Visual Servoing Platform  version 3.6.1 under development (2024-11-11)
vpHistogram Class Reference

#include <visp3/core/vpHistogram.h>

Public Member Functions

 vpHistogram ()
 
 vpHistogram (const vpHistogram &h)
 
VP_EXPLICIT vpHistogram (const vpImage< unsigned char > &I)
 
VP_EXPLICIT vpHistogram (const vpImage< unsigned char > &I, const vpImage< bool > *p_mask)
 
virtual ~vpHistogram ()
 
vpHistogramoperator= (const vpHistogram &h)
 
unsigned operator[] (const unsigned char level) const
 
unsigned operator() (const unsigned char level) const
 
unsigned get (const unsigned char level) const
 
void set (const unsigned char level, unsigned int value)
 
void setMask (const vpImage< bool > *p_mask)
 
void calculate (const vpImage< unsigned char > &I, unsigned int nbins=256, unsigned int nbThreads=1)
 
void equalize (const vpImage< unsigned char > &I, vpImage< unsigned char > &Iout)
 
void display (const vpImage< unsigned char > &I, const vpColor &color=vpColor::white, unsigned int thickness=2, unsigned int maxValue_=0)
 
void smooth (unsigned int fsize=3)
 
unsigned getPeaks (std::list< vpHistogramPeak > &peaks)
 
unsigned getPeaks (unsigned char dist, vpHistogramPeak &peak1, vpHistogramPeak &peak2)
 
bool getPeaks (unsigned char dist, vpHistogramPeak &peakl, vpHistogramPeak &peakr, vpHistogramValey &valey)
 
unsigned getValey (std::list< vpHistogramValey > &valey)
 
bool getValey (const vpHistogramPeak &peak1, const vpHistogramPeak &peak2, vpHistogramValey &valey)
 
unsigned getValey (unsigned char dist, const vpHistogramPeak &peak, vpHistogramValey &valeyl, vpHistogramValey &valeyr)
 
unsigned sort (std::list< vpHistogramPeak > &peaks)
 
bool write (const std::string &filename)
 
bool write (const char *filename)
 
unsigned getSize () const
 
unsigned * getValues ()
 
unsigned int getTotal ()
 

Detailed Description

Class to compute a gray level image histogram.

Here are two examples showing how to use this class to determine the threshold which can be used to segment two objects.

The code below:

...
unsigned char dist = 60;
h.smooth(); // Filter the histogram values
vpHistogramPeak peakl, peakr; // Two highest peaks in the histogram
// - peakl: Peak on the left
// - peakr: Peak on the right
h.getPeaks(dist, peakl, peakr, valey);
unsigned char threshold; // Position of the valey between the two peaks
threshold = valey.getLevel();
Declaration of the peak (maximum value) in a gray level image histogram.
Declaration of the valey (minimum value) in a gray level image histogram.
unsigned char getLevel() const
Class to compute a gray level image histogram.
Definition: vpHistogram.h:106

has the same behaviour than this one:

...
unsigned char dist = 60;
h.smooth(); // Filter the histogram values
vpHistogramPeak peak1, peak2; // Two highest peaks in the histogram
// - peak1: Highest peak
// - peakr: Second highest peak
// Get the two highest peaks
h.getPeaks(dist, peak1, peak2);
// Get the valey between the two highest peaks
h.getValey(peak1, peak2, valey);
unsigned char threshold; // Position of the valey between the two peaks
threshold = valey.getLevel();
Examples
histogram.cpp, and testHistogram.cpp.

Definition at line 105 of file vpHistogram.h.

Constructor & Destructor Documentation

◆ vpHistogram() [1/4]

vpHistogram::vpHistogram ( )

Default constructor for a gray level histogram.

Definition at line 195 of file vpHistogram.cpp.

◆ vpHistogram() [2/4]

vpHistogram::vpHistogram ( const vpHistogram h)

Copy constructor of a gray level histogram.

Definition at line 200 of file vpHistogram.cpp.

◆ vpHistogram() [3/4]

vpHistogram::vpHistogram ( const vpImage< unsigned char > &  I)

Calculates the histogram from a gray level image.

Parameters
I: Gray level image.
See also
calculate()

Definition at line 213 of file vpHistogram.cpp.

References calculate().

