load_bitmap.md

Load Bitmaps into Memory

I recently read an article on how to load bitmaps into memory in an efficient manner from here. Obviously, there are a lot of libraries that can care of this but I felt that knowing the theory behind it was helpful for me and I wanted to summarize the key points from the article here:

• How to load bitmap into Memory: We can decode an image using BitmapFactory class.

  Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.image);
  imageView.setImageBitmap(bitmap);
  

  • The above code will fetch an image from the Drawable folder, load it into memory and display in the ImageView. This means that when you check the image size when loaded into memory, you can see that it will be larger than the image size on disk.
  • You can use bitmap.getByteCount() to check the image size.
  • This is because the image is compressed when it is on disk (stored in a JPG, PNG, or similar format). Once you load the image into memory, it is no longer compressed and takes up as much memory as is necessary for all the pixels.
  • In order to get the size of the image without loading it into memory: we can use BitmapFactory.Options.

    BitmapFactory.Options options = new BitmapFactory.Options();
    options.inJustDecodeBounds = true;   // -> We set inJustDecodeBounds to true to make sure that the image is not loaded to memory
    BitmapFactory.decodeResource(getResources(), R.mipmap.hqimage, options);
    

• How to reduce Image size in Memory: We can use inSampleSize from the BitmapFactory.Options. For instance, if we have an image with size 1000x1000 and if we set the inSampleSize to 2, then the image will be scaled to 500x500.

  BitmapFactory.Options options = new BitmapFactory.Options();
  options.inJustDecodeBounds = true;
  options.inSampleSize = 3;   // -> We can set a value to inSampleSize to scale the image
  BitmapFactory.decodeResource(getResources(), R.mipmap.hqimage, options);
  

  This step is done because: it’s not worth loading a 1024x768 pixel image into memory if it will eventually be displayed in a 128x96 pixel thumbnail in an ImageView. Instead of providing a constant value (which might not work since our images might be of different sizes), we can also calculate the inSampleSize value:

  public static int calculateInSampleSize(
        BitmapFactory.Options options, int reqWidth, int reqHeight) {
      // Raw height and width of image
      final int height = options.outHeight;
      final int width = options.outWidth;
      int inSampleSize = 1;
  
      if (height > reqHeight || width > reqWidth) {
  
          final int halfHeight = height / 2;
          final int halfWidth = width / 2;
  
          // Calculate the largest inSampleSize value that is a power of 2 and keeps both
          // height and width larger than the requested height and width.
          while ((halfHeight / inSampleSize) >= reqHeight
                  && (halfWidth / inSampleSize) >= reqWidth) {
              inSampleSize *= 2;
          }
      }
  
      return inSampleSize;
  }
  

  Now we need to set the inJustDecodeBounds value to true again before we can display it in the ImageView. The full code for the above implementation:

  BitmapFactory.Options options = new BitmapFactory.Options();
  options.inJustDecodeBounds = true;
  options.inSampleSize = calculateInSampleSize(options, 500,500);
  
  options.inJustDecodeBounds = false;
  Bitmap smallBitmap = BitmapFactory.decodeResource(getResources(), R.drawable.image, options);
  imageView.setImageBitmap(smallBitmap);
  

  Now if you check the bitmap size again, you will notice that it will have reduced significantly.

• How to reduce the size of image in disk: We can use the compress method in the Bitmap class. We can pass the bitmap type, the quality and the ByteArrayOutputStream to the compress method.

  ByteArrayOutputStream bos = new ByteArrayOutputStream();
  bitmap.compress(Bitmap.CompressFormat.JPEG, 100, bos);
  byte[] bitmapdata = bos.toByteArray();
  

  Note: The value 100 means the quality is not changed. If we set the value to 50, the image size would reduce significantly. Note: Also, please keep in mind that quality cannot be changed in png format, only in jpeg format.