According to the documentation,"ENVI's shaded relief product is essentially Lambertian reflectance, calculated by taking the cosine of the incidence angle: cos(i) = cos(90-a)*cos(b) + sin(90-a)*sin(b)*cos(c-d) where i=incidence angle (angle between solar rays and the surface normal), a=solar elevation angle, b=slope, c=solar azimuth angle, d=aspect.". However, when I try to caculate the cos(i) by myself, I find the two results are not exact the same,especially in the the shadow region. My caculated result is usually larger then the the product in the shadow areas, sometimes the the two results varies greatly. What's more weird, the two results are mostly same in the light area. I use the slope and aspect products geting from topographic modeling. I can't understand why this situation happens, which result is the right cos(i)?