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graphics – Scale & rotate Bitmap using Matrix in Android

Posted by: admin April 23, 2020 Leave a comment

Questions:

I’m trying to scale and rotate in single operation before creting the final bitmap but the preRotate, postConcat doesn’t seem to work.

Bitmap bmp = ... original image ...

Matrix m = new Matrix()
m.setScale(x, y);
m.preRotate(degrees, (float) width / 2, (float) height / 2);

Bitmap.createBitmap(bmp, 0, 0, bmp.getWidth(), bmp.getHeight(), m, true);

It only applies the scale and not rotation.

How to&Answers:

This is the code

public class Bitmaptest extends Activity {
    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        LinearLayout linLayout = new LinearLayout(this);

        // load the origial BitMap (500 x 500 px)
        Bitmap bitmapOrg = BitmapFactory.decodeResource(getResources(),
               R.drawable.android);

        int width = bitmapOrg.getWidth();
        int height = bitmapOrg.getHeight();
        int newWidth = 200;
        int newHeight = 200;

        // calculate the scale - in this case = 0.4f
        float scaleWidth = ((float) newWidth) / width;
        float scaleHeight = ((float) newHeight) / height;

        // createa matrix for the manipulation
        Matrix matrix = new Matrix();
        // resize the bit map
        matrix.postScale(scaleWidth, scaleHeight);
        // rotate the Bitmap
        matrix.postRotate(45);

        // recreate the new Bitmap
        Bitmap resizedBitmap = Bitmap.createBitmap(bitmapOrg, 0, 0,
                          newWidth, newHeight, matrix, true);

        // make a Drawable from Bitmap to allow to set the BitMap
        // to the ImageView, ImageButton or what ever
        BitmapDrawable bmd = new BitmapDrawable(resizedBitmap);

        ImageView imageView = new ImageView(this);

        // set the Drawable on the ImageView
        imageView.setImageDrawable(bmd);

        // center the Image
        imageView.setScaleType(ScaleType.CENTER);

        // add ImageView to the Layout
        linLayout.addView(imageView,
                new LinearLayout.LayoutParams(
                      LayoutParams.FILL_PARENT, LayoutParams.FILL_PARENT
                )
        );

        // set LinearLayout as ContentView
        setContentView(linLayout);
    }
}

Answer:

The answer was given, but to make things more clear to anyone reading this:

1) if you wish to perform ONE transformation in your bitmap, you CAN use SET (setRotate, setScale etc).

But note that any call to a “set” method OVERRIDES other transformations. It’s like a new matrix. That’s why OP’s rotation was not working. These calls are not performed line by line. It’s like they are scheduled to be done at runtime by the GPU when the new bitmap is being drawn. It’s like when resolving your matrix, GPU rotated it, but then, created a scaled new one, ignoring previous matrix.

2) if you wish to perform more then one transformation, then you MUST use “pre” or “post” methods.

And what is the difference between a postRotate and a preRotate, for example? Well, this matrix math stuff is not my strength, but what I know is that the graphic cards make these transformations using matrix multiplication. It seems to be way more efficient. And as far as I remember from school, when multiplicating matrices the order IS important. A X B != B X A. So, scale a matrix and then rotate it is different from rotate and then scale it.

BUUUUT, as far as the final result in the screen is the same, we high level programmers usually do not need to know these differences. The GPU does.

Well, in that rare cases when you are performing really complicated matrix operations, and results are not what you expected or the performance is terrible and you need to deeply understand these methods to fix your code, well, then android documentation can not be of much help anyway. Instead, a good Linear Algebra book would be your best friend. 😉

Answer:

If you face the issue of OutOfMemory with above answers, than use below:

Bitmap MyFinalBitmap = Bitmap.createBitmap(CurrentBitmap, 0, 0,CurrentBitmap.getWidth()/2, CurrentBitmap.getHeight()/2,matrix, true);

Answer:

Canvas holds a matrix stack und you can use it with the methods:

Canvas.save()

Doc:
/**
* Saves the current matrix and clip onto a private stack.
*

* Subsequent calls to translate,scale,rotate,skew,concat or clipRect,
* clipPath will all operate as usual, but when the balancing call to
* restore() is made, those calls will be forgotten, and the settings that
* existed before the save() will be reinstated.
*
* @return The value to pass to restoreToCount() to balance this save()
*/

