From the test below (R2013a), it appears that the FANBEAM tomographic projection function doesn't satisfy linearity/superposition with high accuracy
>> errfunc(F(I1)+F(I2) , F(I1+I2))
Conversley, the RADON tomographic projector shows a lot more linearity,
>> errfunc(R(I1)+R(I2) , R(I1+I2))
Why is this?