the three materials (blue, yellow, green) in the object. The background is indicated in red. (b) Histogram of attenuation values of pixels at energy $E_1$ (above) and $E_2$ (below). Vertical lines show true material attenuations. (c) Attenuations of the materials (red dots) and computed attenuations by a reconstruction algorithm for each voxel (colors indicate the material they belong to).[]{data-label="fig:1"}](figure1.eps "fig:"){width="29.00000%"}]{} \[0.32\][ ![Elementary example of separation difficulties during segmentation. (a) Distribution of the three materials (blue, yellow, green) in the object. The background is indicated in red. (b) Histogram of attenuation values of pixels at energy $E_1$ (above) and $E_2$ (below). Vertical lines show true material attenuations. (c) Attenuations of the materials (red dots) and computed attenuations by a reconstruction algorithm for each voxel (colors indicate the material they belong to).[]{data-label="fig:1"}](figure2.eps "fig:"){width="35.00000%"} ![Elementary example of separation difficulties during segmentation. (a) Distribution of the three materials (blue, yellow, green) in the object. The background is indicated in red. (b) Histogram of attenuation values of pixels at energy $E_1$ (above) and $E_2$ (below). Vertical lines show true material attenuations. (c) Attenuations of the materials (red dots) and computed attenuations by a reconstruction algorithm for each voxel (colors indicate the material they belong to).[]{data-label="fig:1"}](figure3.eps 