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Working with ImageViews and Bitmaps in Android Application Development

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In this chapter from Android Application Development in 24 Hours, Sams Teach Yourself, 4th Edition, you'll learn the details of handling images and bitmaps, including creating bitmaps, using drawing commands, and handling very large images.
This chapter is from the book

What You’ll Learn in This Hour:

  • Examining ImageView Bitmaps
  • Using Bitmaps and Canvas
  • Introducing Picasso

Images and media can play an important role in creating an exceptional Android app. In this chapter, you look at the details of handling images and bitmaps, including creating bitmaps, using drawing commands, and handling very large images.

Examining ImageView

You learned about different types of views in Hour 10, “More Views and Controls.” An ImageView is a view that displays an image, but you will find that there are unique aspects to working with images. An ImageView can display any drawable image. The source of the image can be a resource, a drawable, or a bitmap.

Displaying an Image

There are four methods available for setting an image in an ImageView. They differ by how the image to display is defined. The image can be a bitmap, drawable, Uri, or resource id. The methods are as follows:

  • setImageDrawable(): Set a drawable as the content of the ImageView.
  • setImageBitmap(): Set a Bitmap as the content of the ImageView.
  • setImageResource(): Use a resource id to set the content of the ImageView.
  • setImageUri(): Use a URI to set the content of the ImageView.

To set an ImageView to an image resource defined by R.drawable.mainImage, you use the following:

ImageView mainImage = (ImageView) findViewById(R.id.imageView1);

To populate a Drawable object from a resource, use the getResources.getDrawable() method:

Drawable myDrawable = getResources().getDrawable(R.drawable.ic_launcher);

In this hour, you populate an ImageView using a resource id as the source and then explore several properties of how an ImageView can display an image.

Using ScaleTypes in ImageView

ImageViews include a ScaleType property. The ScaleType defines how the image will be displayed within the ImageView. Using ScaleType, you can have an image fill the entire ImageView, be centered in the ImageView, or be cropped and centered in the ImageView.

The options for ScaleType are defined in ImageView.ScaleType. For example, ImageView.ScaleType.CENTER refers to a scale type in which the image is centered in the ImageView. The complete set of ScaleTypes are as follows:

  • ImageView.ScaleType.CENTER: Center the image with no scaling. The image dimensions are unchanged.
  • ImageView.ScaleType.CENTER_CROP: Scales the image and keeps the aspect ratio until either the width of height of the image is the same as the width or height of the ImageView. For a small image, this has the effect of enlarging the entire image. For a large image, this has the effect of showing the center of the image.
  • ImageView.ScaleType.CENTER_INSIDE: The image is scaled, and the aspect ratio is maintained. The width and height of the image fit within the ImageView.
  • ImageView.ScaleType.FIT_CENTER: Maintain aspect ratio and fit the image in the center of the ImageView.
  • ImageView.ScaleType.FIT_START: Maintain aspect ratio and fit the image in the left and top edge of the ImageView.
  • ImageView.ScaleType.FIT_END: Maintain aspect ratio and fit the image in the right and bottom edge of the ImageView.
  • ImageView.ScaleType.FIT_END: Maintain aspect ratio and fit the image in the right and bottom edge of the ImageView.
  • ImageView.ScaleType.MATRIX: Scale using a matrix.

You can change scaleType dynamically in your code. Listing 11.1 show the code for an app that displays an ImageView and includes a RadioGroup and set of RadioButtons for changing the scale type. When a radio button is selected, the scaleType for the ImageView is updated.

LISTING 11.1 Changing ScaleType Programatically

 1:  package com.talkingandroid.hour11application;
 2:  import android.app.Activity;
 3:  import android.os.Bundle;
 4:  import android.widget.ImageView;
 5:  import android.widget.RadioGroup;
 7:  public class ScaleActivity extends Activity {
 8:      RadioGroup radioGroup;
 9:      ImageView imageView;
11:      @Override
12:      protected void onCreate(Bundle savedInstanceState) {
13:          super.onCreate(savedInstanceState);
14:          setContentView(R.layout.activity_scale);
15:          radioGroup = (RadioGroup) findViewById(R.id.radioGroup);
16:          imageView = (ImageView) findViewById(R.id.imageView);
17:          radioGroup.setOnCheckedChangeListener(new
18:                           RadioGroup.OnCheckedChangeListener() {
19:              @Override
20:              public void onCheckedChanged(RadioGroup group, int checkedId) {
21:                  switch (checkedId){
22:                    case R.id.radioCenter:
23:                      imageView.setScaleType(ImageView.ScaleType.CENTER);
24:                      break;
25:                    case R.id.radioCenterCrop:
26:                      imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
27:                      break;
28:                    case R.id.radioCenterInside:
29:                      imageView.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
30:                      break;
31:                    case R.id.radioFitCenter:
32:                      imageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
33:                      break;
34:                    case R.id.radioFitStart:
35:                      imageView.setScaleType(ImageView.ScaleType.FIT_START);
36:                      break;
37:                    case R.id.radioFitEnd:
38:                      imageView.setScaleType(ImageView.ScaleType.FIT_END);
39:                      break;
40:                    case R.id.radioFitXY:
41:                      imageView.setScaleType(ImageView.ScaleType.FIT_XY);
42:                      break;
43:                }
44:            }
45:        });
46:    }

