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Problems with MRT in Fragment Shader

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Hi, every one! I'm currently trying to rendering front and back faces of a object to two textures which bind to a FBO during one pass. I successfully made it by the setting the gl_layer = layerid in geometry shader. (layerid is labeled by a calculation with the camera position and the normal of the vertex). The key sentence in my vertex shader vertex shader: float layer = dot( normalize(eyeSpaceNorm), -normalize(eyeSpacePos.xyz) ); gl_TexCoord[0].w = layer; geometry shader: gl_Layer = int(gl_TexCoordIn[0][0].w); fragment shader: Just simply output the color. And the result is correct, I did correctly get the front and back colors of the object during one pass. However, for some reasons, I have to do this by using the fragment shader. After carefully bind the multiple textures with the FBO, GLenum drawbuffers[2] = {GL_COLOR_ATTACHMENT0_EXT, GL_COLOR_ATTACHMENT1_EXT} glDrawBuffers(2, drawbuffers); vertex shader: float layer = dot( normalize(eyeSpaceNorm), -normalize(eyeSpacePos.xyz) ); gl_TexCoord[0].w = layer; fragment shader gl_FragData[layerId] = ..... However, I can only get the front face, There is a wierd output of the second texture. And if I changed the judge sentence, like if(depth>0.5) gl_FragData[0]=.. else gl_FragData[1]=.. The output of the two images is the seperate part of the front face, there is no back face. How did this happened?

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