Canvas.restore()

Doc:
/**
* This call balances a previous call to save(), and is used to remove all
* modifications to the matrix/clip state since the last save call. It is
* an error to call restore() more times than save() was called.
*/

example:
A custom View(Android) which looks like a rotary knob(e.g. potentiometer)

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    viewX = getWidth();     //views width
    viewY = getHeight();    //views height
    setMeasuredDimension(widthMeasureSpec, heightMeasureSpec); //a must call for every custom view
}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    double tempAngel = 3.6 * barValue;
    int deltaX = bitmap.getWidth() / 2;
    int deltaY = bitmap.getHeight() / 2;
    ...
    canvas.save();
    canvas.translate(viewX / 2, viewY / 2);             //translate drawing point to center
    canvas.rotate((float) tempAngel);                   //rotate matrix
    canvas.save();                                      //save matrix. your drawing point is still at (viewX / 2, viewY / 2)
    canvas.translate(deltaX * -1, deltaY * -1);         //translate drawing  point a bit up and left to draw the bitmap in the middle
    canvas.drawBitmap(bitmap, 0,0, bitmapPaint);   // draw bitmap to the tranlated point at 0,0
    canvas.restore();     //must calls...
    canvas.restore();
}

Answer:

Matrix rotateMatrix = new Matrix();
rotateMatrix.postRotate(rotation);
rotatedBitmap = Bitmap.createBitmap(loadedImage, 0, 0,loadedImage.getWidth(), loadedImage.getHeight(),rotateMatrix, false);

Answer:

Refer to the following code, seems to work. In your code you are defining Matrix as m but referring to it as matrix

public class FourthActivity extends Activity {
private static final int WIDTH = 50;
private static final int HEIGHT = 50;
private static final int STRIDE = 64;  

private static int[] createColors() {
    int[] colors = new int[STRIDE * HEIGHT];
    for (int y = 0; y < HEIGHT; y++) {
        for (int x = 0; x < WIDTH; x++) {
            int r = x * 255 / (WIDTH - 1);
            int g = y * 255 / (HEIGHT - 1);
            int b = 255 - Math.min(r, g);
            int a = Math.max(r, g);
            colors[y * STRIDE + x] = (a << 24) | (r << 16) | (g << 8) | b;
        }
    }
    return colors;
}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main2);
    final ImageView view1 = (ImageView) findViewById(R.id.imageView1);

    int[] colors = createColors();
    final Bitmap bmp1 = Bitmap.createBitmap(colors, 0, STRIDE, WIDTH, HEIGHT,
    Bitmap.Config.ARGB_8888);
    view1.setImageBitmap(bmp1);
    Button button = (Button) findViewById(R.id.button1);
    button.setOnClickListener(new OnClickListener(){
        @Override
        public void onClick(View v) {   
            Matrix matrix = new Matrix();
            matrix.setScale(2, 2);
            matrix.preRotate(45, (float) WIDTH / 2, (float) HEIGHT / 2);

            Bitmap bmp2 = Bitmap.createBitmap(bmp1, 0, 0, 
      bmp1.getWidth(), bmp1.getHeight(), matrix, true);
            ImageView view2 = (ImageView) findViewById(R.id.imageView2);
            view2.setImageBitmap(bmp2);
        }
    });
}
}

Answer:

All of the previous answer assume that this change to the bitmap is being made in a view. However in my case I was making the change to be saved out. Figured I would answer it for those in a similar boat.

There are two ways to do translation. Below dx is the translation in the X axis, and dy is the translation in the Y axis. The other variables should be self explanatory.

1 – Translation within the image (without rotation)

val newBitmap = Bitmap.createBitmap(originalBitmap, dx, dy, newWidth, newHeight, matrix, false)

2 – Complex matrix

 matrix.postTranslate(dx, dy)

 val newBitmap = Bitmap.createBitmap(newWidth, newHeight, Bitmap.Config.ARGB_8888)

 val canvas = Canvas(newBitmap)
 canvas.drawBitmap(originalBitmap, matrix, null)

Answer:

Use matrix to scale area of original bitmap to 50% and compress bitmap until it’s size < 200k
Compress bitmap to a specific byte size in Android