Is Conv2 based on direct convolution?
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Is the conv2 function in Matlab 2023a, in particular the way it operates on 1d vectors, based on direct convolution or fft? I understand that you do not want to share all details on how this function is implemented, but for making use of this function in a scientific work where round-off errors and computational cost enter, it would be good to state with confidence that the method is either direct-based or fft-based.
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Walter Roberson on 4 Dec 2023
Historically, MATLAB implemented several different cases according to size and data type (and matters such as complex-valued or not.) The general case was done by direct convolution -- but possibly by calls into high speed libraries. The special cases involved decomposition into kernels and some kind of transform.
The source code for the direct convolution used to be visible. The decomposition etc., was something I do not recall ever seeing the source code for (that is, it might have been present in MATLAB source form in sufficiently old versions in the time before I thought to look for it.)