◆ vpHistogram() [4/4]

vpHistogram::vpHistogram ( const vpImage< unsigned char > &  I,
const vpImage< bool > *  p_mask 
)

Calculates the histogram from a gray level image using a binary mask.

Parameters
I: Gray level image.
p_maskA pointer towards a binary mask where true indicates that the pixel should considered and false that it should be ignored.
See also
calculate() setMask()

Definition at line 228 of file vpHistogram.cpp.

References calculate(), and setMask().

◆ ~vpHistogram()

vpHistogram::~vpHistogram ( )
virtual

Destructor.

Definition at line 238 of file vpHistogram.cpp.

Member Function Documentation

◆ calculate()

void vpHistogram::calculate ( const vpImage< unsigned char > &  I,
unsigned int  nbins = 256,
unsigned int  nbThreads = 1 
)

Calculate the histogram from a gray level image.

Parameters
I: Gray level image.
nbins: Number of bins to compute the histogram.
nbThreads: Number of threads to use for the computation.
Examples
histogram.cpp, and testHistogram.cpp.

Definition at line 299 of file vpHistogram.cpp.

References vpImage< Type >::bitmap, vpImage< Type >::getHeight(), vpImage< Type >::getSize(), and vpImage< Type >::getWidth().

Referenced by vpImageFilter::computeCannyThreshold(), equalize(), VISP_NAMESPACE_NAME::gammaCorrectionProbBasedMethod(), and vpHistogram().

◆ display()

void vpHistogram::display ( const vpImage< unsigned char > &  I,
const vpColor color = vpColor::white,
unsigned int  thickness = 2,
unsigned int  maxValue_ = 0 
)

Display the histogram distribution in an image, the minimal image size is 36x36 px.

Parameters
I: Image with the histogram distribution displayed.
color: Color used for the display.
thickness: Thickness of the line.
maxValue_: Maximum value in the histogram, if 0 it will be computed from the current histogram. Useful to plot a 3 channels histogram for a RGB image for example to keep a coherent vertical scale between the channels.

Definition at line 474 of file vpHistogram.cpp.

References vpDisplay::displayLine(), vpException::divideByZeroError, vpImage< Type >::getHeight(), and vpImage< Type >::getWidth().

◆ equalize()

void vpHistogram::equalize ( const vpImage< unsigned char > &  I,
vpImage< unsigned char > &  Iout 
)

◆ get()

unsigned vpHistogram::get ( const unsigned char  level) const
inline

Return the number of pixels having the gray level.

Parameters
level: Gray level in the histogram.
Returns
Number of pixels having the gray level.
vpImage<unsigned char> I; // A gray level image
h.calculate(I); // Histogram of the gray level image
// Print the histogram values
for (int i=0; i < h.getSize(); ++i)
printf("%d: %d\n", i, h.get(i));
void calculate(const vpImage< unsigned char > &I, unsigned int nbins=256, unsigned int nbThreads=1)
unsigned getSize() const
Definition: vpHistogram.h:278
unsigned get(const unsigned char level) const
Definition: vpHistogram.h:196

Definition at line 196 of file vpHistogram.h.

References vpException::dimensionError.

◆ getPeaks() [1/3]

unsigned vpHistogram::getPeaks ( std::list< vpHistogramPeak > &  peaks)

Build a list of all histogram peaks. This peak list is gray level sorted from 0 to 255. That mean that the first peak has a gray level less than the second one, etc.

To sort this list from highest peak value to the lowest one, you can use sort().

Parameters
peaks: List of peaks.
Returns
The number of peaks in the histogram.
See also
sort()
Examples
histogram.cpp.

Definition at line 568 of file vpHistogram.cpp.

References vpImageException::notInitializedError, and vpHistogramPeak::set().

Referenced by getPeaks(), and getValey().

◆ getPeaks() [2/3]

unsigned vpHistogram::getPeaks ( unsigned char  dist,
vpHistogramPeak peak1,
vpHistogramPeak peak2 
)

Find the two highest peaks in the histogram. Usually, the first one correspond to the image background, the second one to the object.

Parameters
dist: Minimal distance between two significative histogram peaks.
peak1: Highest peak in the histogram.
peak2: Second highest peak in the histogram.
Returns
The number of highest peaks:
  • 2: If the histogram is bimodal
  • 1: If no second highest peak was found.
  • 0: if no peaks were found.

