Arcpy script not working

I am very new to developing tool boxes using arcpy on Arcgis 10.2. I have written the code down to the execution stage but I am hooked up there. Please Help. It is a basic code for band difference operation.

import arcpy import envipy from arcpy import env from arcpy.sa import *

class Toolbox(object): def init(self): """Define the toolbox (the name of the toolbox is the name of the .pyt file).""" self.label = "Image Cal" self.alias = "My Calculator"

    # List of tool classes associated with this toolbox
    self.tools = [Tool]

class Tool(object): def init(self): """Define the tool (tool name is the name of the class).""" self.label = "Image Differencing" self.description = "This is a basic image calculator for differencing" self.canRunInBackground = True

def getParameterInfo(self):
    """Define parameter definitions"""
    #first parameter
    image1 = arcpy.Parameter (
                              displayName = "Input first Raster File",
                              name = "image1",
                              datatype = "Raster Layer",
                              parameterType = "Required",
                              direction = "Input"
                              )
    image2 = arcpy.Parameter (
                              displayName = "Input second Raster File",
                              name = "image2",
                              datatype = "Raster Layer",
                              parameterType = "Required",
                              direction ="Input"
                              )
    output = arcpy.Parameter (
                                    displayName = "Output Raster",
                                    name = "output",
                                    datatype = "Raster Layer",
                                    parameterType = "Required",
                                    direction = "Output"
                                    )
    params = [image1, image2, output]
    return params

def isLicensed(self):
    """Set whether tool is licensed to execute."""
    arcpy.CheckOutExtension("Spatial")
    return True

def updateParameters(self, parameters):
    """Modify the values and properties of parameters before internal
    validation is performed.  This method is called whenever a parameter
    has been changed."""
    image1 = params[0]
    image2 = params[1]
    output = params[2]


    return

def updateMessages(self, parameters):
    """Modify the messages created by internal validation for each tool
    parameter.  This method is called after internal validation."""
    return

def execute(self, parameters, messages):
    """The source code of the tool."""
    image1 = params[0].valueAsText
    image2 = params[1].valueAsText
    output = params[2].valueAsText

    output_map= arcpy.sa.Minus( image1, image2)



    return

Please do help me . you can also help by sending materials that you think might be of help to christad92@gmail.com.