GL_ARB_texture_compression_rgtc http://www.opengl.org/registry/specs/gl/ARB/texture_compression_rgtc.txt GL_ARB_texture_compression_rgtc GL_COMPRESSED_RED_RGTC1 0x8DBB GL_COMPRESSED_SIGNED_RED_RGTC1 0x8DBC GL_COMPRESSED_RG_RGTC2 0x8DBD GL_COMPRESSED_SIGNED_RG_RGTC2 0x8DBE