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Using Bitmaps and Canvas

The Bitmap(android.graphics.Bitmap) class represents a bitmap image. Bitmaps are created via the BitmapFactory(android.graphics.BitmapFactory) class.

Three typical ways use BitmapFactory to create Bitmaps are to create a bitmap from a resource, file, or InputStream. To create a Bitmap from a resource, use the BitmapFactory method decodeResource():

Bitmap = BitmapFactory.decodeResource(getResources(), R.drawable.someImage);

The other two methods are similar to decodeResource(): decodeFile() and decodeStream().

Handling Large Images

There are techniques for avoiding the dreaded out-of-memory (OOM) exception. Large images can have a significant impact on memory use in your app. To demonstrate this, you’ll create an unrealistically large image to display in an ImageView. If the unmodified image is loaded into an ImageView, the app fails with an OOM error. A java.lang.OutOfMemory exception occurs. You’ll fix the memory error for this case by checking the image size and display side.

The idea is to display the image at an appropriate size for the device. There is no point in showing a 10-foot mural in a 6-inch frame. Similarly, there is no point in showing a 20-inch image on a 3-inch device screen. You will scale down the image size and save memory.

The details of your app will influence your memory usage and the techniques that will work best in your case. This example shows how to handle a single large image.

To demonstrate this, you’ll start with an image and increase it to an unrealistic size. You will use a photo that is 72 inches x 54 inches and that has a 28MB file size.

The image is in the drawable resource folder and has the id R.drawable.largeimage.

You can cause the app to fail with an OOM error by trying to set an ImageView to this resource. You have an ImageView named imageView. This line of code that causes the app to fail is this:


Some work is required, but it is possible to handle an image this large. In all cases, it would be better to work with appropriately sized images, but that does not always happen.

The approach is to get the dimensions of the underlying Bitmap without actually rendering it. Getting those dimensions is not a memory-intensive activity. Once you have the Bitmap, you can determine an appropriate size for the Bitmap that will fit in our display. If you have a 20-inch image and a 4-inch display, you’ll request that the Bitmap that is created in memory fill the 4-inch display.

Using BitmapFactory.Options

The BitmapFactory.Options class is used with the BitmapFactory class. It is essential for handling large bitmaps.

You’ll use the following options from the BitmapFactoryOptions class:

  • inJustDecodeBounds: If set to true, this option indicates that the Bitmap dimensions should be determined by the BitmapFactory but that the Bitmap itself should not be created. This is the key to getting the Bitmap dimensions without the memory overhead of creating the Bitmap.
  • outWidth: The width of the image set when inJustDecodeBounds is used.
  • outHeight: The height of the image set when inJustDecodeBounds is used.
  • inSampleSize: This integer indicates how much the dimensions of the Bitmap should be reduced. Given an image of 1000x400, an inSampleSize of 4 will result in a Bitmap of 250x100. The dimensions are reduced by a factor of 4.

Listing 11.3 shows the code to address this.

LISTING 11.3 Displaying a Large Image

 1: package com.talkingandroid.hour11application;
 2: import android.app.Activity;
 3: import android.graphics.Bitmap;
 4: import android.graphics.BitmapFactory;
 5: import android.os.Bundle;
 6: import android.view.Display;
 7: import android.widget.ImageView;
 9: public class LargeImageActivity extends Activity {
11:     @Override
12:     protected void onCreate(Bundle savedInstanceState) {
13:         super.onCreate(savedInstanceState);
14:         setContentView(R.layout.activity_large_image);
15:         ImageView imageView = (ImageView) findViewById(R.id.imageView);
16:         Display display = getWindowManager().getDefaultDisplay();
17:         int displayWidth = display.getWidth();
18:         BitmapFactory.Options options = new BitmapFactory.Options();
19:         options.inJustDecodeBounds = true;
20:         BitmapFactory.decodeResource(getResources(), R.drawable.largeimage,
21:              options);
22:         int width = options.outWidth;
23:         if (width > displayWidth) {
24:             int widthRatio = Math.round((float) width / (float) displayWidth);
25:             options.inSampleSize = widthRatio;
26:         }
27:        options.inJustDecodeBounds = false;
28:        Bitmap scaledBitmap =  BitmapFactory.decodeResource(getResources(),
29:                               R.drawable.largeimage, options);
30:        imageView.setImageBitmap(scaledBitmap);
31:    }
32: }

On lines 16 and 17, you get the size of the device display. You’ll use this as the target size for reducing the image size.

