Geometry Shader : GL_LINE_STRIP_ADJACENCY_EXT

Hi,

I am using GLSL Geometry Shader to manipulate line strip. The input type for my geometry shader is GL_LINE_STRIP_ADJACENCY_EXT, the output type is GL_TRIANGLE_STRIP. In my geometry shader, I use gl_PositionIn[3] which gives me a error : error C1068: array index out of bounds.

To my understanding, gl_PositionIn should have 4 dimensions, so it should be legal to call gl_PositionIn[3]. The call to gl_PositionIn[i] (i<3) is O.K. Am I doing something wrong here?

BTW, when I change the input type to GL_LINES_ADJACENCY_EXT, it works fine.

Thanks.