Cannot generate Mandelbrot matrix to save it to file in c++ threads - c++

Question

I'm starting with my c++ threads and don't understand some basic stuff. That's Mandelbrot example, it generates fractal image.
It's not my code, I just did some changes (here's original: https://rosettacode.org/wiki/Mandelbrot_set#PPM_non_interactive)
I have this function which generates matrix with colors to save to file:
vector<unsigned char *> drawMandelbrot()
{
/* screen ( integer) coordinate */
int iX, iY;
double Cx, Cy;
const double CxMin = -2.5;
const double CxMax = 1.5;
const double CyMin = -2.0;
const double CyMax = 2.0;
double PixelWidth = (CxMax - CxMin) / iXmax;
double PixelHeight = (CyMax - CyMin) / iYmax;
int Index = 0;
const int IterationMax = 200;
unsigned char color[3];
vector<unsigned char *> rows(MaxIndex);
double Zx, Zy;
double Zx2, Zy2;
int Iteration;
const double EscapeRadius = 2;
double ER2 = EscapeRadius * EscapeRadius;
for (iY = 0; iY < iYmax; iY++)
{
Cy = CyMin + iY * PixelHeight;
if (fabs(Cy) < PixelHeight / 2)
Cy = 0.0; /* Main antenna */
for (iX = 0; iX < iXmax; iX++)
{
Cx = CxMin + iX * PixelWidth;
/* initial value of orbit = critical point Z= 0 */
Zx = 0.0;
Zy = 0.0;
Zx2 = Zx * Zx;
Zy2 = Zy * Zy;
/* */
for (Iteration = 0; Iteration < IterationMax && ((Zx2 + Zy2) < ER2); Iteration++)
{
Zy = 2 * Zx * Zy + Cy;
Zx = Zx2 - Zy2 + Cx;
Zx2 = Zx * Zx;
Zy2 = Zy * Zy;
};
/* compute pixel color (24 bit = 3 bytes) */
if (Iteration == IterationMax)
{ /* interior of Mandelbrot set = black */
color[0] = 0;
color[1] = 0;
color[2] = 0;
}
else
{ /* exterior of Mandelbrot set = white */
color[0] = 255; /* Red*/
color[1] = 255; /* Green */
color[2] = 255; /* Blue */
};
rows[Index] = color;
Index++;
}
}
return rows;
}
Here is function to save it to file:
void saveToFile(vector<unsigned char *> matrix, char *filename)
{
char *comment = (char *)"# "; /* comment should start with # */
FILE *file;
file = fopen(filename, "wb"); /* b - binary mode */
fprintf(file, "P6\n %s\n %d\n %d\n %d\n", comment, iXmax, iYmax, MaxColorComponentValue);
for (int Index = 0; Index < MaxIndex; Index++)
{
fwrite(matrix[Index], 1, 3, file);
}
fclose(file);
}
Some global values and main loop:
const int iXmax = 1000;
const int iYmax = 1000;
const int MaxColorComponentValue = 255;
int const MaxIndex = (iXmax * iYmax) - 1;
int main()
{
clock_t start = clock();
vector<unsigned char *> image = drawMandelbrot();
clock_t stop = clock();
cout << (double(stop - start) / CLOCKS_PER_SEC) << " seconds\n";
char *filename = (char *)"new2.ppm";
saveToFile(image,filename);
return 0;
}
Problem is that generateMandelbrot() returns matrix like this:
image matrix
but it should be vector of elements looks like this which is actually color value:
color char
I know the problems is with color and image values types, but have any idea how it should look like.
Thanks!

Answer 1

This:
rows[Index] = color;
Is assigning the unsigned char * in your vector to the same array every time!
In other words it's like if I sell you ten cars and deliver the keys but they are all identical keys to the same car. Wouldn't you be upset?
Change your variables to use std::array:
using Color = std::array<unsigned char, 3>;
Color color;
vector<Color> rows(MaxIndex);
Now you have a vector of triples (Colors), instead of a vector of pointers that all point at the same triple.