	    --curHue;
	    if (curHue < 0) curHue += 360;
	    int h,s,v;
	    curColor.getHsv(&h, &s, &v);
	    QColor tmp;
	    tmp.setHsv(curHue, s, v);
	    setColor(tmp);
	}
	    break;
	case Qt::Key_Right: {
	    ++curHue;
	    if (curHue > 359) curHue -= 360;
	    int h,s,v;
	    curColor.getHsv(&h, &s, &v);
	    QColor tmp;
	    tmp.setHsv(curHue, s, v);
	    setColor(tmp);
	}
	    break;
	case Qt::Key_Up: {
	    int h,s,v;
	    curColor.getHsv(&h, &s, &v);
	    QColor tmp;
	    if (e->modifiers() & Qt::ShiftModifier) {
		if (s > 5) s -= 5;
		else s = 0;
	    } else {
		if (v > 5) v -= 5;
		else v = 0;
	    }
	    tmp.setHsv(curHue, s, v);
	    setColor(tmp);
	}
	    break;
	case Qt::Key_Down: {
	    int h,s,v;
	    curColor.getHsv(&h, &s, &v);
	    QColor tmp;
	    if (e->modifiers() & Qt::ShiftModifier) {
		if (s < 250) s += 5;
		else s = 255;
	    } else {
		if (v < 250) v += 5;
		else v = 255;
	    }
	    tmp.setHsv(curHue, s, v);
	    setColor(tmp);
	}
	    break;
    };
}

/*!
    \internal

    Regenerates the background image and sends an update.
*/
void QtColorTriangle::resizeEvent(QResizeEvent *)
{
    outerRadius = (contentsRect().width() - 1) / 2;
    if ((contentsRect().height() - 1) / 2 < outerRadius)
	outerRadius = (contentsRect().height() - 1) / 2;

    penWidth = (int) floor(outerRadius / 50.0);
    ellipseSize = (int) floor(outerRadius / 12.5);

    double cx = (double) contentsRect().center().x();
    double cy = (double) contentsRect().center().y();

    pa = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 5.0))),
                 cy - (sin(a) * (outerRadius - (outerRadius / 5.0))));
    pb = QPointF(cx + (cos(b) * (outerRadius - (outerRadius / 5.0))),
                 cy - (sin(b) * (outerRadius - (outerRadius / 5.0))));
    pc = QPointF(cx + (cos(c) * (outerRadius - (outerRadius / 5.0))),
                 cy - (sin(c) * (outerRadius - (outerRadius / 5.0))));
    pd = QPointF(cx + (cos(a) * (outerRadius - (outerRadius / 10.0))),
                 cy - (sin(a) * (outerRadius - (outerRadius / 10.0))));

    // Find the current position of the selector
    selectorPos = pointFromColor(curColor);

    mustGenerateBackground = true;
    update();
}

/*! \reimp

First copies a background image of the hue donut and its
    background color onto the frame, then draws the color triangle,
    and finally the selectors.
*/
void QtColorTriangle::paintEvent(QPaintEvent *e)
{
    QPainter p(this);
    if (e->rect().intersects(contentsRect()))
        p.setClipRegion(e->region().intersected(contentsRect()));
    if (mustGenerateBackground) {
	genBackground();
	mustGenerateBackground = false;
    }

    // Blit the static generated background with the hue gradient onto
    // the double buffer.
    QImage buf = bg.copy();

    // Draw the trigon
    int h,s,v;
    curColor.getHsv(&h, &s, &v);

    // Find the color with only the hue, and max value and saturation
    QColor hueColor;
    hueColor.setHsv(curHue, 255, 255);

    // Draw the triangle
    drawTrigon(&buf, pa, pb, pc, hueColor);

    // Slow step: convert the image to a pixmap
    QPixmap pix = QPixmap::fromImage(buf);
    QPainter painter(&pix);
    painter.setRenderHint(QPainter::Antialiasing);

    // Draw an outline of the triangle
    QColor halfAlpha(0, 0, 0, 128);
    painter.setPen(QPen(halfAlpha, 0));
    painter.drawLine(pa, pb);
    painter.drawLine(pb, pc);
    painter.drawLine(pc, pa);
    
    int ri, gi, bi;
    hueColor.getRgb(&ri, &gi, &bi);
    if ((ri * 30) + (gi * 59) + (bi * 11) > 12800)
	painter.setPen(QPen(Qt::black, penWidth));
    else
	painter.setPen(QPen(Qt::white, penWidth));
    painter.drawEllipse((int) (pd.x() - ellipseSize / 2.0),
			(int) (pd.y() - ellipseSize / 2.0),
			ellipseSize, ellipseSize);

    curColor.getRgb(&ri, &gi, &bi);

    // Find a color for painting the selector based on the brightness
    // value of the color.
    if ((ri * 30) + (gi * 59) + (bi * 11) > 12800)
	painter.setPen(QPen(Qt::black, penWidth));
    else
	painter.setPen(QPen(Qt::white, penWidth));

    // Draw the selector ellipse.
    painter.drawEllipse(QRectF(selectorPos.x() - ellipseSize / 2.0,
                               selectorPos.y() - ellipseSize / 2.0,
                               ellipseSize + 0.5, ellipseSize + 0.5));

    // Blit
    p.drawPixmap(contentsRect().topLeft(), pix);
}

/*! \internal

Draws a trigon (polygon with three corners \a pa, \a pb and \a pc
    and three edges), using \a painter.

