How to find center position of a bitmap that rotated and scaled with a matrix?

问题

I want to calculate center of a bitmap that is drawn on a canvas with a matrix, it can be rotated, scaled or translated with a arbitrary value. What is the easiest way to find center of the this bitmap on canvas?

回答1:

You need to apply the matrix to the coordinates of the center of bitmap.

If you use a canvas that has a transformation matrix, you can get the final matrix through Canvas.getMatrix()

If you draw the Bitmap on the Canvas with a Matrix : drawBitmap(bitmap, matrix, paint), then you you need to use that Matrix, or concatenate it to that of the Canvas (in the case it has one).

Then you can finally apply that matrix to the center of the matrix using Matrix.mapPoints.

Something like :

canvas.drawBitmap(bitmap, bitmap_transform, paint);
Matrix full_transform = new Matrix(canvas.getMatrix());
full_transform.postConcat(bitmap_transform);
float[] center = new float[] {bitmap.getHeight()/2.0, bitmap.getWidth()/2.0};
full_transform.mapPoints(center);

Alternatively, if you apply transformations to your bitmap without a matrix, you can use the full_transform.postRotate, full_transform.postScale, etc with the same values. In particular, if you draw your bitmap with drawBitmap(bitmap, left, top, paint) then you need to do a full_transform.postTranslate(left, top).

回答2:

If you're looking for the "easiest" way then just sticking with the translate rotate and scale functions would solve the problem. The reason that those were developed was so developers wouldn't have to do vector calculus for simple animations. Also the only value you would have to actually calculate in that sense it is the translate value after you take into account the original coordinates.

来源：https://stackoverflow.com/questions/29827151/how-to-find-center-position-of-a-bitmap-that-rotated-and-scaled-with-a-matrix