n. Doe February 2016
### 2D transformation, c++, rotate/translate/Scale/centering

I have my code working for translate, the only problem is I cannot seem to center it. I have tried to do "translate = translate * center" and that moves it to the upper right corner it seems like. I am not exactly sure what else I can do. I know my matrix functions are working properly, what am I missing here?

```
oid viewContext::calculate() {
std::cout << "calculate" << std::endl;
/*
* reinitialize these values
*/
translated.clear();
translated[0][0] = 1;
translated[1][1] = 1;
translated[2][2] = 1;
//translated = translated * center;
//if translate is called
if (whichMethod == trans) {
translated = translated * center;
translated = translated * S;
translated = translated * T;
}
```

the if statement in the code above is something I was trying out, I know its un-necessary.

```
matrix viewContext::model_to_device(const matrix& param) {
std::cout << "model to device" << std::endl;
//matrix hold_values = param;
matrix hold_values = translated * param;
return hold_values;
}
```

my methods that translate, rotate, etc look like this. well are formatted in this sort of way, they call calculate() at the end

```
void viewContext::rotator(double degree) {
std::cout << "Rotate" << std::endl;
double theta = ((degree * M_PI) / 180);
R[0][0] = cos(theta);
R[0][1] = -1 * sin(theta);
R[1][0] = sin(theta);
R[1][1] = cos(theta);
calculate();
```

}

```
viewContext::viewContext() :
translated(3, 3), inverted(3, 3), center(3, 3), T(3, 3), S(3, 3), R(3,
3) {
//clear matrix just incase it isn't empty
//Should be empty but this is precautionary
translated.clear();
inverted.clear();
/*
* Constant values
* echelon form
*/
T[0][0] = 1;
T[1][1] = 1;
T[2][2] = 1;
/*
* Constant values
* 300 = x, 400 = y to put at origin.
*/
cen
```

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```### Answers

Anedar February 2016
I assume that your matrix-multiplication is implemented correctly, then your translated matrix is build as

```
translated = 1 * center * S * T
```

where 1 is your initial identity-matrix.

I further assume from your comments that `param`

in `model_to_device()`

is a coordinate vector, resulting in

```
hold_values = translated * param
hold_values = center * S * T * param
```

As you might see, the first operator applied to param is actually T, then S, then center. So first you translate the coordinates, then scale them (with scaling center at (0,0) ) and then use your center-matrix which translates them by (width,height) and inverts y. So basically you first translate, then scale, then center.

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```#### Post Status

Asked in February 2016

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Voted 12

Answered 1 times
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