Alpha blending for better text visibility

In poisson_reconstruct.py is the:

IM_TOP -> text instance
IM_BACK -> background image

There says:

combine images using poission editing.
IM_TOP and IM_BACK should be of the same size.

But what images script combines?

Also in the poisson_reconstruct.py what is these gxs,gys,gxd,gyd? This is X and Y coordinates of something?

[gxs,gys] = get_grads(ims)
[gxd,gyd] = get_grads(imd)

Is the alpha blend done here?

elif mode=='blend': # from recursive call:
           # just do an alpha blend
           gx = gxs+gxd
           gy = gys+gyd

When I replace 'blend' instead 'max' in function blit_images(), nothing happens?