    Fills the trigon with a gradient, where the \a pa point has the
    color \a color, \a pb is black and \a bc is white. Bilinear
    gradient.
*/
void QtColorTriangle::drawTrigon(QImage *buf, const QPointF &pa,
			       const QPointF &pb, const QPointF &pc,
			       const QColor &color)
{
    // Create three Vertex objects. A Vertex contains a double-point
    // coordinate and a color.
    // pa is the tip of the arrow
    // pb is the black corner
    // pc is the white corner
    Vertex aa(color, pa);
    Vertex bb(Qt::black, pb);
    Vertex cc(Qt::white, pc);

    // Sort. Make p1 above p2, which is above p3 (using y coordinate).
    // Bubble sorting is fastest here.
    Vertex *p1 = &aa;
    Vertex *p2 = &bb;
    Vertex *p3 = &cc;
    if (p1->point.y() > p2->point.y()) swap(&p1, &p2);
    if (p1->point.y() > p3->point.y()) swap(&p1, &p3);
    if (p2->point.y() > p3->point.y()) swap(&p2, &p3);

    // All the three y deltas are >= 0
    double p1p2ydist = p2->point.y() - p1->point.y();
    double p1p3ydist = p3->point.y() - p1->point.y();
    double p2p3ydist = p3->point.y() - p2->point.y();
    double p1p2xdist = p2->point.x() - p1->point.x();
    double p1p3xdist = p3->point.x() - p1->point.x();
    double p2p3xdist = p3->point.x() - p2->point.x();

    // The first x delta decides wether we have a lefty or a righty
    // trigon.
    bool lefty = p1p2xdist < 0;

    // Left and right colors and X values. The key in this map is the
    // y values. Our goal is to fill these structures with all the
    // information needed to do a single pass top-to-bottom,
    // left-to-right drawing of the trigon.
    QVarLengthArray<DoubleColor, 2000> leftColors;
    QVarLengthArray<DoubleColor, 2000> rightColors;
    QVarLengthArray<double, 2000> leftX; 
    QVarLengthArray<double, 2000> rightX; 

    leftColors.resize(int(floor(p3->point.y() + 1)));
    rightColors.resize(int(floor(p3->point.y() + 1)));
    leftX.resize(int(floor(p3->point.y() + 1)));
    rightX.resize(int(floor(p3->point.y() + 1)));

     // Scan longy - find all left and right colors and X-values for
    // the tallest edge (p1-p3).
    DoubleColor source;
    DoubleColor dest;
    double r, g, b;
    double rdelta, gdelta, bdelta;
    double x;
    double xdelta;
    int y1, y2;

    // Initialize with known values
    x = p1->point.x();
    source = p1->color;
    dest = p3->color;
    r = source.r;
    g = source.g;
    b = source.b;
    y1 = (int) floor(p1->point.y());
    y2 = (int) floor(p3->point.y());

    // Find slopes (notice that if the y dists are 0, we don't care
    // about the slopes)
    xdelta = p1p3ydist == 0.0 ? 0.0 : p1p3xdist / p1p3ydist;
    rdelta = p1p3ydist == 0.0 ? 0.0 : (dest.r - r) / p1p3ydist;
    gdelta = p1p3ydist == 0.0 ? 0.0 : (dest.g - g) / p1p3ydist;
    bdelta = p1p3ydist == 0.0 ? 0.0 : (dest.b - b) / p1p3ydist;

    // Calculate gradients using linear approximation
    int y;
    for (y = y1; y < y2; ++y) {
	if (lefty) {
	    rightColors[y] = DoubleColor(r, g, b);
	    rightX[y] = x;
	} else {
	    leftColors[y] = DoubleColor(r, g, b);
	    leftX[y] = x;
	}

	r += rdelta;
	g += gdelta;
	b += bdelta;
	x += xdelta;
    }

    // Scan top shorty - find all left and right colors and x-values
    // for the topmost of the two not-tallest short edges.
    x = p1->point.x();
    source = p1->color;
    dest = p2->color;
    r = source.r;
    g = source.g;
    b = source.b;
    y1 = (int) floor(p1->point.y());
    y2 = (int) floor(p2->point.y());

    // Find slopes (notice that if the y dists are 0, we don't care
    // about the slopes)
    xdelta = p1p2ydist == 0.0 ? 0.0 : p1p2xdist / p1p2ydist;
    rdelta = p1p2ydist == 0.0 ? 0.0 : (dest.r - r) / p1p2ydist;
    gdelta = p1p2ydist == 0.0 ? 0.0 : (dest.g - g) / p1p2ydist;
    bdelta = p1p2ydist == 0.0 ? 0.0 : (dest.b - b) / p1p2ydist;

