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Some Terms

2.3.1 in the book
We normally model in world coordinates.
- This is probably a lie, we probably model in modeling coordinates
- or possibly object coordinates
  - Remember, I drew all my stars centered on the origin.
  - Then moved them into the world when I wanted them.
- Then use transformations to move things to world coordinates.
  - in the moving demo, these were the "last" transformations applied.
```
>
ctx.rotate(redStarTheta * Math.PI/180)
ctx.translate(w/4, 0)
Star(ctx, "red", 20) 
```
We normally move a window
- This allows us to look at different parts of the "world".
- Actually, we move the world around
- Again, this is done with transformation matrices
- In the move demo,
```
ctx.scale(worldScale, worldScale)
ctx.rotate(worldRotate*Math.PI/180)
ctx.translate(worldTx, worldTy)
```
Finally we map the window to the "viewport"
- This is how the window shows up on the canvas.
- This involves transformation matrices as well.
- ```
ctx.setTransform(1, 0, 0, -1, 0, height)
ctx.translate(width/2, height/2)
```
The really neat thing is,
- Matrix multiplication "preserves" the order of the transformations
- So we build up a single transformation matrix
- And apply it to all of the primitives.
- ALL these complex transformations are performed by a single vector matrix multiply (3x3 or 4x4) in the end.
- And the ability to "push" the current matrix reduces matrix matrix multipliciations!
This is the foundation for ALL graphics systems that I know of.
With 3d we will add to this a bit.