1.Bitmap总讲

# Bitmap

[Bitmap](https://developer.android.com/reference/android/graphics/Bitmap)

[细说Bitmap](https://juejin.im/post/58c3b29761ff4b005d906730)

[https://blog.csdn.net/HarryWeasley/article/details/51955467](bitmap的六种压缩方式，Android图片压缩)

## 1.先讲讲屏幕密度

### 1.1 屏幕密度

**1.1.1 概念**:一英尺内像素点的个数。 

**1.1.2计算公式**

```

公式：根号下（长的平方+高的平方）➗屏幕尺寸

```


**1.1.3 手机屏幕密度的表示方式和范围**

使用dpi表示的实际屏幕密度 | [0dpi，160dpi) | [160dpi,240dpi) | [240dpi,320dpi) | [320,480dpi) | [480dpi,640dpi) |[640dpi,960dpi)

---|---| --- | --- |---| ---| --- |

DisplayMetrics.densityDpi获取的屏幕密度 | 160dpi | 240dpi | 320dpi | 480dpi | 640dpi |960dpi

DisplayMetrics.density获取的屏幕密度 | 0.75 | 1 | 1.5 | 2.0 | 3.0 | 4.0

使用x hpi的表示方式 | ldpi | mdpi | hdpi | xhdpi | xxhdpi | xxxhdpi|

注意：现在市面上常见的android手机的屏幕密度为xxhdpi

**1.1.4 dpi与dp的关系** 

以mdpi为基准  ,1dp=1px。 

因为hdpi为mdpi的1.5倍，所以在屏幕密度为hdpi的屏幕上 1dp=1.5px

##  2.Bitmap的内存大小

图片的宽 * 图片的高 * （手机屏幕密度/图片存放的分类目录）^2 * 颜色模式

#### 2.1 缩放比例是什么 

比如我们将图片放在mdpi的文件夹下，而手机的屏幕密度为xxhdpi   

则缩放比例=(xxhdpi/mdpi)=3

#### 2.2 颜色模式是什么

颜色模式有4种

1. ALPHA_8 只有透明通道 一个像素占8个二进制位

2. RGB_565 红，绿，蓝三个颜色通道分别占 5，6，5个 二进制位，一个像素点占16个二进制位

3. ARGB_4444  透明通道，红，绿，蓝三个颜色通道都占4个二进制位，一个像素点占16个二进制位。

4. ARGB_8888 透明通道，红，绿，蓝三个颜色通道都占8个二进制位，一个像素点占

32个二进制位。

```

BitmapFactory.Options options=new BitmapFactory.Options();

        options.inPreferredConfig= Bitmap.Config.ARGB_8888;

```

#### 2.3 通过代码获取Bitmap的大小

```

int getByteCount()

```

```

int getAllocationByteCount()

```

通常情况下getByteCount()和getAllocationByteCount（）获取到的值相同， 

但是如果通过复用的Bitmap来创建Bitmap, 

getByteCount()获取到的是新的Bitmap的大小 

getAllocationByteCount（）获取到的是原来的Bitmap的大小

```

BitmapFactory.Options options=new BitmapFactory.Options();

options.inBitmap=bitmap//被复用的Bitmap

```

#### 2.4实例

**实例** 

一个48*48的图片存放到drawable-mdpi目录下，然后在各个屏幕密度下的内存大小为

分类目录  | mdpi | hdpi | xhdpi | xxhdpi 

---| --- | --- |---| ---| --- |

图片所占内存  | 9216=48 * 48*4 | 20736=48 * 48*((1.5/1)^2)*4 | 36864 =48 * 48 *((2/1)^2)*4  | 82944 =48 * 48 *((3/1)^2)*4

##  3.Bitmap的释放

+ android 3.0之前 

Bitmap对象存储在jvm堆中，Bitmap像素数据存放在Native中，不归jvm管理。 所以要通过Bitmap.recycle()来手动释放Bitmap像素数据

+ android3.0(包括android 3.0)之后

  Bitmap对象和像素数据都存储在jvm堆中，由jvm管理，所以不必调用Bitmap.recycle()。

要促进Bitmap的回收，可以将Bitmap的引用置空。

```

Bitmap bitmap=...

bitmap=null;

