1. A method for visualizing 3D image data from tomographic imaging modalities using a rendering technique in which every pixel is calculated by integrating or summing along respectively one ray through a volume surrounded by the 3D image data, the method comprising: additionally introducing a peeling function into the integration or summation, by which, in the integration or summation, the 3D image data on a respective ray only contributes with its full data value to reducing the optical transparency beyond a value of an optical skin depth; and selecting the peeling function such that, in a transition region before the value of the optical skin depth is reached, the 3D image data on the ray still contributes to reducing the optical transparency with a fraction of its full data value such that there is a smooth profile, generated by the peeling function, when an outer layer is peeled off, the optical skin depth being d ⁡ ( l ) = ∫ 0 l ⁢ TF a ⁡ ( s ⁡ ( x ) ) ⁢ ⁢ ⅆ x , wherein TF a (s(x)) is an absorption component of a transfer function for visualizing the 3D image data.