On lines 18–22, you determine the size of the current Bitmap. You do that by creating a BitmapFactory.Options class and setting the inJustDecodeBounds value to true. On line 20, the Bitmap is decoded to get the dimensions. Using this method, you get the dimensions without the memory overhead of creating the Bitmap. The result is available in options.outWidth. On line 22, you assign options.outWidth to the int variable width.

In this example, you use a simple test for the size of the image. On line 23, you check whether the width of the Bitmap is greater than the size of the display. If that is the case, you must determine the inSampleSize to use. That is done on lines 24 and 25. If the width of the Bitmap is 1000 pixels and the size of the display is 250 pixels, you get an inSampleSize of 4 by dividing the width of the Bitmap by the width of the display. For simplicity, you are not checking the height.

With the imSampleSize set to an appropriate value, you can render the image.

On line 27, the inJustDecodeBounds value is set to false. That means the image will be decoded and a Bitmap object will be created.

Lines 28 and 29 use the BitmapFactory.decodeResource() method to actually decode the image and create the Bitmap. The bitmap is assigned to the variable scaledBitmap. It is important to note that in this call, the BitmapFactory.Options variable options is passed as a parameter. That is how you indicate to the BitmapFactory what inSampleSize to use. The value for options.inSampleSize was set on line 25.

It is certainly not recommended to display a 72-inch image on a device, but Figure 11.5 shows that it can be done!

Figure 11.5

FIGURE 11.5 Very large photo displayed on device.

Drawing Directly on a Canvas

There is one more thing that you can do with an ImageView and Bitmap. You’ll create a Bitmap and draw directly on the Canvas that is associated with the Bitmap. A Canvas is an object that you can draw on by calling drawing commands.

You will use an ImageView to display the Bitmap. You will also use an ImageView to determine the dimensions when creating the Bitmap and for drawing. In Listing 11.4, you draw the word “Hello” in the center of the screen.

LISTING 11.4 Drawing on a Canvas

 1: package com.talkingandroid.hour11application;
 2: import android.app.Activity;
 3: import android.graphics.Bitmap;
 4: import android.graphics.Canvas;
 5: import android.graphics.Color;
 6: import android.graphics.Paint;
 7: import android.os.Bundle;
 8: import android.view.View;
 9: import android.widget.Button;
10: import android.widget.ImageView;
12: public class DrawActivity extends Activity {
13:     ImageView imageView;
14:     Button drawButton;
16:     @Override
17:     protected void onCreate(Bundle savedInstanceState) {
18:         super.onCreate(savedInstanceState);
19:         setContentView(R.layout.activity_draw);
20:         imageView = (ImageView) findViewById(R.id.imageView);
21:         drawButton = (Button)findViewById(R.id.button);
22:         drawButton.setOnClickListener(new View.OnClickListener() {
23:             @Override
24:             public void onClick(View v) {
25:                 Bitmap imageBitmap = Bitmap.createBitmap(imageView.getWidth(),
26:                        imageView.getHeight(), Bitmap.Config.ARGB_8888);
27:                 Canvas canvas = new Canvas(imageBitmap);
28:                 float scale = getResources().getDisplayMetrics().density;
29:                 Paint p = new Paint();
30:                 p.setColor(Color.BLUE);
31:                 p.setTextSize(48*scale);
32:                 canvas.drawText("Hello", imageView. getWidth()/2,
33:                                  imageView.getHeight()/2, p);
34:                 imageView.setImageBitmap(imageBitmap);
35:             }
36:         });
37:     }
38: }

A new Bitmap is created on lines 25 and 26 by using the method Bitmap.createBitmap(). Note that the width and height of the bitmap are set using the width and height of the ImageView. The Bitmap.Config (android.graphics.Bitmap.Config) is set to Bitmap.Config.ARGB_8888.

When looking at the documentation for the Bitmap class, there are a number of createBitmap() methods that take different parameters. These methods may return a mutable or an immutable Bitmap. That is important; only a mutable Bitmap can be used for drawing.

On line 27, a Canvas is instantiated based on the Bitmap that you created.

Simple drawing commands are applied to the canvas in lines 28–33. You create a Paint object and set the Color to blue and set the text size. Line 28 gets the density of the display. That is used to set the text size properly. Recall that you previously learned about converting density independent pixels to pixels. On line 32, you draw the word “Hello” in the center of the Canvas.

On line 34, you update the ImageView to show your generated Bitmap.

Figure 11.6 shows the result.

Figure 11.6

FIGURE 11.6 Drawing on a Canvas.

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