A Monte-Carlo ray-tracing method acting on a three-dimensional off-lattice reconstructed porous medium has been developed to simulate and understand the effect of the texture on the spectral emissivity of materials in the semitransparent domain. As the mean pore radius of the ceramics investigated is larger than the wavelength of the thermal radiation, the assumption that the ray trial follows the optical geometric Fresnel laws at each interface (matrix^ pore or pore^ matrix) is made. Both diffuse transmission and reflection parts of the normal incident rays are computed and the spectral emissivity is deduced by the use of Kirchhoff's laws. A discussion of the influence of the refractive index and the absorption coefficient on the radiative properties is given. A comparison between simulated data obtained on alumina ceramics and experimental results is also proposed.