Changes between Version 43 and Version 44 of tutorial

Timestamp:

Jul 1, 2013, 11:37:06 AM (11 years ago)

Author:

leon

Comment:

Calculate normals

tutorial

@@ -677 +677 @@
   }}}
   Result is silhouette as we did't provided vertex normals. 
+ 5. Calculate vertex normals by averaging nearby faces normals that are calculated with 
+  [http://www.opengl.org/wiki/Calculating_a_Surface_Normal cross product]. We need to add normal array 
+  as a global variable
+  {{{
+  #!c
+  float       normal[MaxVertices*3];
+  }}}
+ and add [http://www.opengl.org/sdk/docs/man2/xhtml/glNormalPointer.xml glNormalPointer] with  [http://www.opengl.org/sdk/docs/man2/xhtml/glEnableClientState.xml glEnableClientState(GL_NORMAL_ARRAY)] to the last part of `display()` that now reads:
+  {{{
+  #!c
+  //glutSolidTeapot(0.6);
+  glNormalPointer(GL_FLOAT, 0, normal);
+  glVertexPointer(3, GL_FLOAT, 0, vertex);
+  glEnableClientState(GL_VERTEX_ARRAY);
+  glEnableClientState(GL_NORMAL_ARRAY);
+  glDrawElements(GL_TRIANGLES, faces*3, GL_UNSIGNED_INT, face);
+  glDisableClientState(GL_VERTEX_ARRAY);
+  glDisableClientState(GL_NORMAL_ARRAY);
+  }}}
+ For calculation of normals we quickly invent the following subroutine: 
+ [[Image(motorbike-shadow.png,right)]]
+  {{{
+#!c
+void calculate_normals()
+{
+  int i;
+  for(i = 0; i < vertices*3; ++i)
+    normal[i] = 0.0;
+  for(i = 0; i < faces; ++i)
+    {
+      int p1 = face[i*3]*3;
+      int p2 = face[i*3+1]*3;
+      int p3 = face[i*3+2]*3;
+      float ux = vertex[p3]-vertex[p1];
+      float uy = vertex[p3+1]-vertex[p1+1];
+      float uz = vertex[p3+2]-vertex[p1+2];
+      float vx = vertex[p2]-vertex[p1];
+      float vy = vertex[p2+1]-vertex[p1+1];
+      float vz = vertex[p2+2]-vertex[p1+2];
+      float nx = uy*vz - uz*vy;
+      float ny = uz*vx - ux*vz;
+      float nz = ux*vy - uy*vx;
+      float length = sqrt(nx*nx+ny*ny+nz*nz);
+      normal[p1] += nx/length;
+      normal[p1+1] += ny/length;
+      normal[p1+2] += nz/length;
+      normal[p2] += nx/length;
+      normal[p2+1] += ny/length;
+      normal[p2+2] += nz/length;
+      normal[p3] += nx/length;
+      normal[p3+1] += ny/length;
+      normal[p3+2] += nz/length;
+    }
+}
+}}}
+  called in `main()` right after `read_wavefront("motorBike.obj");`. Now mistery occurs with garbled part of motorbike. Where the problem is? Possible suggestions: pointer problems, normal calculation,  OpenGL bug, shader, data, ... Hint: Observe number of triangles versus number of vertices.