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2021-03-09 16:39:54 +08:00
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@@ -16,7 +16,7 @@ using supervised training.
_Supervised training_ is the natural starting point for **any** DL project. It always,
and we really mean **always** here, makes sense to start with a fully supervised
test using as little data as possible. This will be a pure overfitting test,
but if your network can't quicklyl converge and give a very good performance
but if your network can't quickly converge and give a very good performance
on a single example, then there's something fundamentally wrong
with your code or data. Thus, there's no reason to move on to more complex
setups that will make finding these fundamental problems more difficult.
@@ -28,7 +28,7 @@ and then increase the complexity of the setup.
A nice property of the supervised training is also that it's very stable.
Things won't get any better when we include more complex physical
models, or look at more complicated NN architectures.
models, or look at more complicated ANN architectures.
Thus, again, make sure you can see a nice exponential falloff in your training
loss when starting with the simple overfitting tests. This is a good
@@ -42,10 +42,10 @@ rough estimate of suitable values for $\eta$.
A comment that you'll often hear when talking about DL approaches, and especially
when using relatively simple training methodologies is: "Isn't it just interpolating the data?"
Well, **yes** it is! And that's exactly what the NN should do. In a way - there isn't
Well, **yes** it is! And that's exactly what the ANN should do. In a way - there isn't
anything else to do. This is what _all_ DL approaches are about. They give us smooth
representations of the data seen at training time. Even if we'll use fancy physical
models at training time later on, the NNs just adjust their weights to represent the signals
models at training time later on, the ANNs just adjust their weights to represent the signals
they receive, and reproduce it.
Due to the hype and numerous success stories, people not familiar with DL often have
@@ -54,34 +54,34 @@ and general principles in data sets (["messages from god"](https://dilbert.com/s
That's not what happens with the current state of the art. Nonetheless, it's
the most powerful tool we have to approximate complex, non-linear functions.
It is a great tool, but it's important to keep in mind, that once we set up the training
correctly, all we'll get out of it is an approximation of the function the NN
correctly, all we'll get out of it is an approximation of the function the ANN
was trained for - no magic involved.
An implication of this is that you shouldn't expect the network
to work on data it has never seen. In a way, the NNs are so good exactly
to work on data it has never seen. In a way, the ANNs are so good exactly
because they can accurately adapt to the signals they receive at training time,
but in contrast to other learned representations, they're acutally not very good
at extrapolation. So we can't expect an NN to magically work with new inputs.
but in contrast to other learned representations, they're actually not very good
at extrapolation. So we can't expect an ANN to magically work with new inputs.
Rather, we need to make sure that we can properly shape the input space,
e.g., by normalization and by focusing on invariants. In short, if you always train
your networks for inputs in the range $[0\dots1]$, don't expect it to work
with inputs of $[10\dots11]$. You might be able to subtract an offset of $10$ beforehand,
and re-apply it after evaluating the network.
As a rule of thumb: always make sure you
acutally train the NN on the kinds of input you want to use at inference time.
actually train the ANN on the kinds of input you want to use at inference time.
This is important to keep in mind during the next chapters: e.g., if we
want an NN to work in conjunction with another solver or simulation environment,
want an ANN to work in conjunction with another solver or simulation environment,
it's important to actually bring the solver into the training process, otherwise
the network might specialize on pre-computed data that differs from what is produced
when combining the NN with the solver, i.e _distribution shift_.
when combining the ANN with the solver, i.e _distribution shift_.
### Meshes and grids
The previous airfoil example use Cartesian grids with standard
convolutions. These typically give the most _bang-for-the-buck_, in terms
of performance and stability. Nonetheless, the whole discussion here of course
also holds for less regular convcolutions, e.g., a less regular mesh
also holds for less regular convolutions, e.g., a less regular mesh
in conjunction with graph-convolutions. You will typically see reduced learning
performance in exchange for improved stability when switching to these.