Android: Scale a watermark Bitmap over another Bitmap using the Matrix

What I set out to do was to overlay a logo over a photo and it was surprisingly easy with the use of the Matrix class.

 
No no, not him!

I've littered the code with comments, but if you need any more information leave a comment.

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01./**

02. * Adds a watermark on the given image.

03. */

04.public static Bitmap addWatermark(Resources res, Bitmap source) {

05.  int w, h;

06.  Canvas c;

07.  Paint paint;

08.  Bitmap bmp, watermark;

09. 

10.  Matrix matrix;

11.  float scale;

12.  RectF r;

13. 

14.  w = source.getWidth();

15.  h = source.getHeight();

16. 

17.  // Create the new bitmap

18.  bmp = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);

19. 

20.  paint = new Paint(Paint.ANTI_ALIAS_FLAG | Paint.DITHER_FLAG | Paint.FILTER_BITMAP_FLAG);

21. 

22.  // Copy the original bitmap into the new one

23.  c = new Canvas(bmp);

24.  c.drawBitmap(source, 0, 0, paint);

25. 

26.  // Load the watermark

27.  watermark = BitmapFactory.decodeResource(res, R.drawable.android_mo);

28.  // Scale the watermark to be approximately 10% of the source image height

29.  scale = (float) (((float) h * 0.10) / (float) watermark.getHeight());

30. 

31.  // Create the matrix

32.  matrix = new Matrix();

33.  matrix.postScale(scale, scale);

34.  // Determine the post-scaled size of the watermark

35.  r = new RectF(0, 0, watermark.getWidth(), watermark.getHeight());

36.  matrix.mapRect(r);

37.  // Move the watermark to the bottom right corner

38.  matrix.postTranslate(w - r.width(), h - r.height());

39. 

40.  // Draw the watermark

41.  c.drawBitmap(watermark, matrix, paint);

42.  // Free up the bitmap memory

43.  watermark.recycle();

44. 

45.  return bmp;

46.}

Sources

• Need help with canvas.drawBitmap and matrix
• Get measured size and position of bitmap on canvas after 2D transformation