QCPColorGradient Class Reference
Author: Emanuel Eichhammer

· · · ·
Public Types | Public Functions | Protected Functions
QCPColorGradient Class Reference

Defines a color gradient for use with e.g. QCPColorMap. More...

## Public Types
enum | ColorInterpolation
enum | NanHandling
enum | GradientPreset

## Public Functions
| QCPColorGradient ()
| QCPColorGradient (GradientPreset preset)
bool | operator== (const QCPColorGradient &other) const
bool | operator!= (const QCPColorGradient &other) const
int | levelCount () const
QMap< double, QColor > | colorStops () const
ColorInterpolation | colorInterpolation () const
NanHandling | nanHandling () const
QColor | nanColor () const
bool | periodic () const
void | setLevelCount (int n)
void | setColorStops (const QMap< double, QColor > &colorStops)
void | setColorStopAt (double position, const QColor &color)
void | setColorInterpolation (ColorInterpolation interpolation)
void | setNanHandling (NanHandling handling)
void | setNanColor (const QColor &color)
void | setPeriodic (bool enabled)
void | colorize (const double *data, const QCPRange &range, QRgb *scanLine, int n, int dataIndexFactor=1, bool logarithmic=false)
void | colorize (const double *data, const unsigned char *alpha, const QCPRange &range, QRgb *scanLine, int n, int dataIndexFactor=1, bool logarithmic=false)
QRgb | color (double position, const QCPRange &range, bool logarithmic=false)
void | loadPreset (GradientPreset preset)
void | clearColorStops ()
QCPColorGradient | inverted () const

## Protected Functions
bool | stopsUseAlpha () const
void | updateColorBuffer ()

## Detailed Description

Defines a color gradient for use with e.g. QCPColorMap.

This class describes a color gradient which can be used to encode data with color. For example, QCPColorMap and QCPColorScale have setGradient methods which take an instance of this class. Colors are set with setColorStopAt(double position, const QColor &color) with a position from 0 to 1. In between these defined color positions, the color will be interpolated linearly either in RGB or HSV space, see setColorInterpolation.

Alternatively, load one of the preset color gradients shown in the image below, with loadPreset, or by directly specifying the preset in the constructor.

Apart from red, green and blue components, the gradient also interpolates the alpha values of the configured color stops. This allows to display some portions of the data range as transparent in the plot.

How NaN values are interpreted can be configured with setNanHandling.

The constructor QCPColorGradient(GradientPreset preset) allows directly converting a GradientPreset to a QCPColorGradient. This means that you can directly pass GradientPreset to all the setGradient methods, e.g.:

colorMap->setGradient(QCPColorGradient::gpHot);

The total number of levels used in the gradient can be set with setLevelCount. Whether the color gradient shall be applied periodically (wrapping around) to data values that lie outside the data range specified on the plottable instance can be controlled with setPeriodic.

## Member Enumeration Documentation

## § ColorInterpolation

enum QCPColorGradient::ColorInterpolation

Defines the color spaces in which color interpolation between gradient stops can be performed.

See also
  setColorInterpolation

Enumerator
ciRGB | Color channels red, green and blue are linearly interpolated.
ciHSV | Color channels hue, saturation and value are linearly interpolated (The hue is interpolated over the shortest angle distance)

## § NanHandling

enum QCPColorGradient::NanHandling

Defines how NaN data points shall appear in the plot.

See also
  setNanHandling, setNanColor

Enumerator
nhNone | NaN data points are not explicitly handled and shouldn't occur in the data (this gives slight performance improvement)
nhLowestColor | NaN data points appear as the lowest color defined in this QCPColorGradient.
nhHighestColor | NaN data points appear as the highest color defined in this QCPColorGradient.
nhTransparent | NaN data points appear transparent.
nhNanColor | NaN data points appear as the color defined with setNanColor.