Definition at line 637 of file vpHistogram.cpp.

References vpHistogramPeak::getLevel(), getPeaks(), vpHistogramPeak::set(), and sort().

◆ getPeaks() [3/3]

bool vpHistogram::getPeaks ( unsigned char  dist,
vpHistogramPeak peakl,
vpHistogramPeak peakr,
vpHistogramValey valey 
)

Determine the two highest peaks in the histogram and compute a threshold to separate the two objects. Here we dont know which is the highest peak. It could be peakl or peakr.

Parameters
dist: Distance between two significative histogram maxima
peakl: Position of the left histogram peak.
peakr: Position of the right histogram peak.
valey: Valey between the two peaks peakl and peakr.
Returns
true if the histogram is bimodal, false otherwise.

Definition at line 690 of file vpHistogram.cpp.

References vpHistogramPeak::getLevel(), vpHistogramPeak::getValue(), vpHistogramPeak::set(), and vpHistogramValey::set().

◆ getSize()

unsigned vpHistogram::getSize ( ) const
inline

◆ getTotal()

unsigned int vpHistogram::getTotal ( )
inline

Get the total number of pixels in the input image.

Returns
unsigned int Cumulated number of pixels in the input image.

Definition at line 308 of file vpHistogram.h.

Referenced by vpImageFilter::computeCannyThreshold(), and VISP_NAMESPACE_NAME::gammaCorrectionProbBasedMethod().

◆ getValey() [1/3]

bool vpHistogram::getValey ( const vpHistogramPeak peak1,
const vpHistogramPeak peak2,
vpHistogramValey valey 
)

Find the valey between two peaks. It starts at the lowest peak and works its way up to the highest peak. Along the way, it looks at each point in the histogram to find the location of the smallest histogram point which is the valey point.

Parameters
peak1: A peak in the histogram.
peak2: A other peak in the histogram.
valey: Low point of the valey between two peaks in a histogram.
Returns
true if a valey was found, false otherwise.

Definition at line 909 of file vpHistogram.cpp.

References vpHistogramPeak::getLevel(), vpHistogramPeak::getValue(), and vpHistogramValey::set().

◆ getValey() [2/3]

unsigned vpHistogram::getValey ( std::list< vpHistogramValey > &  valey)

Build a list of all histogram valey. This valey list is gray level sorted from 0 to 255. That mean that the first valey has a gray level less than the second one, etc.

Parameters
valey: List of valey.
Returns
The number of valey in the histogram.
See also
sort()
Examples
histogram.cpp.

Definition at line 842 of file vpHistogram.cpp.

References vpImageException::notInitializedError, and vpHistogramValey::set().

◆ getValey() [3/3]

unsigned vpHistogram::getValey ( unsigned char  dist,
const vpHistogramPeak peak,
vpHistogramValey valeyl,
vpHistogramValey valeyr 
)

Find the two valey around an histogram peak. It starts at the peak position and works its way down and up to find the left and right valey around the peak.

Parameters
dist: Minimal distance between two significative histogram peaks.
peak: A peak in the histogram.
valeyl: The valey on the left of the peak.
valeyr: The valey on the right of the peak.
Returns
0x00 : If no valey was found.
0x01 : If only the right valey was found.
0x10 : If only the left valey was found.
0x11 : If two valeys around the peak were found.

Definition at line 978 of file vpHistogram.cpp.

References vpHistogramPeak::getLevel(), getPeaks(), vpHistogramPeak::getValue(), vpHistogramPeak::set(), and vpHistogramValey::set().

◆ getValues()

unsigned* vpHistogram::getValues ( )
inline

Get the histogram values.

Returns
A pointer to the array of histogram values. The size of this array is given by getSize().
vpImage<unsigned char> I; // A gray level image
h.calculate(I); // Histogram of the gray level image
// Print the histogram values
unsigned char *values = h.getValues();
for (int i=0; i < h.getSize(); ++i)
printf("%d: %d\n", i, values[i]);
unsigned * getValues()
Definition: vpHistogram.h:301
See also
getSize()

Definition at line 301 of file vpHistogram.h.

◆ operator()()

unsigned vpHistogram::operator() ( const unsigned char  level) const
inline

Return the number of pixels having the gray level.

