line intersection formula from GraphicsGems; started Segment class

src/Segment.cpp

@@ -0,0 +1,37 @@
+#include "Segment.hpp"
+
+Segment::Segment() : Segment({0, 0}, {0, 0}) {};
+
+Segment::Segment(const glm::vec2& location) : Segment(location, location) {};
+
+Segment::Segment(const glm::vec2& start, const glm::vec2& end) : start(start), end(end) {};
+
+glm::vec2 Segment::get_intersection()
+{
+}
+
+float Segment::get_dx()
+{
+    return end.x - start.x;
+}
+
+float Segment::get_length()
+{
+    return glm::distance(start, end);
+}
+
+void Segment::move(const glm::vec2& delta)
+{
+    start += delta;
+    end += delta;
+}
+
+glm::vec2 Segment::get_center()
+{
+}
+
+std::ostream& operator<<(std::ostream& out, const Segment& segment)
+{
+    out << "{(" << segment.start.x << ", " << segment.start.y << "), (" << segment.end.x << ", " << segment.end.y << ")}";
+    return out;
+}

src/Segment.hpp

@@ -0,0 +1,27 @@
+#ifndef Segment_h_
+#define Segment_h_
+
+#include <ostream>
+
+#include "glm/vec2.hpp"
+#include "glm/geometric.hpp"
+
+struct Segment
+{
+
+    glm::vec2 start, end;
+
+    Segment();
+    Segment(const glm::vec2&);
+    Segment(const glm::vec2&, const glm::vec2&);
+    glm::vec2 get_intersection();
+    float get_dx();
+    float get_length();
+    void move(const glm::vec2&);
+    glm::vec2 get_center();
+
+};
+
+std::ostream& operator<<(std::ostream&, const Segment&);
+
+#endif

src/extension.cpp

@@ -11,6 +11,70 @@ void sfw::set_magnitude(glm::vec2& vector, float magnitude)
     vector = glm::normalize(vector) * magnitude;
 }
 
+bool sfw::lines_intersect(const Segment& segment_a, const Segment& segment_b)
+{
+    glm::vec2 intersection;
+    return lines_intersect(segment_a, segment_b, intersection);
+}
+
+/*
+  from http://www.realtimerendering.com/resources/GraphicsGems/gemsii/xlines.c
+*/
+bool sfw::lines_intersect(const Segment& segment_a, const Segment& segment_b, glm::vec2& intersection)
+{
+    float x1 = segment_a.start.x, y1 = segment_a.start.y, x2 = segment_a.end.x,
+        y2 = segment_a.end.y, x3 = segment_b.start.x, y3 = segment_b.start.y,
+        x4 = segment_b.end.x, y4 = segment_b.end.y;
+
+    float a1, a2, b1, b2, c1, c2; // Coefficients of line eqns.
+    float r1, r2, r3, r4; // 'Sign' values
+    float denom, num; // Intermediate values
+
+    // Compute a1, b1, c1, where line joining points 1 and 2 is "a1 x  +  b1 y  +  c1  =  0"
+    a1 = y2 - y1;
+    b1 = x1 - x2;
+    c1 = x2 * y1 - x1 * y2;
+
+    // Compute r3 and r4
+    r3 = a1 * x3 + b1 * y3 + c1;
+    r4 = a1 * x4 + b1 * y4 + c1;
+
+    // Check signs of r3 and r4. If both point 3 and point 4 lie on same side of
+    // line 1, the line segments do not intersect
+    if (r3 != 0 && r4 != 0 && std::copysign(1, r3) == std::copysign(1, r4))
+    {
+        return false;
+    }
+
+    // Compute a2, b2, c2
+    a2 = y4 - y3;
+    b2 = x3 - x4;
+    c2 = x4 * y3 - x3 * y4;
+
+    // Compute r1 and r2
+    r1 = a2 * x1 + b2 * y1 + c2;
+    r2 = a2 * x2 + b2 * y2 + c2;
+
+    // Check signs of r1 and r2. If both point 1 and point 2 lie on same side
+    // of second line segment, the line segments do not intersect
+    if (r1 != 0 && r2 != 0 && std::copysign(1, r1) == std::copysign(1, r2))
+    {
+        return false;
+    }
+
+    // Line segments intersect: compute intersection point
+    denom = a1 * b2 - a2 * b1;
+    if (denom == 0)
+    {
+        return false;
+    }
+    num = b1 * c2 - b2 * c1;
+    intersection.x = num / denom;
+    num = a2 * c1 - a1 * c2;
+    intersection.y = num / denom;
+    return true;
+}
+
 Box sfw::get_texture_box(SDL_Texture* texture)
 {
     int w, h;

src/extension.hpp

@@ -9,6 +9,7 @@
 #include <iomanip>
 #include <stdexcept>
 #include <map>
+#include <cmath>
 
 #include "SDL.h"
 #include "SDL_image.h"
@@ -19,6 +20,7 @@
 #include "glm/gtx/vector_angle.hpp"
 
 #include "Box.hpp"
+#include "Segment.hpp"
 #include "filesystem.hpp"
 
 namespace sfw
@@ -27,6 +29,8 @@ namespace sfw
 
     glm::vec2 get_step(glm::vec2, glm::vec2, float);
     void set_magnitude(glm::vec2&, float);
+    bool lines_intersect(const Segment&, const Segment&);
+    bool lines_intersect(const Segment&, const Segment&, glm::fvec2&);
     Box get_texture_box(SDL_Texture*);
     void fill_texture(SDL_Renderer*, SDL_Texture*, Uint8, Uint8, Uint8, Uint8 = 0xff);
     void fill_texture(SDL_Renderer*, SDL_Texture*, SDL_Texture*);