## § GradientPreset

enum QCPColorGradient::GradientPreset

Defines the available presets that can be loaded with loadPreset. See the documentation there for an image of the presets.

Enumerator
gpGrayscale | Continuous lightness from black to white (suited for non-biased data representation)
gpHot | Continuous lightness from black over firey colors to white (suited for non-biased data representation)
gpCold | Continuous lightness from black over icey colors to white (suited for non-biased data representation)
gpNight | Continuous lightness from black over weak blueish colors to white (suited for non-biased data representation)
gpCandy | Blue over pink to white.
gpGeography | Colors suitable to represent different elevations on geographical maps.
gpIon | Half hue spectrum from black over purple to blue and finally green (creates banding illusion but allows more precise magnitude estimates)
gpThermal | Colors suitable for thermal imaging, ranging from dark blue over purple to orange, yellow and white.
gpPolar | Colors suitable to emphasize polarity around the center, with blue for negative, black in the middle and red for positive values.
gpSpectrum | An approximation of the visible light spectrum (creates banding illusion but allows more precise magnitude estimates)
gpJet | Hue variation similar to a spectrum, often used in numerical visualization (creates banding illusion but allows more precise magnitude estimates)
gpHues | Full hue cycle, with highest and lowest color red (suitable for periodic data, such as angles and phases, see setPeriodic)

## Constructor & Destructor Documentation

## § QCPColorGradient() [1/2]

QCPColorGradient::QCPColorGradient | ( |  | ) |

Constructs a new, empty QCPColorGradient with no predefined color stops. You can add own color stops with setColorStopAt.

The color level count is initialized to 350.

## § QCPColorGradient() [2/2]

QCPColorGradient::QCPColorGradient | ( | GradientPreset | preset | ) |

Constructs a new QCPColorGradient initialized with the colors and color interpolation according to preset.

The color level count is initialized to 350.

## Member Function Documentation

## § setLevelCount()

void QCPColorGradient::setLevelCount | ( | int | n | ) |

Sets the number of discretization levels of the color gradient to n. The default is 350 which is typically enough to create a smooth appearance. The minimum number of levels is 2.

## § setColorStops()

void QCPColorGradient::setColorStops | ( | const QMap< double, QColor > & | colorStops | ) |

Sets at which positions from 0 to 1 which color shall occur. The positions are the keys, the colors are the values of the passed QMap colorStops. In between these color stops, the color is interpolated according to setColorInterpolation.

A more convenient way to create a custom gradient may be to clear all color stops with clearColorStops (or creating a new, empty QCPColorGradient) and then adding them one by one with setColorStopAt.

See also
  clearColorStops

## § setColorStopAt()

void QCPColorGradient::setColorStopAt | ( | double | position,
|  | const QColor & | color
| ) |  |

Sets the color the gradient will have at the specified position (from 0 to 1). In between these color stops, the color is interpolated according to setColorInterpolation.

See also
  setColorStops, clearColorStops

## § setColorInterpolation()

void QCPColorGradient::setColorInterpolation | ( | QCPColorGradient::ColorInterpolation | interpolation | ) |

Sets whether the colors in between the configured color stops (see setColorStopAt) shall be interpolated linearly in RGB or in HSV color space.

For example, a sweep in RGB space from red to green will have a muddy brown intermediate color, whereas in HSV space the intermediate color is yellow.

## § setNanHandling()

void QCPColorGradient::setNanHandling | ( | QCPColorGradient::NanHandling | handling | ) |

Sets how NaNs in the data are displayed in the plot.

See also
  setNanColor

## § setNanColor()

void QCPColorGradient::setNanColor | ( | const QColor & | color | ) |

Sets the color that NaN data is represented by, if setNanHandling is set to ref nhNanColor.

See also
  setNanHandling

## § setPeriodic()

void QCPColorGradient::setPeriodic | ( | bool | enabled | ) |

Sets whether data points that are outside the configured data range (e.g. QCPColorMap::setDataRange) are colored by periodically repeating the color gradient or whether they all have the same color, corresponding to the respective gradient boundary color.