```

## 4.创建Bitmap

### 4.1 [BitmapFactory](https://developer.android.com/reference/android/graphics/BitmapFactory)

从各种源中创建Bitmap

```

BitmapFactory.decode***

```

**注意**:

decode***得到的Bitmap默认是immutable(不可更改)的。所以有时候我们decode得到Bitmap后，如果想修改此Bitmap,会报IllegalStateException。

```

        Bitmap bitmap1 = BitmapFactory.decodeResource(getResources(), R.drawable.bitmaptest,options);

        Bitmap bitmapNew=Bitmap.createBitmap(bitmap1);

```

如果想decode***得到的Bitmap是可更改的则Options选项的inMutable要设置为true(默认为false)

  ```

    BitmapFactory.Options options=new BitmapFactory.Options();

    options.inMutable=true;

    Bitmap bitmap1 = BitmapFactory.decodeResource(getResources(), R.drawable.bitmaptest,options);

    Canvas canvas=new Canvas(bitmap1);

  ```

### 4.2 [Bitmap](https://developer.android.com/reference/android/graphics/Bitmap)

从已有Bitmap创建Bitmap(

Bitmap.createBitmap)  ,api分为两类

  ##### 2.2.1 根据已有的Bitmap做Matrix变化,返回的Bitmap为immutable不可修改。且如果新的Bitmap和原来的Bitamp参数一样，则新的Bitmap就是原来的Bitmap

```

    public static Bitmap createBitmap(@NonNull Bitmap src) {

        return createBitmap(src, 0, 0, src.getWidth(), src.getHeight());

    }

    public static Bitmap createBitmap(@NonNull Bitmap source, int x, int y, int width, int height) {

        return createBitmap(source, x, y, width, height, null, false);

    }

    /**

    * Returns an immutable bitmap from subset of the source bitmap,

    * transformed by the optional matrix. The new bitmap may be the

    * same object as source, or a copy may have been made. It is

    * initialized with the same density and color space as the original

    * bitmap.

    *

    * If the source bitmap is immutable and the requested subset is the

    * same as the source bitmap itself, then the source bitmap is

    * returned and no new bitmap is created.

    *

    * @param source  The bitmap we are subsetting

    * @param x        The x coordinate of the first pixel in source

    * @param y        The y coordinate of the first pixel in source

    * @param width    The number of pixels in each row

    * @param height  The number of rows

    * @param m        Optional matrix to be applied to the pixels

    * @param filter  true if the source should be filtered.

    *                  Only applies if the matrix contains more than just

    *                  translation.

    * @return A bitmap that represents the specified subset of source

    * @throws IllegalArgumentException if the x, y, width, height values are

    *        outside of the dimensions of the source bitmap, or width is <= 0,

    *        or height is <= 0

    */

    public static Bitmap createBitmap(@NonNull Bitmap source, int x, int y, int width, int height,

            @Nullable Matrix m, boolean filter) {

    }

```

#### 2.2.2 根据已有的Bitmap做RGB更改,得到的Bitmap可以更改(mutable)

```

  public static Bitmap createBitmap(int width, int height, @NonNull Config config) {

        return createBitmap(width, height, config, true);

  }

  public static Bitmap createBitmap(int width, int height,

            @NonNull Config config, boolean hasAlpha) {

        return createBitmap(null, width, height, config, hasAlpha);

    }

  public static Bitmap createBitmap(@Nullable DisplayMetrics display, int width, int height,

            @NonNull Config config, boolean hasAlpha) {

        return createBitmap(display, width, height, config, hasAlpha,

                ColorSpace.get(ColorSpace.Named.SRGB));

    }

      /**

    * Returns a mutable bitmap with the specified width and height.  Its

    * initial density is determined from the given {@link DisplayMetrics}.

    * The newly created bitmap is in the {@link ColorSpace.Named#SRGB sRGB}

    * color space.

    *

    * @param display  Display metrics for the display this bitmap will be

    *                drawn on.

    * @param width    The width of the bitmap

    * @param height  The height of the bitmap

    * @param config  The bitmap config to create.

    * @param hasAlpha If the bitmap is ARGB_8888 or RGBA_16F this flag can be used to

    *                mark the bitmap as opaque. Doing so will clear the bitmap in black

    *                instead of transparent.

    * @param colorSpace The color space of the bitmap. If the config is {@link Config#RGBA_F16},

    *                  {@link ColorSpace.Named#EXTENDED_SRGB scRGB} is assumed, and if the

    *                  config is not {@link Config#ARGB_8888}, {@link ColorSpace.Named#SRGB sRGB}

    *                  is assumed.

    *

    * @throws IllegalArgumentException if the width or height are <= 0, if

    *        Config is Config.HARDWARE (because hardware bitmaps are always

    *        immutable), if the specified color space is not {@link ColorSpace.Model#RGB RGB},

    *        if the specified color space's transfer function is not an

    *        {@link ColorSpace.Rgb.TransferParameters ICC parametric curve}, or if

    *        the color space is null

    */

    public static Bitmap createBitmap(@Nullable DisplayMetrics display, int width, int height,

            @NonNull Config config, boolean hasAlpha, @NonNull ColorSpace colorSpace) {

    }

