In Android, What kind of transform does mapRect api of Matrix class performs?

Question

I would like to know about the function of mapRect api available under Matrix class in Android. If I have a sample Matrix A and Rectangle R, then for

RectF R = new RectF(t1,t2,t3,t4);

A.mapRect(R);

what kind of transformation is likely to happen to R. It would be more helpful if someone can illustrate the mapRect() api with some suitable examples.

Answer 1

Here's a very simple example:

Let's take a matrix:

Matrix matrix = new Matrix();

Set that matrix to scale everything twice as large:

matrix.setScale(2.0F, 2.0F);

Create a rectangle that is 10x10 with origin in upper left corner:

RectF rect = new RectF(0F, 0F, 10F, 10F);

So when we call

matrix.mapRect(rect);

the input rectangle we created is replaced with the output rectangle, which is the result of transforming the input:

rect.left = 0F;
rect.top = 0F;
rect.right = 20F;
rect.bottom = 20F;

There is another version of the method

matrix.mapRect(RectF dst, RectF src);

that does the same transform without affecting the input rectangle.

---

What is a matrix?

Consider a mirror. The mirror takes your image and creates a horizontally flipped version of your image.

Consider a microphone and an amplifier. They take your voice and create a louder version of your voice.

That's what a matrix is. It's a transformer. It takes an input and creates an output that is based on the input. So a matrix can transform a point, a rectangle, a circle, a polygon...

For more info, see my answer How does Matrix.postScale( sx, sy, px, py) work?

Also check out Affine transformations | Wikipedia. There is an awesome graphic that shows the different affine transforms and their effects.