Function drawLine

src/OpenLoco/src/Graphics/SoftwareDrawingContext.cpp:957–1019

0x00452DA4

        // 0x00452DA4
        static void drawLine(const RenderTarget& rt, Ui::Point a, Ui::Point b, const PaletteIndex_t colour)
        {
            const auto bounding = Rect::fromLTRB(a.x, a.y, b.x, b.y);
            // Check to make sure the line is within the drawing area
            if (!rt.getUiRect().intersects(bounding))
            {
                return;
            }

            // Bresenham's algorithm

            // If vertical plot points upwards
            const bool isSteep = std::abs(a.y - b.y) > std::abs(a.x - b.x);
            if (isSteep)
            {
                std::swap(b.y, a.x);
                std::swap(a.y, b.x);
            }

            // If line is right to left swap direction
            if (a.x > b.x)
            {
                std::swap(a.x, b.x);
                std::swap(b.y, a.y);
            }

            const auto deltaX = b.x - a.x;
            const auto deltaY = std::abs(b.y - a.y);
            auto error = deltaX / 2;
            const auto yStep = a.y < b.y ? 1 : -1;
            auto y = a.y;

            for (auto x = a.x, xStart = a.x, length = 1; x < b.x; ++x, ++length)
            {
                // Vertical lines are drawn 1 pixel at a time
                if (isSteep)
                {
                    drawHorizontalLine(rt, colour, { y, x }, 1);
                }

                error -= deltaY;
                if (error < 0)
                {
                    // Non vertical lines are drawn with as many pixels in a horizontal line as possible
                    if (!isSteep)
                    {
                        drawHorizontalLine(rt, colour, { xStart, y }, length);
                    }

                    // Reset non vertical line vars
                    xStart = x + 1;
                    length = 0; // NB: will be incremented in next iteration
                    y += yStep;
                    error += deltaX;
                }

                // Catch the case of the last line
                if (x + 1 == b.x && !isSteep)