    // Calculate gradients using linear approximation
    for (y = y1; y < y2; ++y) {
	if (lefty) {
	    leftColors[y] = DoubleColor(r, g, b);
	    leftX[y] = x;
	} else {
	    rightColors[y] = DoubleColor(r, g, b);
	    rightX[y] = x;
	}

	r += rdelta;
	g += gdelta;
	b += bdelta;
	x += xdelta;
    }

    // Scan bottom shorty - find all left and right colors and
    // x-values for the bottommost of the two not-tallest short edges.
    x = p2->point.x();
    source = p2->color;
    dest = p3->color;
    r = source.r;
    g = source.g;
    b = source.b;
    y1 = (int) floor(p2->point.y());
    y2 = (int) floor(p3->point.y());

    // Find slopes (notice that if the y dists are 0, we don't care
    // about the slopes)
    xdelta = p2p3ydist == 0.0 ? 0.0 : p2p3xdist / p2p3ydist;
    rdelta = p2p3ydist == 0.0 ? 0.0 : (dest.r - r) / p2p3ydist;
    gdelta = p2p3ydist == 0.0 ? 0.0 : (dest.g - g) / p2p3ydist;
    bdelta = p2p3ydist == 0.0 ? 0.0 : (dest.b - b) / p2p3ydist;

    // Calculate gradients using linear approximation
    for (y = y1; y < y2; ++y) {
	if (lefty) {
	    leftColors[y] = DoubleColor(r, g, b);
	    leftX[y] = x;
	} else {
	    rightColors[y] = DoubleColor(r, g, b);
	    rightX[y] = x;
	}

	r += rdelta;
	g += gdelta;
	b += bdelta;
	x += xdelta;
    }

    // Inner loop. For each y in the left map of x-values, draw one
    // line from left to right.
    const int p3yfloor = int(floor(p3->point.y()));
    for (int y = int(floor(p1->point.y())); y < p3yfloor; ++y) {
	double lx = leftX[y];
	double rx = rightX[y];

	int lxi = (int) floor(lx);
	int rxi = (int) floor(rx);
	DoubleColor rc = rightColors[y];
	DoubleColor lc = leftColors[y];

        // if the xdist is 0, don't draw anything.
	double xdist = rx - lx;
	if (xdist != 0.0) {
            double r = lc.r;
            double g = lc.g;
            double b = lc.b;
            double rdelta = (rc.r - r) / xdist;
            double gdelta = (rc.g - g) / xdist;
            double bdelta = (rc.b - b) / xdist;

            QRgb *scanline = reinterpret_cast<QRgb *>(buf->scanLine(y));
            scanline += lxi;

            // Inner loop 2. Draws the line from left to right.
            for (int i = lxi; i < rxi; ++i) {
                *scanline++ = qRgb((int) r, (int) g, (int) b);
                r += rdelta;
                g += gdelta;
                b += bdelta;
            }
        }
    }
}

/*! \internal

Sets the color of the triangle to \a col.
 */
void QtColorTriangle::setColor(const QColor &col)
{
    if (col == curColor)
	return;

    curColor = col;

    int h, s, v;
    curColor.getHsv(&h, &s, &v);

    // Never use an invalid hue to display colors
    if (h != -1)
	curHue = h;

    a = (((360 - curHue) * TWOPI) / 360.0);
    a += PI / 2.0;
    if (a > TWOPI) a -= TWOPI;

    b = a + TWOPI/3;
    c = b + TWOPI/3;

    if (b > TWOPI) b -= TWOPI;
    if (c > TWOPI) c -= TWOPI;

    double cx = (double) contentsRect().center().x();
    double cy = (double) contentsRect().center().y();
    double innerRadius = outerRadius - (outerRadius / 5.0);
    double pointerRadius = outerRadius - (outerRadius / 10.0);

    pa = QPointF(cx + (cos(a) * innerRadius), cy - (sin(a) * innerRadius));
    pb = QPointF(cx + (cos(b) * innerRadius), cy - (sin(b) * innerRadius));
    pc = QPointF(cx + (cos(c) * innerRadius), cy - (sin(c) * innerRadius));
    pd = QPointF(cx + (cos(a) * pointerRadius), cy - (sin(a) * pointerRadius));

    selectorPos = pointFromColor(curColor);
    update();

    emit colorChanged(curColor);
}

/*! \internal

Returns the current color of the triangle.
 */
QColor QtColorTriangle::color() const
{
    return curColor;
}

/*!
    \internal

  Returns the distance from \a pos to the center of \a rect.
*/
double QtColorTriangle::radiusAt(const QPointF &pos, const QRect &rect) const
{
    double mousexdist = pos.x() - (double) rect.center().x();
    double mouseydist = pos.y() - (double) rect.center().y();
    return sqrt(mousexdist * mousexdist + mouseydist * mouseydist);
}

