Lesson 150. Drawing. PathMeasure - Path object information

Lesson 150. Drawing. PathMeasure – Path object information


In this lesson:

– we use PathMeasure to work with Path

We studied the Path object in detail in Lesson 143. Now let’s look at PathMeasure, a very useful tool in some cases that can:

– calculate the length of Path segments

– define closed or open segment

– get coordinates and angle for the specified Path point

– select part of Path into a separate object

Let’s create a project:

Project name: P1501_PathMeasure
Build Target: Android 2.3.3
Application name: PathMeasure
Package name: en.startandroid.develop.p1501pathmeasure
Create Activity: MainActivity

MainActivity.java:

package ru.startandroid.develop.p1501pathmeasure;

import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.Path;
import android.graphics.PathMeasure;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.view.WindowManager;

public class MainActivity extends Activity {

  final String TAG = "myLogs";

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
        WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(new DrawView(this));
  }

  class DrawView extends View {

    Paint paint;
    Paint paintText;
    Path path;
    PathMeasure pMeasure;
    float length;

    public DrawView(Context context) {
      super(context);
      paint = new Paint(Paint.ANTI_ALIAS_FLAG);
      paint.setStyle(Paint.Style.STROKE);
      paint.setStrokeWidth(3);

      paintText = new Paint(Paint.ANTI_ALIAS_FLAG);
      paintText.setTextSize(30);

      path = new Path();
      path.moveTo(100, 300);
      path.rLineTo(150, 100);
      path.rLineTo(150, -100);
      path.rQuadTo(150, 200, 300, 0);
      path.rLineTo(150, 100);
      path.rLineTo(150, -100);

      pMeasure = new PathMeasure(path, false);
      length = pMeasure.getLength();
    }

    @Override
    protected void onDraw(Canvas canvas) {
      canvas.drawARGB(80, 102, 204, 255);

      canvas.drawPath(path, paint);
      canvas.drawText(String.format("Length: %s", length), 100, 100,
          paintText);
    }
  }

}

IN onCreate we flag the window title and put the application in full-screen mode. I will explain later why this is necessary.

In the constructor DrawView we create a Path consisting of several lines and one curve. Next, we create a PathMeasure for him, with the forceClosed flag set to false – we do not need to close the Path. The getLength method gives the Path length.

IN onDraw we draw Path and display its length.

Now let’s try to get a geometric infa about an arbitrary Path point.

rewrite DrawView:

class DrawView extends View {

    Paint paint;
    Paint paintText;
    Path path;
    PathMeasure pMeasure;
    Matrix matrix;
    Rect rect;

    float[] pos;
    float[] tan;

    float length;
    float distance;

    public DrawView(Context context) {
      super(context);
      paint = new Paint(Paint.ANTI_ALIAS_FLAG);
      paint.setStyle(Paint.Style.STROKE);
      paint.setStrokeWidth(3);

      paintText = new Paint(Paint.ANTI_ALIAS_FLAG);
      paintText.setTextSize(30);

      path = new Path();
      path.moveTo(100, 300);
      path.rLineTo(150, 100);
      path.rLineTo(150, -100);
      path.rQuadTo(150, 200, 300, 0);
      path.rLineTo(150, 100);
      path.rLineTo(150, -100);

      pMeasure = new PathMeasure(path, false);

      length = pMeasure.getLength();
      distance = length / 4;

      matrix = new Matrix();
      pMeasure.getMatrix(distance, matrix,
          PathMeasure.POSITION_MATRIX_FLAG
              + PathMeasure.TANGENT_MATRIX_FLAG);

      pos = new float[2];
      tan = new float[2];
      pMeasure.getPosTan(distance, pos, tan);

      rect = new Rect(-20, -10, 20, 10);
    }

    @Override
    protected void onDraw(Canvas canvas) {
      canvas.drawARGB(80, 102, 204, 255);

      canvas.drawPath(path, paint);
      canvas.drawText(
          String.format("Distance: %s of %s", distance, length), 100,
          100, paintText);
      canvas.drawText(
          String.format("Position: %s. Tangent (cos,sin): %s",
              Arrays.toString(pos), Arrays.toString(tan)), 100,
          150, paintText);

      canvas.setMatrix(matrix);
      canvas.drawRect(rect, paint);
    }
  }

In the constructor DrawView create the same Path, then create a PathMeasure for it, measure the length and put a value equal to a quarter of the length. Next we use the getMatrix method and pass it:

– the distance from the beginning of the Path to the point we need information about

– a matrix that will be filled with values ​​relevant to the specified point

– flags. Their two POSITION_MATRIX_FLAG – the matrix will get data only at the point position, TANGENT_MATRIX_FLAG – the matrix will get data only by turning at the point. We use both flags at once.

