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copied!<p>OpenGL front/back detection is based on winding, not the normal. The normal vector does not have any effect on whether the polygon is considered front or back facing.</p> <p>I think what you want to do is set the <code>GL_LIGHT_MODEL_TWO_SIDE</code> option of <a href="http://www.opengl.org/sdk/docs/man/xhtml/glLightModel.xml" rel="noreferrer">glLightModel</a>. </p> <blockquote> <p>GL_LIGHT_MODEL_TWO_SIDE</p> <p>params is a single integer or floating-point value that specifies whether one- or two-sided lighting calculations are done for polygons. It has no effect on the lighting calculations for points, lines, or bitmaps. If params is 0 (or 0.0), one-sided lighting is specified, and only the front material parameters are used in the lighting equation. Otherwise, two-sided lighting is specified. In this case, vertices of back-facing polygons are lighted using the back material parameters <strong>and have their normals reversed before the lighting equation is evaluated</strong>. Vertices of front-facing polygons are always lighted using the front material parameters, with no change to their normals. The initial value is 0.</p> </blockquote> <p>Note that this only applies to the fixed pipeline. </p> <p>===EDIT===</p> <p>Using custom shaders (GLES2.0 or OpenGL3+), then in the fragment shader you have access to the special boolean <code>gl_FrontFacing</code>. To emulate two sided lighting in a shader just test for <code>gl_FrontFacing</code> and multiply normal by negative one if <code>false</code>.</p>

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