/*!
    \internal

  With origin set to the center of \a rect, this function returns
    the angle in radians between the line that starts at (0,0) and
    ends at (1,0) and the line that stars at (0,0) and ends at \a pos.
*/
double QtColorTriangle::angleAt(const QPointF &pos, const QRect &rect) const
{
    double mousexdist = pos.x() - (double) rect.center().x();
    double mouseydist = pos.y() - (double) rect.center().y();
    double mouserad = sqrt(mousexdist * mousexdist + mouseydist * mouseydist);
    if (mouserad == 0.0)
        return 0.0;

    double angle = acos(mousexdist / mouserad);
    if (mouseydist >= 0)
	angle = TWOPI - angle;

    return angle;
}

/*! \internal

Returns a * a.
 */
inline double qsqr(double a)
{
    return a * a;
}

/*! \internal

Returns the length of the vector (x,y).
 */
inline double vlen(double x, double y)
{
    return sqrt(qsqr(x) + qsqr(y));
}

/*! \internal

Returns the vector product of (x1,y1) and (x2,y2).
 */
inline double vprod(double x1, double y1, double x2, double y2)
{
    return x1 * x2 + y1 * y2;
}

/*! \internal

Returns true if the point cos(p),sin(p) is on the arc between
    cos(a1),sin(a1) and cos(a2),sin(a2); otherwise returns false.
*/
bool angleBetweenAngles(double p, double a1, double a2)
{
    if (a1 > a2) {
	a2 += TWOPI;
	if (p < PI) p += TWOPI;
    }

    return p >= a1 && p < a2;
}

/*! \internal

    A line from a to b is one of several lines in an equilateral
    polygon, and they are drawn counter clockwise. This line therefore
    has one side facing in and one facing out of the polygon. This
    function determines wether (x,y) is on the inside or outside of
    the given line, defined by the "from" coordinate (ax,ay) and the
    "to" coordinate (bx,by).

    The point (px,py) is the intersection between the a-b line and the
    perpendicular projection of (x,y) onto that line.

    Returns true if (x,y) is above the line; otherwise returns false.

    If ax and bx are equal and ay and by are equal (line is a point),
    this function will return true if (x,y) is equal to this point.
*/
static bool pointAbovePoint(double x, double y, double px, double py,
			    double ax, double ay, double bx, double by)
{
    bool result = false;

    if (floor(ax) > floor(bx)) {
	if (floor(ay) < floor(by)) {
	    // line is draw upright-to-downleft
	    if (floor(x) < floor(px) || floor(y) < floor(py))
		result = true;
	} else if (floor(ay) > floor(by)) {
            // line is draw downright-to-upleft
	    if (floor(x) > floor(px) || floor(y) < floor(py))
		result = true;
	} else {
	    // line is flat horizontal
	    if (y < ay) result = true;
	}
    } else if (floor(ax) < floor(bx)) {
	if (floor(ay) < floor(by)) {
	    // line is draw upleft-to-downright
	    if (floor(x) < floor(px) || floor(y) > floor(py))
		result = true;
	} else if (floor(ay) > floor(by)) {
	    // line is draw downleft-to-upright
	    if (floor(x) > floor(px) || floor(y) > floor(py))
		result = true;
	} else {
	    // line is flat horizontal
	    if (y > ay)
                result = true;
	}
    } else {
	// line is vertical
	if (floor(ay) < floor(by)) {
	    if (x < ax) result = true;
	} else if (floor(ay) > floor(by)) {
	    if (x > ax) result = true;
	} else {
	    if (!(x == ax && y == ay))
		result = true;
	}
    }

    return result;
}

/*! \internal

if (ax,ay) to (bx,by) describes a line, and (x,y) is a point on
    that line, returns -1 if (x,y) is outside the (ax,ay) bounds, 1 if
    it is outside the (bx,by) bounds and 0 if (x,y) is within (ax,ay)
    and (bx,by).
*/
static int pointInLine(double x, double y, double ax, double ay,
		       double bx, double by)
{
    if (ax > bx) {
	if (ay < by) {
	    // line is draw upright-to-downleft

	    // if (x,y) is in on or above the upper right point,
	    // return -1.
	    if (y <= ay && x >= ax)
		return -1;

	    // if (x,y) is in on or below the lower left point,
	    // return 1.
	    if (y >= by && x <= bx)
		return 1;
	} else {
	    // line is draw downright-to-upleft

	    // If the line is flat, only use the x coordinate.
	    if (floor(ay) == floor(by)) {
		// if (x is to the right of the rightmost point,
		// return -1. otherwise if x is to the left of the
		// leftmost point, return 1.
		if (x >= ax)
		    return -1;
		else if (x <= bx)
		    return 1;
	    } else {
		// if (x,y) is on or below the lower right point,
		// return -1.
		if (y >= ay && x >= ax)
		    return -1;

