How to map java's AffineTransform to android's Matrix?

Question

Suppose I initialize an AffineTransform as below:

AffineTransform af = new AffineTransform(2, 3, 4, 5, 6, 7);

How would I create an equivalent Matrix using a

Answer 1

From AffineTransform's javadoc:

[ x']   [  m00  m01  m02  ] [ x ]   [ m00x + m01y + m02 ]
[ y'] = [  m10  m11  m12  ] [ y ] = [ m10x + m11y + m12 ]
[ 1 ]   [   0    0    1   ] [ 1 ]   [         1         ]

Not sure, but perhaps

Matrix m = new Matrix();
m.setValues(new float[]{2,3,4,5,6,7,0,0,1});

EDIT: Commenter points out that the order should be

m.setValues(new float[]{2,4,6,3,5,7,0,0,1});

Answer 2

The order provided in AffineTransform is provided as:

java.awt.geom.AffineTransform.AffineTransform(float m00, float m10, float m01, float m11, float m02, float m12)

Constructs a new AffineTransform from 6 floating point values representing the 6 specifiable entries of the 3x3 transformation matrix.

Parameters:
m00 the X coordinate scaling element of the 3x3 matrix
m10 the Y coordinate shearing element of the 3x3 matrix
m01 the X coordinate shearing element of the 3x3 matrix
m11 the Y coordinate scaling element of the 3x3 matrix
m02 the X coordinate translation element of the 3x3 matrix
m12 the Y coordinate translation element of the 3x3 matrix

An example implementation:

new AffineTransform(
    q0, q1, q2,
    q3, q4, q5);

counter-intuitively yields:

[  m00  m01  m02  ]   [ q0  q2  q4 ]
[  m10  m11  m12  ] = [ q1  q3  q5 ]
[   0    0    1   ]   [  0   0   1 ]

To accomplish the same result with android.graphics.Matrix:

Matrix m = new Matrix();
m.setValues(new float[] {
    q0, q2, q4,
    q1, q3, q5,
     0,  0,  1
}

I think Matrix's setValues() method documentation could be improved; it should reflect that the order of its parameters is:

void android.graphics.Matrix.setValues(float[] values)

Copy 9 values from the array into the matrix. Depending on the implementation of Matrix, these may be transformed into 16.16 integers in the Matrix, such that a subsequent call to getValues() will not yield exactly the same values.

The values are provided into the 3x3 matrix in the following order:

float[] {  
    m00, m01, m02,
    m10, m11, m12,
    m20, m21, m22
}

Where:

m00 the X coordinate scaling element of the 3x3 matrix (Matrix.MSCALE_X)
m01 the X coordinate shearing element of the 3x3 matrix (Matrix.MSKEW_X)
m02 the X coordinate translation element of the 3x3 matrix (Matrix.MTRANS_X)
m10 the Y coordinate shearing element of the 3x3 matrix (Matrix.MSKEW_Y)
m11 the Y coordinate scaling element of the 3x3 matrix (Matrix.MSCALE_Y)
m12 the Y coordinate translation element of the 3x3 matrix (Matrix.MTRANS_Y)
m20 the first perspective element of the 3x3 matrix (Matrix.MPERSP_0)
m21 the second perspective element of the 3x3 matrix (Matrix.MPERSP_1)
m22 the third perspective element of the 3x3 matrix (Matrix.MPERSP_2)

Answer 3

This is a demo Java code:

 public static float[] createMatrixValues(AffineTransform Tx) {
    double[] at = new double[9];
    Tx.getMatrix(at);
    float[] f = new float[at.length];
    f[0] = (float) at[0];
    f[1] = (float) at[2];
    f[2] = (float) at[4];
    f[3] = (float) at[1];
    f[4] = (float) at[3];
    f[5] = (float) at[5];
    f[6] = 0;
    f[7] = 0;
    f[8] = 1;
    return f;
}

and to create the Matrix object:

public static Matrix createMatrixObj(AffineTransform Tx) {
    Matrix m = new Matrix();
    m.reset();
    m.setValues(createMatrix(Tx));
    return m;
}

I hope this helper methods solves your question.