C++ 2D Graphics: Flat Bottom Triangle Rasterization - c++

Question

I'm trying to build a simple 2D application on the Windows Console using C++ to display various primitives so I started with the most basic one: a triangle. I can display the triangle's outline and vertices correctly but I'm having problems with filling it. I have come across some rasterization algorithms presented here: http://www.sunshine2k.de/coding/java/TriangleRasterization/TriangleRasterization.html and decided I would go for the first approach. I copied the code for the flat bottom triangle case, however this doesn't appear to work correctly on my application and I have no idea what's the cause of the problem.
My result:
main()
screen.fillFlatBottomTriangle(30,10, 10, 140/4, 256/2, 140/4, block_char, FG_CYAN);
screen.drawTriangle(30,10, 10, 140/4, 256/2, 140/4, block_char, BG_DARK_GREY);
screen.drawPixel(30,10,block_char,FG_RED);
screen.drawPixel(10,140/4,block_char,FG_GREEN);
screen.drawPixel(256/2,140/4,block_char,FG_MAGENTA);
fillFlatBottomTriangle
//http://www.sunshine2k.de/coding/java/TriangleRasterization/TriangleRasterization.html
void Screen::fillFlatBottomTriangle(int x1, int y1, int x2, int y2, int x3, int y3, short character, short color)
{
float invslope1 = (x2 - x1) / (y2 - y1);
float invslope2 = (x3 - x1) / (y3 - y1);
float curx1 = x1;
float curx2 = x1;
for (int scanlineY = y1; scanlineY <= y2; scanlineY++)
{
this->drawLine((int)curx1, scanlineY, (int)curx2, scanlineY, character, color);
curx1 += invslope1;
curx2 += invslope2;
}
}
drawLine
// https://en.wikipedia.org/wiki/Digital_differential_analyzer_(graphics_algorithm)
void Screen::drawLine(int x1, int y1, int x2, int y2, short character, short color)
{
float delta_x = x2 - x1;
float delta_y = y2 - y1;
float step;
if (abs(delta_x) >= abs(delta_y))
step = abs(delta_x);
else
step = abs(delta_y);
delta_x = delta_x / step;
delta_y = delta_y / step;
float x = x1;
float y = y1;
for (int i = 1; i <= step; i++)
{
this->drawPixel((int)x, (int)y, character, color);
x += delta_x;
y += delta_y;
}
}
DrawTriangle
void Screen::drawTriangle(int x1, int y1, int x2, int y2, int x3, int y3, short character, short color)
{
this->drawLine(x1,y1, x2, y2, character, color);
this->drawLine(x1,y1, x3, y3, character, color);
this->drawLine(x2,y2, x3, y3, character, color);
}

Answer 1

See this part: (x2 - x1) / (y2 - y1)
Since all variables here are integers, this is integer division. That is, result of the division is being rounded towards 0. Assigning the result to a float doesn't change that.
To make this float division, you should convert at least one of the operands to float: (x2 - x1) / (float) (y2 - y1)
Your current code rounds one of the inverse slopes to 0, resulting in the vertical line at the left side. x values at the other side also don't increase fast enough, since that part was also rounded below.