		// if (x,y) is on or above the upper left point,
		// return 1.
		if (y <= by && x <= bx)
		    return 1;
	    }
	}
    } else {
	if (ay < by) {
	    // line is draw upleft-to-downright

            // If (x,y) is on or above the upper left point, return
	    // -1.
	    if (y <= ay && x <= ax)
		return -1;

	    // If (x,y) is on or below the lower right point, return
	    // 1.
	    if (y >= by && x >= bx)
		return 1;
	} else {
	    // line is draw downleft-to-upright

	    // If the line is flat, only use the x coordinate.
	    if (floor(ay) == floor(by)) {
		if (x <= ax)
		    return -1;
		else if (x >= bx)
		    return 1;
	    } else {
		// If (x,y) is on or below the lower left point, return
		// -1.
		if (y >= ay && x <= ax)
		    return -1;

		// If (x,y) is on or above the upper right point, return
		// 1.
		if (y <= by && x >= bx)
		    return 1;
	    }
	}
    }

    // No tests proved that (x,y) was outside [(ax,ay),(bx,by)], so we
    // assume it's inside the line's bounds.
    return 0;
}

/*! \internal

    \a a, \a b and \a c are corner points of an equilateral triangle.
    (\a x,\a y) is an arbitrary point inside or outside this triangle.

    If (x,y) is inside the triangle, this function returns the double
    point (x,y).

    Otherwise, the intersection of the perpendicular projection of
    (x,y) onto the closest triangle edge is returned, unless this
    intersection is outside the triangle's bounds, in which case the
    corner closest to the intersection is returned instead.

    Yes, it's trigonometry.
*/
QPointF QtColorTriangle::movePointToTriangle(double x, double y, const Vertex &a,
					       const Vertex &b, const Vertex &c) const
{
    // Let v1A be the vector from (x,y) to a.
    // Let v2A be the vector from a to b.
    // Find the angle alphaA between v1A and v2A.
    double v1xA = x - a.point.x();
    double v1yA = y - a.point.y();
    double v2xA = b.point.x() - a.point.x();
    double v2yA = b.point.y() - a.point.y();
    double vpA = vprod(v1xA, v1yA, v2xA, v2yA);
    double cosA = vpA / (vlen(v1xA, v1yA) * vlen(v2xA, v2yA));
    double alphaA = acos(cosA);

    // Let v1B be the vector from x to b.
    // Let v2B be the vector from b to c.
    double v1xB = x - b.point.x();
    double v1yB = y - b.point.y();
    double v2xB = c.point.x() - b.point.x();
    double v2yB = c.point.y() - b.point.y();
    double vpB = vprod(v1xB, v1yB, v2xB, v2yB);
    double cosB = vpB / (vlen(v1xB, v1yB) * vlen(v2xB, v2yB));
    double alphaB = acos(cosB);

    // Let v1C be the vector from x to c.
    // Let v2C be the vector from c back to a.
    double v1xC = x - c.point.x();
    double v1yC = y - c.point.y();
    double v2xC = a.point.x() - c.point.x();
    double v2yC = a.point.y() - c.point.y();
    double vpC = vprod(v1xC, v1yC, v2xC, v2yC);
    double cosC = vpC / (vlen(v1xC, v1yC) * vlen(v2xC, v2yC));
    double alphaC = acos(cosC);

    // Find the radian angles between the (1,0) vector and the points
    // A, B, C and (x,y). Use this information to determine which of
    // the edges we should project (x,y) onto.
    double angleA = angleAt(a.point, contentsRect());
    double angleB = angleAt(b.point, contentsRect());
    double angleC = angleAt(c.point, contentsRect());
    double angleP = angleAt(QPointF(x, y), contentsRect());

    // If (x,y) is in the a-b area, project onto the a-b vector.
    if (angleBetweenAngles(angleP, angleA, angleB)) {
	// Find the distance from (x,y) to a. Then use the slope of
	// the a-b vector with this distance and the angle between a-b
	// and a-(x,y) to determine the point of intersection of the
	// perpendicular projection from (x,y) onto a-b.
	double pdist = sqrt(qsqr(x - a.point.x()) + qsqr(y - a.point.y()));

        // the length of all edges is always > 0
	double p0x = a.point.x() + ((b.point.x() - a.point.x()) / vlen(v2xB, v2yB)) * cos(alphaA) * pdist;
	double p0y = a.point.y() + ((b.point.y() - a.point.y()) / vlen(v2xB, v2yB)) * cos(alphaA) * pdist;

	// If (x,y) is above the a-b line, which basically means it's
	// outside the triangle, then return its projection onto a-b.
	if (pointAbovePoint(x, y, p0x, p0y, a.point.x(), a.point.y(), b.point.x(), b.point.y())) {
	    // If the projection is "outside" a, return a. If it is
	    // outside b, return b. Otherwise return the projection.
	    int n = pointInLine(p0x, p0y, a.point.x(), a.point.y(), b.point.x(), b.point.y());
	    if (n < 0)
		return a.point;
	    else if (n > 0)
		return b.point;

	    return QPointF(p0x, p0y);
	}
    } else if (angleBetweenAngles(angleP, angleB, angleC)) {
	// If (x,y) is in the b-c area, project onto the b-c vector.
	double pdist = sqrt(qsqr(x - b.point.x()) + qsqr(y - b.point.y()));

