function defineG(input_nc, output_nc, ngf)
    local netG = nil
    if     opt.which_model_netG == "encoder_decoder" then netG = defineG_encoder_decoder(input_nc, output_nc, ngf)
    elseif opt.which_model_netG == "unet" then netG = defineG_unet(input_nc, output_nc, ngf)
    elseif opt.which_model_netG == "unet_128" then netG = defineG_unet_128(input_nc, output_nc, ngf)
    elseif opt.which_model_netG == "unet_512" then netG = defineG_unet_512(input_nc, output_nc, ngf)
    else error("unsupported netG model")
    end

    netG:apply(weights_init)

    return netG
end