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# Matrix inverse

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Am I correct in saying that a matrix multiplied by its inverse = an identity matrix? Then why don''t I get an identity matrix back when I use D3DXMatrixInverse and D3DXMatrixDeterminant to calculate the inverse then multiply the two together? My origional matrix equals camera view * a projection matrix which is then transposed.

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2 * -2 = -4; NOT 0

Just qhat I guess.

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The inverse of 2 is 0.5 meaning 2 * 0.5 = 1 which you could say is the identity matrix of a 1x1 matrix.

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then I''m just me = dumb

.lick

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Well to be fair, numbers have additive inverses as well as multiplicative inverses.

2 and -2 are additive inverses.
2 and 0.5 are multiplicative inverses.

Generally, when referring to matrices, you''re referring to the multiplicative inverse (the additive inverse isn''t as useful, especially with 3D work).

Josh

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There are a couple of things to remember here. First, any square matrix which has a determinant of 0 has an undefined inverse. Second, double-check the order in which you multiply the inverse and the original.

It should be: A*inv(A) = I

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Well I have got my code working. I put an extra unneeded transformation somewhere though that still doesn''t clear up why the previous part didn''t work considering none of the checking code has been changed.

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