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Simple Matrix Question
February 21, 2003 07:20 PM
How does post matrix multiplication differ from regular matrix multiplication?
1,060
February 22, 2003 04:35 PM
And just what do you think 'regular' matrix multiplication is? There are 2 forms of matrix multiplication, pre- and post-multiplication. This is because unlike scalars order matters. For example
2*3=3*2=6
BUT
So A*B!=B*A
If you start with a matrix A, the C*A would be pre-multiplication and A*C would be post-multiplication. At a guess your 'regular' matrix multiplication is multiplying each element, i.e. using Matlab notation
Which isn't a valid matrix operation but does get used sometimes but in data processing (not matrix maths).
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[edited by - joanusdmentia on February 22, 2003 5:36:13 PM]
2*3=3*2=6
BUT
[ 1 2 ] * [ 4 3 ] = [ 8 5 ][ 3 4 ] [ 2 1 ] [ 20 13 ][ 4 3 ] * [ 1 2 ] = [ 13 20 ][ 2 1 ] [ 3 4 ] [ 5 8 ] So A*B!=B*A
If you start with a matrix A, the C*A would be pre-multiplication and A*C would be post-multiplication. At a guess your 'regular' matrix multiplication is multiplying each element, i.e. using Matlab notation
[ 4 3 ] .* [ 1 2 ] = [ 4 6 ][ 2 1 ] [ 3 4 ] [ 6 4 ] Which isn't a valid matrix operation but does get used sometimes but in data processing (not matrix maths).
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[edited by - joanusdmentia on February 22, 2003 5:36:13 PM]
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158
February 22, 2003 04:48 PM
You should provide some context. In computer graphics, pre-multiplication and post-multiplication are equivalent (with a simple modification --- Google for matrix transpose), and the choice is a matter of convenience.
Cédric
Cédric
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