As shown in the image above, gradients that have the same start and end color are especially suitable for a periodic gradient mapping, since they produce smooth color transitions throughout the color map. A preset that has this property is gpHues.

In practice, using periodic color gradients makes sense when the data corresponds to a periodic dimension, such as an angle or a phase. If this is not the case, the color encoding might become ambiguous, because multiple different data values are shown as the same color.

## § colorize() [1/2]

void QCPColorGradient::colorize | ( | const double * | data,
|  | const QCPRange & | range,
|  | QRgb * | scanLine,
|  | int | n,
|  | int | dataIndexFactor = `1`,
|  | bool | logarithmic = `false`
| ) |  |

This is an overloaded function.

This method is used to quickly convert a data array to colors. The colors will be output in the array scanLine. Both data and scanLine must have the length n when passed to this function. The data range that shall be used for mapping the data value to the gradient is passed in range. logarithmic indicates whether the data values shall be mapped to colors logarithmically.

if data actually contains 2D-data linearized via `[row*columnCount + column]`, you can set dataIndexFactor to `columnCount` to convert a column instead of a row of the data array, in scanLine. scanLine will remain a regular (1D) array. This works because data is addressed `data[i*dataIndexFactor]`.

Use the overloaded method to additionally provide alpha map data.

The QRgb values that are placed in scanLine have their r, g, and b components premultiplied with alpha (see QImage::Format_ARGB32_Premultiplied).

## § colorize() [2/2]

void QCPColorGradient::colorize | ( | const double * | data,
|  | const unsigned char * | alpha,
|  | const QCPRange & | range,
|  | QRgb * | scanLine,
|  | int | n,
|  | int | dataIndexFactor = `1`,
|  | bool | logarithmic = `false`
| ) |  |

This is an overloaded function.

Additionally to the other overload of colorize, this method takes the array alpha, which has the same size and structure as data and encodes the alpha information per data point.

The QRgb values that are placed in scanLine have their r, g and b components premultiplied with alpha (see QImage::Format_ARGB32_Premultiplied).

## § color()

QRgb QCPColorGradient::color | ( | double | position,
|  | const QCPRange & | range,
|  | bool | logarithmic = `false`
| ) |  |

This method is used to colorize a single data value given in position, to colors. The data range that shall be used for mapping the data value to the gradient is passed in range. logarithmic indicates whether the data value shall be mapped to a color logarithmically.

If an entire array of data values shall be converted, rather use colorize, for better performance.

The returned QRgb has its r, g and b components premultiplied with alpha (see QImage::Format_ARGB32_Premultiplied).

## § loadPreset()

void QCPColorGradient::loadPreset | ( | GradientPreset | preset | ) |

Clears the current color stops and loads the specified preset. A preset consists of predefined color stops and the corresponding color interpolation method.

The available presets are:

## § clearColorStops()

void QCPColorGradient::clearColorStops | ( |  | ) |

Clears all color stops.

See also
  setColorStops, setColorStopAt

## § inverted()

QCPColorGradient QCPColorGradient::inverted | ( |  | ) | const

Returns an inverted gradient. The inverted gradient has all properties as this QCPColorGradient, but the order of the color stops is inverted.

See also
  setColorStops, setColorStopAt

## § stopsUseAlpha()

bool QCPColorGradient::stopsUseAlpha | ( |  | ) | const
| protected

Returns true if the color gradient uses transparency, i.e. if any of the configured color stops has an alpha value below 255.

## § updateColorBuffer()

void QCPColorGradient::updateColorBuffer | ( |  | ) |
| protected

Updates the internal color buffer which will be used by colorize and color, to quickly convert positions to colors. This is where the interpolation between color stops is calculated.

The documentation for this class was generated from the following files:

- src/colorgradient.h
- src/colorgradient.cpp