Thus, we obtain a matrix that describes the position and rotation of the object, which is at a point at a distance from the start.

The getPosTan method has a similar meaning, but it is filled not by a matrix, but by two arrays: pos – position, tan – slope (cos and sin angles).

In the method onDraw draw Path, output the distance values ​​and information obtained from the getPosTan method.

Next we apply to the canvas the matrix obtained from the getMatrix method and draw a small rectangle. It will be located at a point that is at a distance, and its angle will correspond to the angle of Path at that point.

If header flags and fullscreen mode are removed from MainActivity.onCreate, the resulting matrix will contain incorrect offset values ​​that do not take into account the header height and top bar. I don’t know if it’s a bug or a feature.

Path can consist of several contours and PathMeasure can distinguish them.

rewrite DrawView:

class DrawView extends View {

    Paint paint;
    Path path;
    PathMeasure pMeasure;

    public DrawView(Context context) {
      super(context);
      paint = new Paint(Paint.ANTI_ALIAS_FLAG);
      paint.setStyle(Paint.Style.STROKE);
      paint.setStrokeWidth(3);

      path = new Path();
      path.moveTo(100, 300);
      path.rLineTo(150, 150);
      path.rLineTo(150, -100);

      path.rMoveTo(0, 0);
      path.rQuadTo(150, 200, 300, 0);
      path.close();

      path.rMoveTo(0, 0);
      path.rLineTo(150, 100);
      path.rLineTo(150, -150);
      path.close();

      pMeasure = new PathMeasure(path, false);
      do {
        Log.d(TAG,
            String.format("Length: %s, isClosed: %s",
                pMeasure.getLength(), pMeasure.isClosed()));
      } while (pMeasure.nextContour());

    }

    @Override
    protected void onDraw(Canvas canvas) {
      canvas.drawARGB(80, 102, 204, 255);
      canvas.drawPath(path, paint);
    }
  }

When creating Path, we use the moveTo and rMoveTo methods. These methods start a new path in Path. So we got three outlines. With the close method, we close the second and third paths, leaving the first one open.

Next we use nextContour to iterate over the contours and getLength and isClosed methods get the length and find out if the closed contour is. We log all this information.

The screen shows that the second and third contours are closed.

The list says the same and shows the length of each outline (subject to closing lines):

Length: 392.4096, isClosed: false
Length: 673.3855, isClosed: true
Length: 696.5477, isClosed: true

Select part of one Path into another Path.

rewrite DrawView:

class DrawView extends View {

    Paint paint;
    Path path;
    Path path1;
    PathMeasure pMeasure;

    public DrawView(Context context) {
      super(context);
      paint = new Paint(Paint.ANTI_ALIAS_FLAG);
      paint.setStyle(Paint.Style.STROKE);
      paint.setStrokeWidth(3);

      path = new Path();
      path.moveTo(100, 300);
      path.rLineTo(150, 150);
      path.rLineTo(150, -100);
      path.rQuadTo(150, 200, 300, 0);
      path.rLineTo(150, 100);
      path.rLineTo(150, -150);

      pMeasure = new PathMeasure(path, false);

      path1 = new Path();
      pMeasure.getSegment(150, 850, path1, true);
    }

    @Override
    protected void onDraw(Canvas canvas) {
      canvas.drawARGB(80, 102, 204, 255);
      canvas.drawPath(path1, paint);
    }
  }

We use the getSegment method. Pass the distance there to the point of beginning (150) and to the end point (850) of the path we need. The path1 object will contain a cut-out part of the path object. The fourth parameter is true, so that the resulting figure starts from the starting point. Otherwise it will start with (0,0).

We got part of Path between points 150 and 850 from the beginning of the figure.

In the next lesson:

– consider PathEffect objects




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