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# uglybdavis

Member Since 06 Mar 2012
Offline Last Active Jan 15 2016 02:06 PM

### In Topic: Vector4 W Component

02 January 2016 - 04:04 PM

Thanks guys! That pretty much answers all my questions!

### In Topic: Confused about row major matrix and pre multiplication

25 February 2015 - 01:18 AM

Thank you!

That has answered all of my questions. Thanks again, you have no idea how helpful that all was!

### In Topic: Confused about row major matrix and pre multiplication

25 February 2015 - 12:03 AM

If i understand what you said correctly, if two matrices (A and B) together; to find element 2,3 of the resulting matrix

Row major multiplication: take the dot product row 2 of matrix A and column 3 of matrix B

Column major multiplication: take the dot product of column 2 of matrix A, and row 3 of matrix B

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So, last question (i hope). Does the above mean that the formula for matrix product (For matrices F & G):

This will always take dot product the i-th row and j-th column of the two matrices, the only difference being that row major = FG, while column major = GF

OR, do the subscripts get a different meaning? Like in row major it's the i-th row and j-th column, where with column major it's the j-th row and i-th column?

### In Topic: Confused about row major matrix and pre multiplication

24 February 2015 - 07:48 PM

I see, thank you for clearing that up.

If i understand correctly, the transpose is a result of treating the vector as a column vector; hence the matrix is never actually transposed. Is that correct?

I have one more question regarding pre / post multiplication. I always assumed that the whole row v column order was what determined if pre or post multiplication is used (Due largely to my understanding of linear algebra being based on the differences in OpenGL & Direct X).

Now that i'm trying to build some more solid mathematical foundations i find my old understanding very chalenged.

If we have Matrix T (a translation matrix that translates by 10, 20 and 30)
and matrix S (A scale matrix that is a uniform scale of two)

And we want to concatinate the transformation so that the matrix translates first, and scales second we must calculate it as S x T.
This is assuming the same multiplication method as in the first post. I think because we scaled first then translated this is using post multiplication?

If correct, how come post multiplying two row major matrices works in math but Direct 3D uses pre-multiplication? Is this due to some implicit like the vector X matrix multiplication? Am i missing something trivial in my mental model of how this works?

Or perhaps is my pre / post terminology wrong when it comes to multiplying matrices?

### In Topic: Game Engine for Linux

20 February 2015 - 07:16 PM

@phantom

I totally forgot UE4 is on linux now!

@alnite

It's just in blender. You're gonna have to do some googling. Tough as phantom pointed out, UE4 man! Should be super quick to prototype with blueprints.

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