GraphicsMagick

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# Magick::Color Class

Color is the base color class in Magick++. It is a simple container class for the pixel red, green, blue, and alpha values scaled to fit GraphicsMagick's Quantum (number of bits in a color component value) size. Normally users will instantiate a class derived from Color which supports the color model that fits the needs of the application. The Color class may be constructed directly from an X11-style color string. As a perhaps odd design decision, the value transparent black is considered to represent an unset value (invalid color) in many cases. This choice was made since it avoided using more memory. The default Color constructor constructs an invalid color (i.e. transparent black) and may be used as a parameter in order to remove a color setting.

# Quantum

The base type used to represent color samples in GraphicsMagick is the Quantum type. Pixels are represented by a structure of Quantum values. For example, an RGB pixel contains red, green, and blue quantums, while an RGBA pixel contains red, green, blue, and opacity quantums. The maximum value that a Quantum can attain is specified by a constant value represented by the MaxRGB define, which is itself determined by the number of bits in a Quantum. The QuantumDepth build option determines the number of bits in a Quantum.

# PixelPacket

PixelPacket is the internal representation of a pixel in GraphicsMagick. GraphicsMagick may be compiled to support 32, 64, or 128 bit pixels of type PixelPacket. This is controlled by the value of the QuantumDepth define. The default is 32 bit pixels (QuantumDepth=8), which provides the best performance and the least resource consumption. If additional color precision or range is desired, then GraphicsMagick may be compiled with QuantumDepth=16 or QuantumDepth=32. The following table shows the relationship between QuantumDepth, the type of Quantum, and the overall PixelPacket size:

Effect Of QuantumDepth Values
QuantumDepth | Quantum Typedef | PixelPacket Size
--- | --- | ---
8 | unsigned char | 32 bits
16 | unsigned short | 64 bits
32 | unsigned int | 128 bits

# Color Class

The Color base class is not intended to be used directly. Normally a user will construct a derived class or inherit from this class. Color arguments are must be scaled to fit the Quantum size according to the range of MaxRGB. The Color class contains a pointer to a PixelPacket, which may be allocated by the Color class, or may refer to an existing pixel in an image.

An alternate way to construct the class is via an X11-compatible color specification string (e.g. Color(“red”) or Color (“#FF0000”)). Since the class may be constructed from a string, convenient strings may be passed in place of an explicit Color object in methods which accept a reference to Color. Color may also be converted to a std::string for convenience in user interfaces, and for saving settings to a text file.

The following is the definition of the Color class:

    // Compare two Color objects regardless of LHS/RHS
    int MagickDLLDecl operator == ( const Magick::Color& left_, const Magick::Color& right_ );
    int MagickDLLDecl operator != ( const Magick::Color& left_, const Magick::Color& right_ );
    int MagickDLLDecl operator >  ( const Magick::Color& left_, const Magick::Color& right_ );
    int MagickDLLDecl operator <  ( const Magick::Color& left_, const Magick::Color& right_ );
    int MagickDLLDecl operator >= ( const Magick::Color& left_, const Magick::Color& right_ );
    int MagickDLLDecl operator <= ( const Magick::Color& left_, const Magick::Color& right_ );

    // Base color class stores RGB components scaled to fit Quantum
    class MagickDLLDecl Color
    {
    public:
      Color ( Quantum red_,
              Quantum green_,
              Quantum blue_ );
      Color ( Quantum red_,
              Quantum green_,
              Quantum blue_,
              Quantum alpha_ );
      Color ( const std::string &x11color_ );
      Color ( const char * x11color_ );
      Color ( void );
      virtual        ~Color ( void );
      Color ( const Color & color_ );

      // Red color (range 0 to MaxRGB)
      void           redQuantum ( Quantum red_ );
      Quantum        redQuantum ( void ) const;

      // Green color (range 0 to MaxRGB)
      void           greenQuantum ( Quantum green_ );
      Quantum        greenQuantum ( void ) const;

      // Blue color (range 0 to MaxRGB)
      void           blueQuantum ( Quantum blue_ );
      Quantum        blueQuantum ( void ) const;

      // Alpha level (range OpaqueOpacity=0 to TransparentOpacity=MaxRGB)
      void           alphaQuantum ( Quantum alpha_ );
      Quantum        alphaQuantum ( void ) const;

      // Scaled (to 1.0) version of alpha for use in sub-classes
      // (range opaque=0 to transparent=1.0)
      void           alpha ( double alpha_ );
      double         alpha ( void ) const;

      // Does object contain valid color?
      void           isValid ( bool valid_ );
      bool           isValid ( void ) const;

