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# Modelmatrix and translation/scale/rotation

## 4 posts in this topic

I think I missunderstand how to correctly apply translation/scale/rotation to my model matrix.

My SceneNode has a Mat4 called "mTransform". I get translation to work as expected, but applying rotation and scaling somehow causes the mesh not to be rendered on the screen.

Scale/Translate/RotateTransform corresponds to the glm:: methods.

Am I correct in the way I do this?

    void SceneNode::Scale(Vec3 scaleVec)
{
mTransform = ScaleTransform(mTransform, scaleVec);
}

void SceneNode::Translate(Vec3 translateVec)
{
mTransform = TranslateTransform(mTransform, translateVec);
}

void SceneNode::Rotate(Vec3 rotateVec, const float angle)
{
mTransform = RotateTransform(mTransform, angle, rotateVec);
}

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I don't know if those methods are correct, but on glm, you include

#include <glm\gtc\matrix_transform.hpp>

and then you can use the following methods

glm::translate(glm::mat4 startMatrix, glm::vec3 transVector);

glm::rotate(glm::mat4 startMatrix, float angle, glm::vec3 rotVector);

glm::scale(glm::mat4 startMatrix, glm::vec3 scaleVector);

which return the respective transformed 'startMatrix'.

They have the same functionality as the following deprecated OpenGL functions glTranslatef(), glRotatef(), and glScalef() respectively.

Also note that the glm::mat4 does not have to be a matrix of floats, it can be doubles, etc, which affect the arguments/parameters.

And i'm not sure, but i think the order in which you transform the matrices matters,for example translating the matrix and then rotating it is not the same as doing it the other way around.

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And i'm not sure, but i think the order in which you transform the matrices matters,for example translating the matrix and then rotating it is not the same as doing it the other way around.

This. Matrix don't work like conventional numbers, their multiplication is not commutative. ie, For matrix A and matrix B, A*B =/= B*A.

I think you had to reverse the order to get things working. ie, I have a vertex, a perspective matrix and a rotation. I want to rotate the vertex and then use the perspective projection on it, thus the order would be:

perspective * rotation * vertex

Scaling comes before a rotation otherwise you'd scale along the wrong axis (ie, deform your model), so in the multiplication you reverse it by putting it after the rotation.

Thus you'd have: perspective * rotation * scaling * vertex.

Edited by TheChubu
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Can't I combine all my translation, rotation and scaling into one model matrix? And then if I want to translate it further I could just do translate on the previous matrix the next frame?

OR do I have to save each translation, rotate and scale as separate matrices for the model? Edited by KaiserJohan
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I believe you can combine all the transformations on the same matrix, provided they are done in the right order, however if you wish to transform again, you must apply all transformations again on  an identity matrix (the default matrix).

Because if the order in which you apply the transformations matter, by applying another transformation on top of other you're effectively changing the order (eg: scale, rotate, translate and then another rotate).

I think the exception to this, is if you make the same type of transformation (example: scale, translate and then another translate), thnit would be like the last transformations combined (again: the last too translations could be combined as one).

But i ask that someone confirms this.

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