graphics - In gouraud shading, what is the T-junction issure and how to demonstrate it with OpenGL -

i noticed here in gouraud shading part, said "t-junctions adjoining polygons can result in visual anomalies. in general, t-junctions should avoided".

it seems t-junction 3 surfaces in picture below share edges , point may have different normal vector due belongs different surfaces.

but effect when t-junction happened , how use opengl implement it? tried set different normal each vertex of each rectangle , put light in scene, however, didn't see strange in junction point a.

here code:

glcolor3f(1.0f, 0.0f, 0.0f); glbegin(gl_quads); glnormal3f(0, 0,1); glvertex3f(-5.0f, 5.0f, 0.0f); glnormal3f(0, 1,1); glvertex3f(5.0f, 5.0f, 0.0f); glnormal3f(1, 1,1); glvertex3f(5.0f, 0.0f, 0.0f); glnormal3f(0, -1,1); glvertex3f(-5.0f, 0.0f, 0.0f); glend();  glcolor3f(0.0f, 1.0f, 0.0f); glbegin(gl_quads); glnormal3f(1, 0,1); glvertex3f(-5.0f, 0.0f, 0.0f); glnormal3f(1, 2,1); glvertex3f(0.0f, 0.0f, 0.0f); glnormal3f(0, 0,1); glvertex3f(0.0f, -5.0f, 0.0f); glnormal3f(0, 1, 2); glvertex3f(-5.0f, -5.0f, 0.0f); glend();  glcolor3f(0.0f, 0.0f, 1.0f); glbegin(gl_quads); glnormal3f(1, 1, 3); glvertex3f(0.0f, 0.0f, 0.0f); glnormal3f(0, -2, 5); glvertex3f(5.0f, 0.0f, 0.0f); glnormal3f(-1, 1, 1); glvertex3f(5.0f, -5.0f, 0.0f); glnormal3f(1, -2, 0); glvertex3f(0.0f, -5.0f, 0.0f); glend(); 

the point light in (0, 0, 10) camera. result below has no visual anomaly think. maybe normals should kind of special?

is there wrong did? give me hints make happen?

t-junction bad gouraud shading , in geometry in general.

first remember goraud shading, method light interpolation used in fixed pipeline era light interpolated accross vertices, making mesh tesselation (the number , connectivity) of vertices directly affect shading. having t-junction give sudden discontinuity in how final interpolated color looks (keep in mind gouraud shading has other problems, under-sampling).

gouraud shading directly use vertex normals unlike phong shading, , note don't confuse phong shading phong lighting different

note case presenting t-junction won't notice shading problem because mesh not tessellated enough , (it seems) not using light. try testing on sphere t-junction.

regarding geometry t-junction considered degenerate case. because @ edge/polygon geometric mesh loses consistency, no longer have 2 edges connected @ ends, , lose polygon loop property (read: directed edges). it's tricky problem solve, solution triangulate polygons t-juction edge connected.

http://en.wikipedia.org/wiki/gouraud_shading