(Uni homework) Draw filled shapes on a canvas in C

filledShapes.c

#include <stdio.h>
#include <stdlib.h>
#define NUMROW 40
#define NUMCOL 40

void drawCircle (char *canvas, int r, int xCenter, int yCenter);
void drawEllipse (char *canvas, int width, int height, int xCenter, int yCenter);
void drawRhombus (char *canvas, int a, int b, int xCenter, int yCenter);

void fillEmpty (char *canvas, int nRow, int nCol);
void drawLine (char *canvas, int nRow, int nCol, int x1, int x2, int y);
void drawPixel (char *canvas, int nRow, int nCol, int x, int y);
void printCanvas (char *canvas, int nRow, int nCol);

int main ()
{
    char *pCanvas = (char*) malloc(NUMROW * NUMCOL* sizeof(char));
    if (!pCanvas)
        {
            printf ("Errore nell'allocazione della memoria\n");
            exit (EXIT_FAILURE);
        }
    
    //Empty out the canvas
    fillEmpty (pCanvas, NUMROW, NUMCOL);

    //Draw a circle
    drawCircle (pCanvas, 15, 20, 20);

    //Print the result
    printCanvas (pCanvas, NUMROW, NUMCOL);

    //Empty out the canvas
    fillEmpty (pCanvas, NUMROW, NUMCOL);

    //Draw an ellipse
    drawEllipse (pCanvas, 15, 10, 20, 20);
    
    //Print the result
    printCanvas (pCanvas, NUMROW, NUMCOL);

    //Empty out the canvas
    fillEmpty (pCanvas, NUMROW, NUMCOL);

    //Draw a Rhombus
    drawRhombus (pCanvas, 15, 10, 20, 20);

    //Print the result
    printCanvas (pCanvas, NUMROW, NUMCOL);

    exit (EXIT_SUCCESS);
}

void drawRhombus (char *canvas, int a, int b, int xCenter, int yCenter)
{
    // a is the horizontal SEMIaxis, b the vertical SEMIaxis
    int x = a;

    for (int y = 0; y <= b; y++)
    {
        while (a * y + (x - a) * b > 0)
            x--;

        drawLine (canvas, NUMROW, NUMCOL, xCenter - x, xCenter + x, yCenter + y);
        drawLine (canvas, NUMROW, NUMCOL, xCenter - x, xCenter + x, yCenter - y);
    }
}

void drawEllipse (char *canvas, int a, int b, int xCenter, int yCenter)
{
    // a is the horizontal SEMIaxis, b the vertical SEMIaxis
    int a2 = a * a, b2 = b * b, a2b2 = a2 * b2;
    int x = a;

    for (int y = 0; y <= b; y++)
    {
        while (x * x * b2 + y * y * a2 > a2b2)
            x--;

        drawLine (canvas, NUMROW, NUMCOL, xCenter - x, xCenter + x, yCenter + y);
        drawLine (canvas, NUMROW, NUMCOL, xCenter - x, xCenter + x, yCenter - y);
    }
}

void drawCircle (char *canvas, int r, int xCenter, int yCenter)
{
    int r2 = r * r;
    int x = r;

    for (int y = 0; y <= r; y++)
    {
        while (x * x + y * y > r2)
            x--;
        
        drawLine (canvas, NUMROW, NUMCOL, xCenter - x, xCenter + x, yCenter + y);
        drawLine (canvas, NUMROW, NUMCOL, xCenter - x, xCenter + x, yCenter - y);
    }
}

void fillEmpty (char *canvas, int nRow, int nCol)
{
    for (int i = 0; i < nRow*nCol; i++)
        *canvas++ = ' ';
}

void drawLine (char *canvas, int nRow, int nCol, int x1, int x2, int y)
{
    if (x1 < 0 || x2 < 0 || x1 > nCol || x2 > nCol || y < 0 || y > nRow || x1 > x2)
        return;

    for (int i = x1; i <= x2 && i < nCol; i++)
        *(canvas + y * nCol + i) = '*';
}

void printCanvas (char *canvas, int nRow, int nCol)
{
    for (int i = 0; i < NUMROW*NUMCOL; i++)
    {
        if (i != 0 && i % NUMCOL == 0) printf ("\n");
        printf ("%c ", *canvas++);
    }
    printf ("\n");
}