On line 17 of Listing 11.1, an OnCheckChangeListener() is set for the RadioGroup. When the change is detected, the select RadioButton id is checked, and the appropriate scaleType is set on the image.

The image used in the code for Listing 11.1 is shown in Figure 11.1. The image is 900 pixels wide and 200 pixels high. It is used in several other examples in this chapter.

Figure 11.1

FIGURE 11.1 Base image for showing ScaleType (scaletest.png).

By using this simple image with four circles of different colors, it is easy to see the effect of the changing ScaleType.

The ImageView is set to match the parent width and height. When the image scaleType is set to CENTER_INSIDE, the image is shown taking the full width of the ImageView and is centered with a height that is proportional to the width.

Figure 11.2 shows the base image using the scaleTypes set to CENTER, CENTER_CROP, and CENTER_INSIDE. Using CENTER shows the image in actual size. Because the size of the image is larger than the ImageView, the green and blue circles in the center are shown. CENTER_CROP shows half of the green and blue circle. The height of the image fills the ImageView. CENTER_INSIDE shows the entire image centered in the ImageView.

Figure 11.2


Figure 11.3 shows the base image using the ScaleTypes FIT_CENTER, FIT_START, FIT_END, and FIT_XY. The aspect ratio is maintained in the first three, but when using FIT_XY, the image fills the ImageView and “stretches” the image to fit.

Figure 11.3


Rotating an Image

An ImageView contains several methods for rotating an image. When you rotate an image, you must set the point in the image to rotate around. That is the pivot point. The method setPivotX() and setPivotY() are used to set the pivot point.

Once the pivot point is set, you can call the setRotation() method to make the image actually rotate.

The idea in Listing 11.2 is to set the pivot point to the center of the ImageView and to rotate the image 30 degrees each time the button is clicked. The ImageView is defined to have height and width set to match_parent. The ImageView occupies the entire screen.

To get the center of the ImageView, the width and height are divided by 2. To continuously rotate, the number of clicks count is kept. The angle to rotate is 30 times the number of clicks. So, if the button is clicked twice, the image is rotated 60 degrees.

Figure 11.4 shows the rotated image.

LISTING 11.2 Rotating an Image

 1: package com.talkingandroid.hour11application;
 2: import android.app.Activity;
 3: import android.os.Bundle;
 4: import android.view.View;
 5: import android.widget.Button;
 6: import android.widget.ImageView;
 8: public class RotateActivity extends Activity {
 9:     Button rotateButton;
10:     ImageView imageView;
11:     int numClicks = 1;
13:     @Override
14:     protected void onCreate(Bundle savedInstanceState) {
15:         super.onCreate(savedInstanceState);
16:         setContentView(R.layout.activity_rotate);
17:         imageView = (ImageView)findViewById(R.id.imageView);
18:         rotateButton = (Button) findViewById(R.id.button);
19:         rotateButton.setOnClickListener(new View.OnClickListener() {
20:             @Override
21:             public void onClick(View v) {
22:                 imageView.setPivotX(imageView.getWidth()/2);
23:                 imageView.setPivotY(imageView.getHeight() / 2);
24:                 imageView.setRotation(30*numClicks);
25:                 numClicks++;
26:             }
27:         });
28:     }
29: }
Figure 11.4

FIGURE 11.4 Rotated image.

Setting Alpha

Alpha level indicates the opacity of an image. An image can be completely transparent, completely opaque, or somewhere in the middle. The alpha level can be set on an ImageView using the setAlpha() method or, since API level 11, the setImageAlpha() method. These methods take an integer parameter. A parameter of 0 indicates complete transparency and 255 for complete opacity.

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