MATLAB conv2 giving wrong answer as compare to manual with Valid

When I use conv2 the 'valid' its gives negative answers.. like

A= 3 0 1 2 7 4

1 5 8 9 3 1

2 7 2 5 1 3

0 1 3 1 7 8

4 2 1 6 2 8

2 4 5 2 3 9

b= 1 0 -1

1 0 -1

1 0 -1

manually conv of A with b =

-5 -4 0 8

-10 -2 2 3

0 -2 -4 -7

-3 -2 - 3 -16

but MATLAB i get

5 4 0 -8

10 2 -2 -3

0 2 4 7

3 2 3 16

i cant understand the reason

Answer 1

[Walter Roberson]

You did not fliplr() the convolution kernel when you calculated by hand. MATLAB uses the convention that the convolution kernel is mirrored; this convention fits with the definition of integral convolution involving integral of f(tau)*g(t-tau) -- notice the implicit time reversal of g

[Raza]

Thank you.