Mandelbrot draw method - c++

Question

I have the assignment to finish a mandelbrot program in C++. I'm not that good in C++, I prefer Java or C# but this has to be done in C++. I got some sample code which I have to finish. I'm trying to put the drawing code in the main (between the works comments) into a method (draw_Mandelbrot). The code in the main method works and gives me a nice mandelbrot image but when I use the draw_Mandelbrot method (and comment the draw code in main) I get a grey rectangle image as output. How can I make the draw_Mandelbrot method work? The code above the draw_Mandelbrot method is all sample code and not created by myself.
// mandelbrot.cpp
// compile with: g++ -std=c++11 mandelbrot.cpp -o mandelbrot
// view output with: eog mandelbrot.ppm
#include <fstream>
#include <complex> // if you make use of complex number facilities in C++
#include <iostream>
#include <cstdlib>
#include <complex>
using namespace std;
template <class T> struct RGB { T r, g, b; };
template <class T>
class Matrix {
public:
Matrix(const size_t rows, const size_t cols) : _rows(rows), _cols(cols) {
_matrix = new T*[rows];
for (size_t i = 0; i < rows; ++i) {
_matrix[i] = new T[cols];
}
}
Matrix(const Matrix &m) : _rows(m._rows), _cols(m._cols) {
_matrix = new T*[m._rows];
for (size_t i = 0; i < m._rows; ++i) {
_matrix[i] = new T[m._cols];
for (size_t j = 0; j < m._cols; ++j) {
_matrix[i][j] = m._matrix[i][j];
}
}
}
~Matrix() {
for (size_t i = 0; i < _rows; ++i) {
delete [] _matrix[i];
}
delete [] _matrix;
}
T *operator[] (const size_t nIndex)
{
return _matrix[nIndex];
}
size_t width() const { return _cols; }
size_t height() const { return _rows; }
protected:
size_t _rows, _cols;
T **_matrix;
};
// Portable PixMap image
class PPMImage : public Matrix<RGB<unsigned char> >
{
public:
PPMImage(const size_t height, const size_t width) : Matrix(height, width) { }
void save(const std::string &filename)
{
std::ofstream out(filename, std::ios_base::binary);
out <<"P6" << std::endl << _cols << " " << _rows << std::endl << 255 << std::endl;
for (size_t y=0; y<_rows; y++)
for (size_t x=0; x<_cols; x++)
out << _matrix[y][x].r << _matrix[y][x].g << _matrix[y][x].b;
}
};
void draw_Mandelbrot(PPMImage image, const unsigned width, const unsigned height, double cxmin, double cxmax, double cymin, double cymax,unsigned int max_iterations)
{
for (std::size_t ix = 0; ix < width; ++ix)
for (std::size_t iy = 0; iy < height; ++iy)
{
std::complex<double> c(cxmin + ix / (width - 1.0)*(cxmax - cxmin), cymin + iy / (height - 1.0)*(cymax - cymin));
std::complex<double> z = 0;
unsigned int iterations;
for (iterations = 0; iterations < max_iterations && std::abs(z) < 2.0; ++iterations)
z = z*z + c;
image[iy][ix].r = image[iy][ix].g = image[iy][ix].b = iterations;
}
}
int main()
{
const unsigned width = 1600;
const unsigned height = 1600;
PPMImage image(height, width);
//image[y][x].r = image[y][x].g = image[y][x].b = 255; // white pixel
//image[y][x].r = image[y][x].g = image[y][x][b] = 0; // black pixel
//image[y][x].r = image[y][x].g = image[y][x].b = 0; // black pixel
//// red pixel
//image[y][x].r = 255;
//image[y][x].g = 0;
//image[y][x].b = 0;
draw_Mandelbrot(image, width, height, -2.0, 0.5, -1.0, 1.0, 10);
//works
//double cymin = -1.0;
//double cymax = 1.0;
//double cxmin = -2.0;
//double cxmax = 0.5;
//unsigned int max_iterations = 100;
//for (std::size_t ix = 0; ix < width; ++ix)
// for (std::size_t iy = 0; iy < height; ++iy)
// {
// std::complex<double> c(cxmin + ix / (width - 1.0)*(cxmax - cxmin), cymin + iy / (height - 1.0)*(cymax - cymin));
// std::complex<double> z = 0;
// unsigned int iterations;
// for (iterations = 0; iterations < max_iterations && std::abs(z) < 2.0; ++iterations)
// z = z*z + c;
// image[iy][ix].r = image[iy][ix].g = image[iy][ix].b = iterations;
// }
//works
image.save("mandelbrot.ppm");
return 0;
}
Output image when using the code in the main method

Answer 1

You're passing the image by value, so the function works on a separate image to the one in main, which is left in its initial state.
Either pass by reference:
void draw_Mandelbrot(PPMImage & image, ...)
or return a value:
PPMImage draw_Mandelbrot(...) {
PPMImage image(height, width);
// your code here
return image;
}
// in main
PPMImage image = draw_Mandelbrot(...);