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I am creating an android application for L-Systems and am getting differing line lengths after rotating my points with a rotation-matrix, which results in wonky fractals.

Example:

Misaligned fractal

As you can see, the length of the rotated lines varies, the line with a rotation of 120° is shorter than the line with a rotation of 240°.

.

For rotating the points i am using these Android Matrix Operations based on this answer here. I am rotating the end-point of a line around its start-point.

Code:

Matrix transform;

public Point rotate(Point angleP, Point p, float degrees) {
    // This rotates around the previous Point by degrees
    transform.setRotate(degrees, angleP.x, angleP.y);

    // Create new float[] to hold the rotated coordinates
    float[] pts = new float[2];

    // Initialize the array with our Coordinate
    pts[0] = p.x;
    pts[1] = p.y;

    // Use the Matrix to map the points
    transform.mapPoints(pts);

    // NOTE: pts will be changed by transform.mapPoints call
    // after the call, pts will hold the new cooridnates

    // Now, create a new Point from our new coordinates
    Point newPoint = new Point((int)pts[0], (int)pts[1]);

    // Return the new point
    return newPoint;
}

The corresponding points+rotations for the first five points based on the above code:

- forward by 30pt, angle is: 0.0°, rotate/translate  Point(384, 433) -> Point(384, 433)
- changed rotation by120.0 from 0.0->120.0
- forward by 30pt, angle is: 120.0°, rotate/translate  Point(384, 403) -> Point(409, 447)
- changed rotation by-120.0 from 120.0->0.0
- forward by 30pt, angle is: 0.0°, rotate/translate  Point(409, 417) -> Point(409, 417)
- changed rotation by-120.0 from 0.0->-120.0
- forward by 30pt, angle is: -120.0°, rotate/translate  Point(409, 387) -> Point(383, 431)
- changed rotation by120.0 from -120.0->0.0
- forward by 30pt, angle is: 0.0°, rotate/translate  Point(383, 401) -> Point(383, 401)

Debug-Log for the first five lines (up -> right -> up -> left -> up, same as the picture): 

line#     line-start  line-end         line-length

line0 from 384/463 to 384/433, length: 30.0
line1 from 384/433 to 409/447, length: 28.653097563788805
line2 from 409/447 to 409/417, length: 30.0
line3 from 409/417 to 383/431, length: 29.5296461204668
line4 from 383/431 to 383/401, length: 30.0

The Problem im having is that i want/need the resulting lines to be of equal length, but for some reason the line rotated by 120° is smaller than a line rotated by -120°.

My guess is that the differing line lengths are a result of rounding errors, but i dont know how i would start fixing this.

Any help or pointer would be appreciated.

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Annegrim
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1 Answers1

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Im not sure if i completely solved it, but it appears that there are two different classes for coordinate points in Android:

A integer coordinate point and a float coordinate point. Guess which one i used? Right, the integer one.. Using float coordinates for my points got rid of the rounding error inaccuracy.

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Solution: Use float PointF instead of int Point.

https://developer.android.com/reference/android/graphics/Point.html

https://developer.android.com/reference/android/graphics/PointF.html

Annegrim
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