The Colour Chart Generator in C++

Introduction

This is a C++ coding test. It is expected that this task should take a few hours to complete.

Task description

Overview

When designing graphics hardware it is useful to be able to display various test patterns and colour ramps. The task is to write a command-line application to generate some of these test patterns. In the simplest case this program will produce a colour ramp starting with one colour on one side of the display and changing smoothly to a second colour on the other side. In the most complex case there will be a different colour in each corner of the display and each pixel on the display will show the appropriate mix of these four colours.

Input

The program should be invoked by the following command line:

./ramp.exe display tl tr [bl] [br]

where

display is the name of the display device
tl is the top left colour value
tr is the top right colour value
bl is the bottom left colour value [optional, defaults to tl]
br is the bottom right colour value [optional, defaults to tr]

The colour values are specified as 16-bit RGB565 pixels in hex or decimal. The bits for each colour are assigned as follows:

Bit	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
Colour	R4	R3	R2	R1	R0	G5	G4	G3	G2	G1	G0	B4	B3	B2	B1	B0

For example,

White is 0xffff
Black is 0x0000
Pure blue is 0x001f
Pure green is 0x07e0
Pure red is 0xf800

The following inputs to the program are all valid:

ramp.exe display 0x0 0x2
ramp.exe display 65 255
ramp.exe display 200 0 30
ramp.exe display 0 0 3200 1800
etc

Output

The program must fill the display with a colour ramp as defined by the input values. Each corner of the display must have the specified colour and the pixels in the middle must be a linear mix of those colours.

The image is output on the display using the Display class which is provided. The size method returns the size of the display and the draw method is used to draw a row of 16-bit RGB565 pixels on the display. Finally, the present method is called to display the pixels.

Assessment

The results of this task will be assessed in the following areas:

Correct program operation for all input values
Clarity of design
Consistent coding style and demonstrated use of C++ features
Testing strategy (Supplied unit tests, etc)
Appropriate error handling

You must provide full source code and a brief description. The program will be tested with a variety of input values. The implementation of the Display class is provided below.

Notes

Correct behaviour is much more important than good performance.

This task is harder than it appears: there are a number of pitfalls and difficult cases to allow for. In particular, make sure that the colours are spread evenly when there are only a few different colours across the display. For example, a ramp from 0 to 0xf, over a width of 16 pixels, should attain each value exactly once.

Display Class implementation

Display.h

#ifndef DISPLAY_H_INCLUDED
#define DISPLAY_H_INCLUDED
 
#include <vector>
#include <string>
#include <cstdint>
 
struct Point
{
  int16_t x;
  int16_t y;
};
 
struct Dimension
{
  uint16_t width;
  uint16_t height;
};
 
class Display
{
public:
  Display(const std::string& name);
  ~Display() = default;
 
  Dimension size() const;
  void draw(Point point, const uint16_t* pixels, uint16_t width);
  void present();
 
private:
  const std::string m_name;
  std::vector<uint16_t> m_frameBuffer;
};
 
#endif

#ifndef DISPLAY_H_INCLUDED
#define DISPLAY_H_INCLUDED

#include <vector>
#include <string>
#include <cstdint>

struct Point
{
  int16_t x;
  int16_t y;
};

struct Dimension
{
  uint16_t width;
  uint16_t height;
};

class Display
{
public:
  Display(const std::string& name);
  ~Display() = default;

  Dimension size() const;
  void draw(Point point, const uint16_t* pixels, uint16_t width);
  void present();

private:
  const std::string m_name;
  std::vector<uint16_t> m_frameBuffer;
};

#endif

Display.cpp

#include "Display.h"
 
#include <cstdio>
#include <cstring>
 
namespace {
  constexpr uint16_t Width = 16;
  constexpr uint16_t Height = 9;
}
 
Display::Display(const std::string& name)
  : m_name(name)
  , m_frameBuffer(Width * Height)
{
}
 
Dimension Display::size() const
{
  return { Width, Height };
}
 
void Display::draw(const Point point,
 const uint16_t *pixels,
 const uint16_t width)
{
  std::memcpy(&m_frameBuffer[point.y * Width + point.x],
  pixels,
  width * sizeof(uint16_t));
}
 
void Display::present()
{
  auto pixels = m_frameBuffer.data();
  for (int y = 0; y < Height; ++y) {
    for (int x = 0; x < Width; ++x) {
      printf("%04X ", *pixels++);
    }
    printf("\n");
  }
}

#include "Display.h"

#include <cstdio>
#include <cstring>

namespace {
  constexpr uint16_t Width = 16;
  constexpr uint16_t Height = 9;
}

Display::Display(const std::string& name)
  : m_name(name)
  , m_frameBuffer(Width * Height)
{
}

Dimension Display::size() const
{
  return { Width, Height };
}

void Display::draw(const Point point,
 const uint16_t *pixels,
 const uint16_t width)
{
  std::memcpy(&m_frameBuffer[point.y * Width + point.x],
  pixels,
  width * sizeof(uint16_t));
}

void Display::present()
{
  auto pixels = m_frameBuffer.data();
  for (int y = 0; y < Height; ++y) {
    for (int x = 0; x < Width; ++x) {
      printf("%04X ", *pixels++);
    }
    printf("\n");
  }
}

Colour Chart Ramp ScreenShot

displaylink coding exercise