        // the length of all edges is always > 0
        double p0x = b.point.x() + ((c.point.x() - b.point.x()) / vlen(v2xC, v2yC)) * cos(alphaB) * pdist;
	double p0y = b.point.y() + ((c.point.y() - b.point.y()) / vlen(v2xC, v2yC)) * cos(alphaB) * pdist;

	if (pointAbovePoint(x, y, p0x, p0y, b.point.x(), b.point.y(), c.point.x(), c.point.y())) {
	    int n = pointInLine(p0x, p0y, b.point.x(), b.point.y(), c.point.x(), c.point.y());
	    if (n < 0)
		return b.point;
	    else if (n > 0)
		return c.point;
	    return QPointF(p0x, p0y);
	}
    } else if (angleBetweenAngles(angleP, angleC, angleA)) {
	// If (x,y) is in the c-a area, project onto the c-a vector.
	double pdist = sqrt(qsqr(x - c.point.x()) + qsqr(y - c.point.y()));

        // the length of all edges is always > 0
        double p0x = c.point.x() + ((a.point.x() - c.point.x()) / vlen(v2xA, v2yA)) * cos(alphaC) * pdist;
	double p0y = c.point.y() + ((a.point.y() - c.point.y()) / vlen(v2xA, v2yA)) * cos(alphaC) * pdist;

	if (pointAbovePoint(x, y, p0x, p0y, c.point.x(), c.point.y(), a.point.x(), a.point.y())) {
	    int n = pointInLine(p0x, p0y, c.point.x(), c.point.y(), a.point.x(), a.point.y());
	    if (n < 0)
		return c.point;
	    else if (n > 0)
		return a.point;
	    return QPointF(p0x, p0y);
	}
    }

    // (x,y) is inside the triangle (inside a-b, b-c and a-c).
    return QPointF(x, y);
}

/*! \internal

    Given the color \a col, this function determines the point in the
    equilateral triangle defined with (pa, pb, pc) that displays this
    color. The function assumes the color at pa has a hue equal to the
    hue of \a col, and that pb is black and pc is white.

    In this certain type of triangle, we observe that saturation grows
    from the black-color edge towards the black-white edge. The value
    grows from the black corner towards the white-color edge. Using
    the intersection of the saturation and value points on the three
    edges, we are able to determine the point with the same saturation
    and value as \a col.
*/
QPointF QtColorTriangle::pointFromColor(const QColor &col) const
{
    // Simplifications for the corner cases.
    if (col == Qt::black)
	return pb;
    else if (col == Qt::white)
	return pc;

    // Find the x and y slopes
    double ab_deltax = pb.x() - pa.x();
    double ab_deltay = pb.y() - pa.y();
    double bc_deltax = pc.x() - pb.x();
    double bc_deltay = pc.y() - pb.y();
    double ac_deltax = pc.x() - pa.x();
    double ac_deltay = pc.y() - pa.y();

    // Extract the h,s,v values of col.
    int hue,sat,val;
    col.getHsv(&hue, &sat, &val);

    // Find the line that passes through the triangle where the value
    // is equal to our color's value.
    double p1 = pa.x() + (ab_deltax * (double) (255 - val)) / 255.0;
    double q1 = pa.y() + (ab_deltay * (double) (255 - val)) / 255.0;
    double p2 = pb.x() + (bc_deltax * (double) val) / 255.0;
    double q2 = pb.y() + (bc_deltay * (double) val) / 255.0;

    // Find the line that passes through the triangle where the
    // saturation is equal to our color's value.
    double p3 = pa.x() + (ac_deltax * (double) (255 - sat)) / 255.0;
    double q3 = pa.y() + (ac_deltay * (double) (255 - sat)) / 255.0;
    double p4 = pb.x();
    double q4 = pb.y();

    // Find the intersection between these lines.
	double x = 0;
	double y = 0;
	if (p1 != p2) {
		double a = (q2 - q1) / (p2 - p1);
		double c = (q4 - q3) / (p4 - p3);
		double b = q1 - a * p1;
		double d = q3 - c * p3;

		x = (d - b) / (a - c);
		y = a * x + b;
	}
	else {
		x = p1;
		y = q3 + (x - p3) * (q4 - q3) / (p4 - p3);
	}
	
    return QPointF(x, y);
}

/*! \internal

    Determines the color in the color triangle at the point \a p. Uses
    linear interpolation to find the colors to the left and right of
    \a p, then uses the same technique to find the color at \a p using
    these two colors.
*/
QColor QtColorTriangle::colorFromPoint(const QPointF &p) const
{
    // Find the outer radius of the hue gradient.
    int outerRadius = (contentsRect().width() - 1) / 2;
    if ((contentsRect().height() - 1) / 2 < outerRadius)
	outerRadius = (contentsRect().height() - 1) / 2;

