The notation B * D - T is simply there to denote the threshold, but nevertheless combining B * D - T can done. A convolution with a constant yields another constant[1]. You can find another constant T' such that T' * D = T, and with the linearity of convolution operator, you get B' = B - T'.
Combining two kernels is straightforward indeed. You just convolve them together. This is possible because convolution is commutative.
The problem is when the thresholding operation is introduced. This makes the whole thing nonlinear. So far, the best way to calculate it (now I'm talking about 2D convolution) get a derivative form out of the kernel in order to apply Kelvin-Stokes theorem by tracing along the contour of the thresholded convolution.
Combining two kernels is straightforward indeed. You just convolve them together. This is possible because convolution is commutative.
The problem is when the thresholding operation is introduced. This makes the whole thing nonlinear. So far, the best way to calculate it (now I'm talking about 2D convolution) get a derivative form out of the kernel in order to apply Kelvin-Stokes theorem by tracing along the contour of the thresholded convolution.
[1] https://math.stackexchange.com/questions/1054165/convolution...