  The dependence of the bispectrum on the size and shape of the triangle
contains a wealth of cosmological information. Here we consider a triangle
parameterization which allows us to separate the size and shape dependence. We
have implemented an FFT based fast estimator for the three dimensional (3D) bin
averaged bispectrum, and we demonstrate that it allows us to study the
variation of the bispectrum across triangles of all possible shapes (and also
sizes). The computational requirement is shown to scale as $\sim N_{\rm
g}^3~\log{N_{\rm g}^3}$ where $N_g$ is the number of grid points along each
side of the volume. We have validated the estimator using a non-Gaussian field
for which the bispectrum can be analytically calculated. The estimated
bispectrum values are found to be in good agreement ($< 10 \%$ deviation) with
the analytical predictions across much of the triangle-shape parameter space.
We also introduce linear redshift space distortion, a situation where also the
bispectrum can be analytically calculated. Here the estimated bispectrum is
found to be in close agreement with the analytical prediction for the monopole
of the redshift space bispectrum.
