Understanding matrices (in shader)

[Christi258]

Hi everyone,
I have problems to understand matrices, espacially when it comes to shader.
For example what does the function mul() exaclty do? When you multiply a float4 with float4x4 how can there result a float4 ?
And what does the fourth value of a vector exactly (w).
I don't expect an description here but perhaps a link to a tutorial or something like that would be nice.

[tothex]

Hi shaders are mostly math so its hard to get started
in hlsl:
struct_name.v_pos = mul(float4(v_pos, 1.0f), wvp_matrix)
we use mul to transform vector (v_pos) from 3D space to our 2d screen.

v_pos has x y z.
but for the shader vectors have 4 components: (x,y,z,w)

w=1 indicates that it is a vector with position only (vertex)
w=0 its a vector (which can only be rotated )

It results to a vector=[x,y,z,w] because you multiply each vector row by each matrix column
(check some linear algebra examples)

[BitPuffin]

Hey!

I recommend Khan Academy: https://www.khanacademy.org/math/linear-algebra excellent free video explanations on linear algebra (and other topics)!

While I didn't learn from that mainly, instead I read this book: http://gamemath.com/ but you are maybe not in the market for buying a book. If you are I really recommend it. It's good!

Hope that helps, good luck!

[Christi258]

Thanks, that should be enough to start

[Christi258]

we use mul to transform vector (v_pos) from 3D space to our 2d screen.

Does that mean if I use:
outPos = mul(v_pos, ViewProjectionWorldMatrix);
I have as result in outPos the 2D position on the screen of the vertex?

[tothex]

we use mul to transform vector (v_pos) from 3D space to our 2d screen.

Does that mean if I use:
outPos = mul(v_pos, ViewProjectionWorldMatrix);
I have as result in outPos the 2D position on the screen of the vertex?

its still a vector with 4 components with normalized values from -1 to 1
the gpu and the pixel shader do the rest.

some good shader books: (directx & opengl )

• Introduction to 3D Game Programming with DirectX 9.0c: A Shader Approach
• OpenGL SuperBible