    // Find the center coordinates
    double cx = (double) contentsRect().center().x();
    double cy = (double) contentsRect().center().y();

    // Find the a, b and c from their angles, the center of the rect
    // and the radius of the hue gradient donut.
    QPointF pa(cx + (cos(a) * (outerRadius - (outerRadius / 5.0))),
		   cy - (sin(a) * (outerRadius - (outerRadius / 5.0))));
    QPointF pb(cx + (cos(b) * (outerRadius - (outerRadius / 5.0))),
		   cy - (sin(b) * (outerRadius - (outerRadius / 5.0))));
    QPointF pc(cx + (cos(c) * (outerRadius - (outerRadius / 5.0))),
		   cy - (sin(c) * (outerRadius - (outerRadius / 5.0))));

    // Find the hue value from the angle of the 'a' point.
    double angle = a - PI/2.0;
    if (angle < 0) angle += TWOPI;
    double hue = (360.0 * angle) / TWOPI;

    // Create the color of the 'a' corner point. We know that b is
    // black and c is white.
    QColor color;
    color.setHsv(360 - (int) floor(hue), 255, 255);

    // See also drawTrigon(), which basically does exactly the same to
    // determine all colors in the trigon.
    Vertex aa(color, pa);
    Vertex bb(Qt::black, pb);
    Vertex cc(Qt::white, pc);

    // Make sure p1 is above p2, which is above p3.
    Vertex *p1 = &aa;
    Vertex *p2 = &bb;
    Vertex *p3 = &cc;
    if (p1->point.y() > p2->point.y()) swap(&p1, &p2);
    if (p1->point.y() > p3->point.y()) swap(&p1, &p3);
    if (p2->point.y() > p3->point.y()) swap(&p2, &p3);

    // Find the slopes of all edges in the trigon. All the three y
    // deltas here are positive because of the above sorting.
    double p1p2ydist = p2->point.y() - p1->point.y();
    double p1p3ydist = p3->point.y() - p1->point.y();
    double p2p3ydist = p3->point.y() - p2->point.y();
    double p1p2xdist = p2->point.x() - p1->point.x();
    double p1p3xdist = p3->point.x() - p1->point.x();
    double p2p3xdist = p3->point.x() - p2->point.x();

    // The first x delta decides wether we have a lefty or a righty
    // trigon. A lefty trigon has its tallest edge on the right hand
    // side of the trigon. The righty trigon has it on its left side.
    // This property determines wether the left or the right set of x
    // coordinates will be continuous.
    bool lefty = p1p2xdist < 0;

    // Find whether the selector's y is in the first or second shorty,
    // counting from the top and downwards. This is used to find the
    // color at the selector point.
    bool firstshorty = (p.y() >= p1->point.y() && p.y() < p2->point.y());

    // From the y value of the selector's position, find the left and
    // right x values.
    double leftx;
    double rightx;
    if (lefty) {
        if (firstshorty) {
            leftx = p1->point.x();
            if (floor(p1p2ydist) != 0.0) {
                leftx += (p1p2xdist * (p.y() - p1->point.y())) / p1p2ydist;
            } else {
                leftx = qMin(p1->point.x(), p2->point.x());
            }
        } else {
            leftx = p2->point.x();
            if (floor(p2p3ydist) != 0.0) {
                leftx += (p2p3xdist * (p.y() - p2->point.y())) / p2p3ydist;
            } else {
                leftx = qMin(p2->point.x(), p3->point.x());
            }
        }

        rightx = p1->point.x();
        rightx += (p1p3xdist * (p.y() - p1->point.y())) / p1p3ydist;
    } else {
        leftx = p1->point.x();
        leftx += (p1p3xdist * (p.y() - p1->point.y())) / p1p3ydist;

        if (firstshorty) {
            rightx = p1->point.x();
            if (floor(p1p2ydist) != 0.0) {
                rightx += (p1p2xdist * (p.y() - p1->point.y())) / p1p2ydist;
            } else {
                rightx = qMax(p1->point.x(), p2->point.x());
            }
        } else {
            rightx = p2->point.x(); 
            if (floor(p2p3ydist) != 0.0) {
                rightx += (p2p3xdist * (p.y() - p2->point.y())) / p2p3ydist;
            } else {
                rightx = qMax(p2->point.x(), p3->point.x());
            }
        }
    }

