draw does not use input image about piff HOT 3 CLOSED

I can't imagine a use case for wanting to add to the existing pixels. (Although that is an option for GalSim's drawImage using add_to_image=True.) So I think we just need to remove the copy()'s.

And add some unit tests to make sure an existing image is actually used if provided.

from piff.

The issue is in the model.draw method. It looks like in gsobject and optical models it takes a copy of the image and prof.drawImage draws onto that copy. Pixelgrid does a similar thing with the star setData method: it replaces a copied image array with the drawn pixelgrid image array.

OK so there are a few things we could change about this, and I'm not sure what we want:

1. We could remove the image.copy() piece from the model draws, and probably change a few lines in the fitting bit
2. Do we want the drawing to overwrite the pixels, or add to them? I think prof.drawImage overwrites the values (happy to be corrected)

thoughts, @rmjarvis and @jmeyers314 ?

from piff.

Done via PR #72. Thanks @cpd!

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