Parameters
level: Gray level in the histogram.
Returns
Number of pixels having the gray level.
vpImage<unsigned char> I; // A gray level image
h.calculate(I); // Histogram of the gray level image
// Print the histogram values
for (int i=0; i < h.getSize(); ++i)
printf("%d: %d\n", i, h(i));

Definition at line 166 of file vpHistogram.h.

References vpException::dimensionError.

◆ operator=()

vpHistogram & vpHistogram::operator= ( const vpHistogram h)

Copy operator.

Parameters
h: Histogram to copy.

Definition at line 260 of file vpHistogram.cpp.

◆ operator[]()

unsigned vpHistogram::operator[] ( const unsigned char  level) const
inline

Return the number of pixels having the gray level.

Parameters
level: Gray level in the histogram.
Returns
Number of pixels having the gray level.
vpImage<unsigned char> I; // A gray level image
h.calculate(I); // Histogram of the gray level image
// Print the histogram values
for (int i=0; i < h.getSize(); ++i)
printf("%d: %d\n", i, h[i]);

Definition at line 136 of file vpHistogram.h.

References vpException::dimensionError.

◆ set()

void vpHistogram::set ( const unsigned char  level,
unsigned int  value 
)
inline

Set the number of pixels having the gray level.

Parameters
level: Gray level in the histogram. Level is in [0:255]
value: Number of pixels having the gray level.
// Set histogram values
for (int i=0; i < h.getSize(); ++i)
h.set(i, i*2); // for each level i, set a value of 2*i
void set(const unsigned char level, unsigned int value)
Definition: vpHistogram.h:224

Definition at line 224 of file vpHistogram.h.

References vpException::dimensionError.

Referenced by VISP_NAMESPACE_NAME::computeThresholdTriangle().

◆ setMask()

void vpHistogram::setMask ( const vpImage< bool > *  p_mask)
inline

Set a mask to ignore pixels for which the mask is false.

Warning
The mask must be reset manually by the user (either for another mask or set to nullptr ) before computing the histogram of another image.
Parameters
p_maskIf different of nullptr , a mask of booleans where true indicates that a pixel must be considered and false that the pixel should be ignored.

Definition at line 246 of file vpHistogram.h.

Referenced by vpImageFilter::computeCannyThreshold(), VISP_NAMESPACE_NAME::equalizeHistogram(), VISP_NAMESPACE_NAME::gammaCorrectionProbBasedMethod(), and vpHistogram().

◆ smooth()

void vpHistogram::smooth ( unsigned int  fsize = 3)

Smoothes the histogram.

A simple average scheme is used where each value $h(i)$ in the histogram is replaced by the average of itself and the neighbors.

\[h(i) = \sum_{j=i-\frac{fsize}{2}}^{i+\frac{fsize}{2}} h(j) \]

Parameters
[in]fsize: Filter size. Corresponds to the number of values around each point used to compute the mean value.
Exceptions
vpImageException::notInitializedError: Histogram array not initialised. Means that the histogram was not calculated before.
See also
calculate()
Examples
histogram.cpp.

Definition at line 529 of file vpHistogram.cpp.

References vpImageException::notInitializedError.

◆ sort()

unsigned vpHistogram::sort ( std::list< vpHistogramPeak > &  peaks)

Sort a list of histogram peaks from highest to the lowest.

Parameters
peaks: List of histogram peaks.
Returns
The number of peaks in the histogram.
See also
getPeak()
Examples
histogram.cpp.

Definition at line 1144 of file vpHistogram.cpp.

Referenced by getPeaks().

◆ write() [1/2]

bool vpHistogram::write ( const char *  filename)

Write the histogram values in a file.

In the file, on each line you will find first the gray level and than the number of pixels which have this gray level.

Parameters
filename: File name to write with histogram values.
Returns
true if the file could be written, false otherwise.

Definition at line 1182 of file vpHistogram.cpp.

◆ write() [2/2]

bool vpHistogram::write ( const std::string &  filename)

Write the histogram values in a file.

In the file, on each line you will find first the gray level and than the number of pixels which have this gray level.

Parameters
filename: File name to write with histogram values.
Returns
true if the file could be written, false otherwise.
Examples
histogram.cpp.

Definition at line 1168 of file vpHistogram.cpp.