Geometry shader: generate tris facing the camera

 - Modified the geometry shader to generate triangles that are oriented
   to sit on the camera plane.

 - Made the geometry shader actually use the "geometry_thickness"
   uniform value.

l_system_3d.c

@@ -16,7 +16,7 @@
 #define DEFAULT_WINDOW_WIDTH (800)
 #define DEFAULT_WINDOW_HEIGHT (600)
 #define DEFAULT_FPS (60.0)
-#define DEFAULT_GEOMETRY_THICKNESS (0.5)
+#define DEFAULT_GEOMETRY_THICKNESS (0.125)
 
 static ApplicationState* AllocateApplicationState(void) {
   ApplicationState *to_return = NULL;
@@ -325,9 +325,8 @@ static int WaitNextFrame(ApplicationState *s) {
 
 static int RunMainLoop(ApplicationState *s) {
   glEnable(GL_DEPTH_TEST);
-  // TODO: Re-enable face culling after getting the geometry shader working.
-  //glEnable(GL_CULL_FACE);
-  //glCullFace(GL_BACK);
+  glEnable(GL_CULL_FACE);
+  glCullFace(GL_BACK);
   glClearColor(0, 0, 0, 1.0);
   while (!glfwWindowShouldClose(s->window)) {
     s->frame_start = glfwGetTime();

pipes_shader.geom

@@ -1,6 +1,6 @@
 #version 330 core
 layout (lines) in;
-layout (line_strip, max_vertices = 4) out;
+layout (triangle_strip, max_vertices = 3) out;
 
 in VS_OUT {
   vec3 position;
@@ -28,35 +28,44 @@ float timeOffset() {
   return (sin(shared_uniforms.current_time * 3) + 1.0) * 0.5;
 }
 
+// Takes a forward-facing vector and returns a vector that points to its right,
+// along the camera plane.
+vec3 cameraRight(vec3 position, vec3 forward) {
+  vec3 to_camera = shared_uniforms.camera_position.xyz - position;
+  return normalize(cross(forward, to_camera));
+}
+
 void main() {
-  gs_out.forward = normal * gs_in[0].forward;
-  gs_out.up = normal * gs_in[0].up;
-  gs_out.right = normal * gs_in[0].right;
+  gs_out.forward = normalize(normal * gs_in[0].forward);
+  gs_out.up = normalize(normal * gs_in[0].up);
+  gs_out.right = normalize(normal * gs_in[0].right);
   gs_out.color = gs_in[1].color;
 
   mat4 projView = shared_uniforms.projection * shared_uniforms.view;
   vec4 pos_tmp = model * vec4(gs_in[0].position, 1);
-  pos_tmp += normalize(vec4(gs_out.forward, 1.0)) * timeOffset();
-  gs_out.frag_position = pos_tmp.xyz;
+  vec3 frag_pos_start = pos_tmp.xyz;
+  gs_out.frag_position = frag_pos_start;;
   gl_Position = projView * pos_tmp;
   EmitVertex();
 
   pos_tmp = model * vec4(gs_in[1].position, 1);
-  pos_tmp += normalize(vec4(gs_out.forward, 1.0)) * timeOffset();
-  gs_out.frag_position = pos_tmp.xyz;
+  vec3 frag_pos_end = pos_tmp.xyz;
+  gs_out.frag_position = frag_pos_end;
   gl_Position = projView * pos_tmp;
   EmitVertex();
-  EndPrimitive();
 
-  // Add a second primitive: a line starting at each midpoint and pointing in
-  // the "up" direction.
+  // TODO (next): Continue the same thing, except put points to the left and
+  // right of both endpoints rather than a single triangle point to the left of
+  // the midpoint.
+
+  // The third vertex is in the "up" direction from the midpoint of the
+  // original segment.
   vec3 dir = gs_in[1].position - gs_in[0].position;
   vec3 midpoint = gs_in[0].position + dir * 0.5;
-  pos_tmp = model * vec4(midpoint, 1.0);
-  gs_out.frag_position = pos_tmp.xyz;
-  gl_Position = projView * pos_tmp;
-  EmitVertex();
-  pos_tmp = model * vec4(midpoint + gs_out.up * (length(dir) * 0.5), 1);
+  pos_tmp = model * vec4(midpoint, 1);
+  vec3 plane_right = cameraRight(pos_tmp.xyz, gs_out.forward);
+  pos_tmp.xyz = pos_tmp.xyz - plane_right * (length(frag_pos_end -
+    frag_pos_start) * shared_uniforms.geometry_thickness);
   gs_out.frag_position = pos_tmp.xyz;
   gl_Position = projView * pos_tmp;
   EmitVertex();