factors with dynamic sized arguments #446
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How do I create a factor that has an argument with an arbitrary length? I've looked through the README and tutorials, but so far all I'm seeing is fixed size matrices. For context, I'm interested in eventually implementing a spline camera distortion model, so as a toy problem, I'm doing a 1D function fit right now to see how it might work. It'd be nice to have a variable number of knots and coefficients, but it seems that might require new code generation every time the value changes? Am I going about it wrong using Matrix for the coefficients? |
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Replies: 1 comment 9 replies
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You can't generate a function with arbitrary size, but you can actually create dynamic size factors, for example if you wanted to call a generated function N times, there are examples of dynamic size factors in One approach that might work for you is picking a maximum number of knots/coefficients, and setting any unused ones to zero? I'm not sure if what would cover what you need without knowing more about your setup. Otherwise the generated code is probably going to be specific to a fixed number of knots/coefficients, yeah |
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Oh, right, there is currently a limit up to 9x9 matrices. If you have more, you can split them into multiple arguments, e.g. you could have one argument per knot point |
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Ah ok. Does that limit apply for both Python and C++? |
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Yeah, it does |
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Will this limit cause any issues when solving for camera calibration coefficients (like if I have >9x9 knots)? I'm still not sure exactly how the camera calibration works, but as far as I can tell it's just treated like another Value? It looks like distortion coefficients are normally stored in a matrix. Sorry for all the follow up questions! I just want to make sure my understanding is correct. |
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For the cameras it's fine, because each camera is its own type (it's not a size limitation, it's that we don't currently have a place set aside to store additional metadata for the size of arbitrary size matrices or vectors). The list of types you can store in a Values is more-or-less the types in type_t here: symforce/lcmtypes/symforce_types.lcm Line 12 in d6afc27 Except that VectorX can't actually be stored |
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You can't generate a function with arbitrary size, but you can actually create dynamic size factors, for example if you wanted to call a generated function N times, there are examples of dynamic size factors in
symforce_factor_test.cc. I don't think this is what you want though, since I'm assuming you want to e.g. sample the spline, and compute other symbolic functions of the sampled point, and whatnot.One approach that might work for you is picking a maximum number of knots/coefficients, and setting any unused ones to zero? I'm not sure if what would cover what you need without knowing more about your setup. Otherwise the generated code is probably going to be specific to a fixed number…