Loading smaller images with subsampling

How to load images efficiently in android using sub-sampling

In this performance tutorial we use the BitmapFactory.Options inSampleSize for loading smaller images with subsampling therefore saving memory & cpu resources, resulting in more responsiveness especially in scrolling.

Method

Create a scale factor algorithm

  private int calculateInSampleSize(BitmapFactory.Options bmOptions) {
        final int photoWidth = bmOptions.outWidth;
        final int photoHeight = bmOptions.outHeight;
        int scaleFactor = 1;

        if(photoWidth > TARGET_IMAGE_VIEW_WIDTH || photoHeight > TARGET_IMAGE_VIEW_HEIGHT) {
            final int halfPhotoWidth = photoWidth/2;
            final int halfPhotoHeight = photoHeight/2;
            while(halfPhotoWidth/scaleFactor > TARGET_IMAGE_VIEW_WIDTH
                    || halfPhotoHeight/scaleFactor > TARGET_IMAGE_VIEW_HEIGHT) {
                scaleFactor *= 2;
            }
        }
        return scaleFactor;
    }

Pass the scale factor to the BitmapFactory.Option inSampleSize

  private Bitmap decodeBitmapFromFile(File imageFile) {
        BitmapFactory.Options bmOptions = new BitmapFactory.Options();
        bmOptions.inJustDecodeBounds = true;
        BitmapFactory.decodeFile(imageFile.getAbsolutePath(), bmOptions);
        bmOptions.inSampleSize = calculateInSampleSize(bmOptions);
        bmOptions.inJustDecodeBounds = false;
        return BitmapFactory.decodeFile(imageFile.getAbsolutePath(), bmOptions);
    }

Call the new decode method

@Override
    protected Bitmap doInBackground(File... params) {
        // return BitmapFactory.decodeFile(params[0].getAbsolutePath());
        mImageFile = params[0];
        return decodeBitmapFromFile(params[0]);
    }

Run the app