Question

There is an ImageView and it has an Image, which has been zoomed and rotated.

See the picture:

The image is scaled by android.graphics.Matrix.

You can see there is red point which is the center point of the image, and also a blue border. How to calculate them?

(Update: I want to operate on the image of the girl inside the blue border, not the whole picture, that’s background)

Answer 1

There is a mathematical dependency between two. If there is other information available:
-assuming rectangle has right angles for all corners-.

center.X = (aCorner.X + oppositeCorner.X)/2;
center.Y = (aCorner.Y + oppositeCorner.Y)/2;

Where aCorner is a arbitrary corner and oppositeCorner is opposite corner to aCorner.

This was trivial, a little more hard work included to calculate borders (and a bit more of information; center position, width and the height of the picture and rotation angle).
Assuming image’s width is “w”, height is “h”, angle is “a”, and center “cX” and “cY”.
First corner;

length = sqrt(w^2+h^2)/2;
x = (length)*(cos(a)*(-w/length) - (h/length)*sin(a)) + cX;
y = (length)*(sin(a)*(-w/length) + (h/length)*cos(a)) + cY;

Second corner;

x = (length)*(cos(a)*(w/length) + sin(a)*(h/length)) + cX;
y = (length)*(cos(a)*(h/length) - sin(a)*(w/length)) + cY;

Third;

x = -(length)*(cos(a)*(-w/length) + (h/length)*sin(a)) + cX;
y = -(length)*(sin(a)*(-w/length) - (h/length)*cos(a)) + cY;

Fourth;

x = -(length)*(cos(a)*(w/length) - sin(a)*(h/length)) + cX;
y = (length)*(cos(a)*(h/length) - sin(a)*(w/length)) + cY;

Length is a half of diagonal of the rectangle. The inner part with cos and sin is result of trigonometric transformation:

sin(a+b) = sin(a)*cos(b) + cos(a)*sin(b)
[....]

And cX and cY is used to translate corners from a arbitrary coordinate system to a specific coordinate system.

I know, I know this was kind of overkill. Matrix class may have this functions on its own. I believe if it has, the method used in it can be broken into method I described here.

NOTE: Angle a -actually even sin(a) and cos(a), which is better- can be accessed via
Matrix.getValues(float[] values)

Most 2D matrices use this scheme:

| sin(a) 0       0    |
| 0      -cos(a) 0    |
| 0      0       scale|

I am not sure about particular implementation of Android API.

BTW, there may have been some signature errors up there so be careful.