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Comment by rsp1984 | Hacker News Reader

rsp1984

I think I need a higher level explanation.

Is the goal to tweak the rendering parameters until it matches a given input image? If yes that would be inference, not learning. What am I missing?

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Yeah, you can do that as an inference step, like you described, and that has its uses without any learning required. But you can also make the output of the renderer depend (differentiably) on parameters apart from whatever you have as input - and those parameters can be learned via gradient descent. For example, you could make a conventional CNN take an image of say, a cube, as an input, and predict its 3D pose wrt to the camera, feed that prediction to the renderer, and have the renderer render the cube. Then, as training, e.g., a pixelwise error can be computed and backpropagated to the CNN. At the end of the process, ideally you would have a network capable of predicting the pose of a cube (and rendering a close match) in one shot, without iterative parameter tweaking.

It sounds like the description of an auto-encoder. You encode the image of a cube to a representation of 3D points, then there is a decoder that transforms it back to an image of a cube. Am I correct ?

Edit : nevermind, the article does state that it is similar to an autoencoder

So, in that case, we could learn without the need of a differentiable renderer neither graphics operators, right? Maybe the throughput of communicating with external renderer is too much when compared with iterative parameter tweaking happening inside of loss functions on CNN?

> we could learn without the need of a differentiable renderer neither graphics operators, right?

No. The sentence of the parent post "a pixelwise error can be computed and backpropagated to the CNN" is possible only if the renderer is differentiable.

Got it. So let's suppose we have an external renderer, then we could learn parameters to tweak rendered scene and then get rendered pixels, so we can calculate pixelwise errors from it and some target image we're trying to optimize for. In this way, do we still need differentiable renderer in your opinion?

Update:

It would require more training cycles and would not be as "atomic" as iterative tweaks but seems possible.

At same time, I wonder about making loss function talk with some external renderer would make it possible to mix both approaches.

How would you learn parameters to tweak the rendered scene if the renderer is not differentiable, and you can't backpropagate through the renderer to calculate the appropriate parameter adjustments from the pixelwise errors?

I suppose you theoretically could do it with some trial and error method or grid search or something like that, but it's going to be absolutely computationally unfeasible in the general case; the pixelwise errors only become practically useful if you have an uninterrupted differentiable/'backpropagatable' path from your parameters to the pixels.

Yes, throughput would be larger and we would loose the backprop path like you said, but it seems practical in some ways and actually guiding approaches like https://nv-tlabs.github.io/meta-sim/

That's one use case - quoting the article, "[...] analysis by synthesis where the vision system extracts the scene parameters and the graphics system renders back an image based on them. If the rendering matches the original image, the vision system has accurately extracted the scene parameters." - so this allows you to learn a image-analysis system from unlabeled data in an iterative manner, rendering what you "understood", looking for differences, and adjusting your understanding.

There are other use cases, for example, rendering "what-if" scenarios for reinforcement learning - e.g. you may have a model that predicts that if an agent does action sequence ABC then it will result in a world state X, and it can render an image Y which it expects to perceive. When it actually performs these actions, it actually obtains a different image Y' ... and needs to update its prediction, so it'd need differentiation/backpropagation through that rendering (and beyond) to figure out how it could have predicted the correct world state; so this feature is needed to allow learning the predictive model.

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