python - numpy broadcast from first dimension -


in numpy, there easy way broadcast 2 arrays of dimensions e.g. (x,y) , (x,y,z)? numpy broadcasting typically matches dimensions last dimension, usual broadcasting not work (it require first array have dimension (y,z)).

background: i'm working images, of rgb (shape (h,w,3)) , of grayscale (shape (h,w)). generate alpha masks of shape (h,w), , want apply mask image via mask * im. doesn't work because of above-mentioned problem, end having e.g. 

mask = mask.reshape(mask.shape + (1,) * (len(im.shape) - len(mask.shape)))

which ugly. other parts of code operations vectors , matrices, run same issue: fails trying execute m + v m has shape (x,y) , v has shape (x,). it's possible use e.g. atleast_3d, have remember how many dimensions wanted.

how use transpose:

(a.t + c.t).t

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