Understanding Using ImageView Matrix

Question

I know that SO is full of Matrix questions, but I cannot find a question where this is fully explained. I assume that any ImageView has a matrix that is responsible for scaling, rotation and position. But why can't I rotate the image using the matrix like this:

ImageView img = (ImageView)findViewById(R.id.some_imageview); img.setScaleType(ScaleType.Matrix); Rect bounds = img.getDrawable.getBounds(); img.getImageMatrix().postRotate(180f, bounds.width() / 2, bounds.height() / 2); 

several answers suggest doing this as follows:

 ImageView img = (ImageView)findViewById(R.id.some_imageview); img.setScaleType(ScaleType.Matrix); Rect bounds = img.getDrawable.getBounds(); Matrix rotationMatrix = new Matrix(); rotationMatrix.postRotate(180f, bounds.width() / 2, bounds.height() / 2); img.setImageMatrix(rotationMatrix); 

WHY do I have to create a new matrix every time I want to rotate? Also, if I install the Matrix from the second example, why doesn't it rotate again (to its original degree) if I install rotationMatrix again? If I want to get an initial degree, I can set a simple matrix. but again, I don’t understand why

 img.getImageMatrix().postRotate(180f, bounds.width() / 2, bounds.height() / 2); 

will not work.

Note. I also tried the setRotate method without any differences

EDIT: due to comment

I asked why I have to create a new matrix every time, which implies the question of why I cannot use the Matrix in place. I also suspect that it was (which actually won't happen either):

 ImageView img = (ImageView)findViewById(R.id.some_imageview); img.setScaleType(ScaleType.Matrix); Rect bounds = img.getDrawable.getBounds(); Matrix rotationMatrix = new Matrix(); rotationMatrix.postRotate(180f, bounds.width() / 2, bounds.height() / 2); img.setImageMatrix(rotationMatrix); //works until here. //Then after that successful call //assumed to get my Matrix back, which is rotated by 180 degrees Matrix matrix = img.getImageMatrix(); Rext bounds = img.getDrawable().getBounds() //rotate again at 90 degree. It should be now rotated 270 degrees (180 from before, plus 90 now) matrix.postRotate(90f, bounds.width() / 2, bounds.height() / 2); //unfortunately NO effect! img.setImageMatrix(matrix); 

Answer 1

Considering it looks like the setImageMatrix () method uses matrix calculation. Using this matrix after (that is, after it is received by the getter) will have no effect if the calculation is performed in the setter method.