      // Set color via X11 color specification string
      const Color& operator= ( const std::string &x11color_ );
      const Color& operator= ( const char * x11color_ );

      // Assignment operator
      Color& operator= ( const Color& color_ );

      // Return X11 color specification string
      /* virtual */ operator std::string() const;

      // Return ImageMagick PixelPacket
      operator PixelPacket() const;

      // Construct color via ImageMagick PixelPacket
      Color ( const PixelPacket &color_ );

      // Set color via ImageMagick PixelPacket
      const Color& operator= ( const PixelPacket &color_ );

    };

# Color Derived Classes

Available derived color specification classes are shown in the following table:

Color Derived Classes
Class | Description
--- | ---
ColorRGB | Representation of RGB color with red, green, and blue specified as ratios (0 to 1)
ColorGray | Representation of grayscale RGB color (equal parts red, green, and blue) specified as a ratio (0 to 1)
ColorMono | Representation of a black/white color (true/false)
ColorHSL | Representation of a color in Hue/Saturation/Luminosity (HSL) colorspace
ColorYUV | Representation of a color in the YUV colorspace

# ColorRGB

Representation of an RGB color in floating point. All color arguments have a valid range of 0.0 - 1.0. Note that floating point alpha may be set via the alpha() method in the Color base class:

    class ColorRGB : public Color
    {
    public:
      ColorRGB ( double red_, double green_, double blue_ );
      ColorRGB ( void );
      ColorRGB ( const Color & color_ );
      /* virtual */  ~ColorRGB ( void );

      void           red ( double red_ );
      double         red ( void ) const;

      void           green ( double green_ );
      double         green ( void ) const;

      void           blue ( double blue_ );
      double         blue ( void ) const;

      // Assignment operator from base class
      ColorRGB& operator= ( const Color& color_ );

    protected:
      // Constructor to construct with PixelPacket*
      ColorRGB ( PixelPacket* rep_, PixelType pixelType_ );
    };

# ColorGray

Representation of a floating point grayscale color (in RGB colorspace). Grayscale is simply RGB with equal parts of red, green, and blue. All double arguments have a valid range of 0.0 - 1.0:

    class ColorGray : public Color
    {
    public:
      ColorGray ( double shade_ );
      ColorGray ( void );
      ColorGray ( const Color & color_ );
      /* virtual */ ~ColorGray ();

      void           shade ( double shade_ );
      double         shade ( void ) const;

      // Assignment operator from base class
      ColorGray& operator= ( const Color& color_ );

    };

# ColorMono

Representation of a black/white pixel (in RGB colorspace). Color arguments are constrained to 'false' (black pixel) and 'true' (white pixel):

    class ColorMono : public Color
    {
    public:
      ColorMono ( bool mono_ );
      ColorMono ( void );
      ColorMono ( const Color & color_ );
      /* virtual */ ~ColorMono ();

      void           mono ( bool mono_ );
      bool           mono ( void ) const;

      // Assignment operator from base class
      ColorMono& operator= ( const Color& color_ );

    };

# ColorHSL

Representation of a color in Hue/Saturation/Luminosity (HSL) colorspace:

    class MagickDLLDecl ColorHSL : public Color
    {
    public:
      ColorHSL ( double hue_, double saturation_, double luminosity_ );
      ColorHSL ( void );
      ColorHSL ( const Color & color_ );
      /* virtual */  ~ColorHSL ( );

      void           hue ( double hue_ );
      double         hue ( void ) const;

      void           saturation ( double saturation_ );
      double         saturation ( void ) const;

      void           luminosity ( double luminosity_ );
      double         luminosity ( void ) const;

      // Assignment operator from base class
      ColorHSL& operator= ( const Color& color_ );

    };

# ColorYUV

Representation of a color in YUV colorspace (used to encode color for television transmission). Argument ranges are

> Y | 0.0 through 1.0
> U | -0.5 through 0.5
> V | -0.5 through 0.5

    class ColorYUV : public Color
    {
    public:
      ColorYUV ( double y_, double u_, double v_ );
      ColorYUV ( void );
      ColorYUV ( const Color & color_ );
      /* virtual */ ~ColorYUV ( void );

      void           u ( double u_ );
      double         u ( void ) const;

      void           v ( double v_ );
      double         v ( void ) const;

      void           y ( double y_ );
      double         y ( void ) const;

      // Assignment operator from base class
      ColorYUV& operator= ( const Color& color_ );

    };

Copyright © Bob Friesenhahn 1999 - 2019