Improve shadows for static walls

shader/wallShadow.geom

@@ -1,29 +1,47 @@
 #version 430 core
 layout (points) in;
-layout (triangle_strip, max_vertices = 4) out;
+layout (triangle_strip, max_vertices = 8) out;
 
-//in vec4 vBackPoss[];
+in vec4 vBackPoss[];
 uniform vec2 lightPos;
 uniform mat4 worldMat;
 
+void emitLine (vec2 pa)
+{
+    gl_Position = vec4 (pa, 0, 1);
+    EmitVertex();
+    gl_Position = vec4 (pa + (200 * (pa - lightPos)), 0 , 1);
+    EmitVertex();
+}
+
+float isLHS (vec2 startV, vec2 testV)
+{
+    return sign( -startV.x * testV.y + startV.y * testV.x);
+}
+
 void main()
 {
-  vec4 posaa = worldMat * vec4(gl_in[0].gl_Position.xy, 0 ,1);
-  vec4 posba = worldMat * vec4(gl_in[0].gl_Position.zw, 0 ,1);
-  vec2 posa = posaa.xy;
-  vec2 posb = posba.xy;
+  vec4 frontL4 = worldMat * vec4(gl_in[0].gl_Position.xy, 0 ,1);
+  vec4 frontR4 = worldMat * vec4(gl_in[0].gl_Position.zw, 0 ,1);
+  vec2 frontL = frontL4.xy;
+  vec2 frontR = frontR4.xy;
+  vec4 backR4 = worldMat * vec4(vBackPoss[0].xy, 0 ,1);
+  vec4 backL4 = worldMat * vec4(vBackPoss[0].zw, 0 ,1);
+  vec2 backL = backL4.xy;
+  vec2 backR = backR4.xy;
 
-//  vec2 posc = vBackPoss[0].xy;
-//  vec2 posd = vBackPoss[0].zw;
-
-  gl_Position = vec4 (posb, 0 , 1);
-  EmitVertex();
-  gl_Position = vec4 (posb + (200 * (posb - lightPos)), 0 , 1);
-  EmitVertex();
-  gl_Position = vec4 (posa, 0 , 1);
-  EmitVertex();
-  gl_Position = vec4 (posa + (200 * (posa - lightPos)), 0 , 1);
-  EmitVertex();
+  if (isLHS( lightPos - frontL, frontR - frontL) < 0)
+  {
+  emitLine (backL);
+  emitLine (frontL);
+  emitLine (frontR);
+  emitLine (backR);
+  }
+  else
+  {
+  emitLine (backR);
+  emitLine (backL);
+  }
 
   EndPrimitive();
 }

shader/wallShadow.vert

@@ -1,8 +1,8 @@
 #version 430 core
 layout (location = 0) in vec4 poss;
-//layout (location = 1) in vec4 backPoss;
+layout (location = 1) in vec4 backPoss;
 
-//out vec4 vBackPoss;
+out vec4 vBackPoss;
 
 //uniform vec2 winSize;
 //uniform float zoom;
@@ -13,5 +13,5 @@ layout (location = 0) in vec4 poss;
 void main()
 {
   gl_Position = poss;
-//  vBackPoss = backPoss;
+  vBackPoss = backPoss;
 }