Displaying Bitmaps Efficiently -Loading Large Bitmaps Efficiently

Images come in all shapes and sizes. In many cases they are larger than required for a typical application user interface (UI). For example, the system Gallery application displays photos taken using your Android
devices's camera which are typically much higher resolution than the screen density of your device.
Given that you are working with limited memory, ideally you only want to load a lower resolution version in memory. The lower resolution version should match the size of the UI component that displays it. An image
with a higher resolution does not provide any visible benefit, but still takes up precious memory and incurs additional performance overhead due to additional on the fly scaling.
This lesson walks you through decoding large bitmaps without exceeding the per application memory limit by loading a smaller subsampled version in memory.

Read Bitmap Dimensions and Type

The

BitmapFactory

class
provides several decoding methods (

, int, int, android.graphics.BitmapFactory.Options)]decodeByteArray()

,

decodeFile()

,

decodeResource()

,
etc.) for creating a

Bitmap

from
various sources. Choose the most appropriate decode method based on your image data source. These methods attempt to allocate memory for the constructed bitmap and therefore can easily result in an

OutOfMemory

exception.
Each type of decode method has additional signatures that let you specify decoding options via the

BitmapFactory.Options

class.
Setting the

inJustDecodeBounds

property
to

true

while decoding avoids memory allocation, returning

null

for the bitmap object but setting

outWidth

,

outHeight

and

outMimeType

.
This technique allows you to read the dimensions and type of the image data prior to construction (and memory allocation) of the bitmap.

BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(getResources(), R.id.myimage, options);
int imageHeight = options.outHeight;
int imageWidth = options.outWidth;
String imageType = options.outMimeType;

To avoid

java.lang.OutOfMemory

exceptions, check the dimensions of a bitmap before decoding it, unless you absolutely trust the source to
provide you with predictably sized image data that comfortably fits within the available memory.

Load a Scaled Down Version into Memory

Now that the image dimensions are known, they can be used to decide if the full image should be loaded into memory or if a subsampled version should be loaded instead. Here are some factors to consider:

Estimated memory usage of loading the full image in memory.
Amount of memory you are willing to commit to loading this image given any other memory requirements of your application.
Dimensions of the target

ImageView

or
UI component that the image is to be loaded into.
Screen size and density of the current device.

For example, 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

.
To tell the decoder to subsample the image, loading a smaller version into memory, set

inSampleSize

to

true

in
your

BitmapFactory.Options

object.
For example, an image with resolution 2048x1536 that is decoded with an

inSampleSize

of
4 produces a bitmap of approximately 512x384. Loading this into memory uses 0.75MB rather than 12MB for the full image (assuming a bitmap configuration of

ARGB_8888

).
Here’s a method to calculate a sample size value that is a power of two based on a target width and height:

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;
}

Note: A power of two value is calculated because the decoder uses a final value by rounding down to the nearest power of two, as per the

inSampleSize

documentation.
To use this method, first decode with

inJustDecodeBounds

set
to

true

, pass the options through and then decode again using the new

inSampleSize

value
and

inJustDecodeBounds

set
to

false

:

public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
int reqWidth, int reqHeight) {

// First decode with inJustDecodeBounds=true to check dimensions
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(res, resId, options);

// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

// Decode bitmap with inSampleSize set
options.inJustDecodeBounds = false;
return BitmapFactory.decodeResource(res, resId, options);
}

This method makes it easy to load a bitmap of arbitrarily large size into an

ImageView

that
displays a 100x100 pixel thumbnail, as shown in the following example code:

mImageView.setImageBitmap(
decodeSampledBitmapFromResource(getResources(), R.id.myimage, 100, 100));

You can follow a similar process to decode bitmaps from other sources, by substituting the appropriate

, int, int, android.graphics.BitmapFactory.Options)]BitmapFactory.decode*

method
as needed.

NEXT: PROCESSING
BITMAPS OFF THE UI THREAD