    // Find the r,g,b values of the points on the trigon's edges that
    // are to the left and right of the selector.
    double rshort = 0, gshort = 0, bshort = 0;
    double rlong = 0, glong = 0, blong = 0;
    if (firstshorty) {
        if (floor(p1p2ydist) != 0.0) {
            rshort  = p2->color.r * (p.y() - p1->point.y()) / p1p2ydist;
            gshort  = p2->color.g * (p.y() - p1->point.y()) / p1p2ydist;
            bshort  = p2->color.b * (p.y() - p1->point.y()) / p1p2ydist;
            rshort += p1->color.r * (p2->point.y() - p.y()) / p1p2ydist;
            gshort += p1->color.g * (p2->point.y() - p.y()) / p1p2ydist;
            bshort += p1->color.b * (p2->point.y() - p.y()) / p1p2ydist;
        } else {
            if (lefty) {
                if (p1->point.x() <= p2->point.x()) {
                    rshort  = p1->color.r;
                    gshort  = p1->color.g;
                    bshort  = p1->color.b;
                } else {
                    rshort  = p2->color.r;
                    gshort  = p2->color.g;
                    bshort  = p2->color.b;
                }
            } else {
                if (p1->point.x() > p2->point.x()) {
                    rshort  = p1->color.r;
                    gshort  = p1->color.g;
                    bshort  = p1->color.b;
                } else {
                    rshort  = p2->color.r;
                    gshort  = p2->color.g;
                    bshort  = p2->color.b;
                }
            }
        }
    } else {
        if (floor(p2p3ydist) != 0.0) {
            rshort  = p3->color.r * (p.y() - p2->point.y()) / p2p3ydist;
            gshort  = p3->color.g * (p.y() - p2->point.y()) / p2p3ydist;
            bshort  = p3->color.b * (p.y() - p2->point.y()) / p2p3ydist;
            rshort += p2->color.r * (p3->point.y() - p.y()) / p2p3ydist;
            gshort += p2->color.g * (p3->point.y() - p.y()) / p2p3ydist;
            bshort += p2->color.b * (p3->point.y() - p.y()) / p2p3ydist;
        } else {
            if (lefty) {
                if (p2->point.x() <= p3->point.x()) {
                    rshort  = p2->color.r;
                    gshort  = p2->color.g;
                    bshort  = p2->color.b;
                } else {
                    rshort  = p3->color.r;
                    gshort  = p3->color.g;
                    bshort  = p3->color.b;
                }
            } else {
                if (p2->point.x() > p3->point.x()) {
                    rshort  = p2->color.r;
                    gshort  = p2->color.g;
                    bshort  = p2->color.b;
                } else {
                    rshort  = p3->color.r;
                    gshort  = p3->color.g;
                    bshort  = p3->color.b;
                }
            }
        }
    }

    // p1p3ydist is never 0
    rlong  = p3->color.r * (p.y() - p1->point.y()) / p1p3ydist;
    glong  = p3->color.g * (p.y() - p1->point.y()) / p1p3ydist;
    blong  = p3->color.b * (p.y() - p1->point.y()) / p1p3ydist;
    rlong += p1->color.r * (p3->point.y() - p.y()) / p1p3ydist;
    glong += p1->color.g * (p3->point.y() - p.y()) / p1p3ydist;
    blong += p1->color.b * (p3->point.y() - p.y()) / p1p3ydist;

    // rshort,gshort,bshort is the color on one of the shortys.
    // rlong,glong,blong is the color on the longy. So depending on
    // wether we have a lefty trigon or not, we can determine which
    // colors are on the left and right edge.
    double rl, gl, bl, rr, gr, br;
    if (lefty) {
	rl = rshort; gl = gshort; bl = bshort;
	rr = rlong; gr = glong; br = blong;
    } else {
	rl = rlong; gl = glong; bl = blong;
	rr = rshort; gr = gshort; br = bshort;
    }

    // Find the distance from the left x to the right x (xdist). Then
    // find the distances from the selector to each of these (saxdist
    // and saxdist2). These distances are used to find the color at
    // the selector.
    double xdist = rightx - leftx;
    double saxdist = p.x() - leftx;
    double saxdist2 = xdist - saxdist;

    // Now determine the r,g,b values of the selector using a linear
    // approximation.
    double r, g, b;
    if (xdist != 0.0) {
	r = (saxdist2 * rl / xdist) + (saxdist * rr / xdist);
	g = (saxdist2 * gl / xdist) + (saxdist * gr / xdist);
	b = (saxdist2 * bl / xdist) + (saxdist * br / xdist);
    } else {
	// In theory, the left and right color will be equal here. But
	// because of the loss of precision, we get an error on both
	// colors. The best approximation we can get is from adding
	// the two errors, which in theory will eliminate the error
	// but in practise will only minimize it.
	r = (rl + rr) / 2;
        g = (gl + gr) / 2;
	b = (bl + br) / 2;
    }

    // Now floor the color components and fit them into proper
    // boundaries. This again is to compensate for the error caused by
    // loss of precision.
    int ri = (int) floor(r);
    int gi = (int) floor(g);
    int bi = (int) floor(b);
    if (ri < 0) ri = 0;
    else if (ri > 255) ri = 255;
    if (gi < 0) gi = 0;
    else if (gi > 255) gi = 255;
    if (bi < 0) bi = 0;
    else if (bi > 255) bi = 255;

    // Voila, we have the color at the point of the selector.
    return QColor(ri, gi, bi);
}