Errors about invalid MSB and LSB in Verilog function

I have following function in my verilog module. I am getting error that invalid MSB and LSB for first_image_final and second_image_final.

Please help me out

function [7:0] difference;
input [20479:0] first_image_final;
input [20479:0] second_image_final;
input control_second_image;
input control_second_signal;
input control_first_signal;
reg [7:0] temp_difference;
integer i,j,k;

begin
for(j=0; j< window * window; j=j+8)
begin
for(i=0; i< window * window; i=i+8)
begin
temp_difference [7:0] = temp_difference [7:0] + (first_image_final [i+j+k+control_first_signal+7:i+j+k+control_first_signal] - second_image_final [i+j+k+control_second_signal+7:i+j+k+control_second_image]);
end
k = k + 5120;
end
difference [control_second_signal+7:control_second_signal] = temp_difference [7:0];
end

endfunction

Re: function in verilog

This is the problem!
difference [control_second_signal+7:control_second_signal] = temp_difference [7:0];

difference is defined as [7:0]
What ur trying to do is shift the return value as per input parameter.
Other problem is k is not initialize!