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POPython/numpy: Splitting an index into coordinates
 

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  1. POPython/numpy: Splitting an index into coordinates
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    <h2>Context</h2> <p>I'm working in Python and numpy, using an array to represent a 3D terrain. The array is of shape ((0.0, 0.0, 0.0),terrainLength) where terrainLength - for this question's purposes - is usually 400. I'm addressing by (x,z) and y is irrelevant for now. It seems that the best way to do this in numpy is as above: a one-dimensional array of length-three arrays (ie vertices). This means that if I want to pull out the vertex at (x=5,y=?,z=10), I have to reference terrain_array[x*terrainLength+z] ie terrain_array[2010].</p> <p>The "arbitrary base" mentioned in the title is terrainLength: I figure that numpy's coordinate system, if I'm switching between 1D numpy and 2D flat grid reference, is just like having a base-400 system where z is the rightmost column and x is the next left column.</p> <h2>Question</h2> <p>I'm writing a pair of functions to convert between the intuitive (x,?,z) coordinate system and numpy's one-dimensional coordinate system. As mentioned above, if I want to pull out the vertex at (x=5,y=?,z=10), I have to reference terrain_array[x*terrainLength+z] ie terrain_array[2010]. It seems like my functions will benefit from use of / and %, but I'm wondering if this is best practice or not, assuming positive integer coordinates:</p> <pre><code>def toGridCoords( matrix_index ): global terrainLength return floor(matrix_index/terrainLength), matrix_index%terrainLength def toArrayCoord( x, z ): global terrainLength return x*terrainLength+z </code></pre> <p>Thanks!</p> <p>PS - to pre-empt "why not have an array of shape ((0.0),3) which only contains the y-coord and assumes the x- and z-coords from position", I'm still very much writing the terrain system and that's caused all kinds of havoc already (made it difficult to introduce variable terrain resolution, for example...).</p>
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    1. COI am confused by the array shape: is this a 3 x terrainLength array? Is there one y value for a given (x, z) pair? Are you working with, say, a uniform 20x20 grid of (x, z) values?
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    2. COTo clarify: I'm working with a uniform 400x400 grid of (x,y,z) values, and I'm seeking to address a given tuple using (x, z) values. terrain_array[20] will return the tuple at (x=20,z=0), which will be (20.0,?,0.0). This is only for the generation function: after that, I'll be moving vertices manually and so the (x,z) pairs won't necessarily correlate. But in generation, it's perfectly workable. The key part is that the array is addressed with a 1D index, so I need to convert my 2D index into 1D. Converting back from 1D-2D will probably involve / and %, hence my question about best practice.
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      1. USDavid Lord
    3. COWhy aren't you using a 2-D array, then? If you need to swap from 1-D to 2-D, you can use np.reshape.
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