```

[项目实践](http://note.youdao.com/noteshare?id=29e492903df9db67f868e062d27a9bf3) 

## 5. 将Bitmap保存到文件 

质量压缩:质量压缩会减小图片在手机上的存储大小，但是不会减少对应的Bitmap占用的内存大小（见第3点，因为图片高宽没变）

```

  /**

    * Write a compressed version of the bitmap to the specified outputstream.

    * If this returns true, the bitmap can be reconstructed by passing a

    * corresponding inputstream to BitmapFactory.decodeStream(). Note: not

    * all Formats support all bitmap configs directly, so it is possible that

    * the returned bitmap from BitmapFactory could be in a different bitdepth,

    * and/or may have lost per-pixel alpha (e.g. JPEG only supports opaque

    * pixels).

    *

    * @param format  The format of the compressed image

    * @param quality  Hint to the compressor, 0-100. 0 meaning compress for

    *                small size, 100 meaning compress for max quality. Some

    *                formats, like PNG which is lossless, will ignore the

    *                quality setting

    * @param stream  The outputstream to write the compressed data.

    * @return true if successfully compressed to the specified stream.

    */

Bitmap.compress(CompressFormat format, int quality, OutputStream stream)

```

```

File file=new File(Environment.getExternalStorageDirectory()+File.separator+"new.png");

        try {

            OutputStream outputStream=new FileOutputStream(file);

            bitmap1.compress(Bitmap.CompressFormat.PNG,100,outputStream);

            outputStream.flush();

            outputStream.close();

        } catch (FileNotFoundException e) {

            e.printStackTrace();

        } catch (IOException e) {

            e.printStackTrace();

        }

```

## 6. 加载超大图 

[参考](http://www.jcodecraeer.com/a/anzhuokaifa/androidkaifa/2015/1021/3607.html) 

通过BitmapRegionDecoder只加载大图的一部分，而不把整张图都加载到内存中

而且可以通过options.inBitmap复用同一个Bimtmap进行加载。

```

InputStream inputStream = getAssets().open("tangyan.jpg");

            //获得图片的宽、高

            BitmapFactory.Options tmpOptions = new BitmapFactory.Options();

            tmpOptions.inJustDecodeBounds = true;

            BitmapFactory.decodeStream(inputStream, null, tmpOptions);

            int width = tmpOptions.outWidth;

            int height = tmpOptions.outHeight;

            //设置显示图片的中心区域

            BitmapRegionDecoder bitmapRegionDecoder = BitmapRegionDecoder.newInstance(inputStream, false);

            BitmapFactory.Options options = new BitmapFactory.Options();

            options.inPreferredConfig = Bitmap.Config.RGB_565;

            Bitmap bitmap = bitmapRegionDecoder.decodeRegion(new Rect(width / 2 - 100, height / 2 - 100, width / 2 + 100, height / 2 + 100), options);

            mImageView.setImageBitmap(bitmap);

```

## 补充

### 1.BitmapFactory.Option

```

        /**

        * If set, decode methods will always return a mutable Bitmap instead of

        * an immutable one. This can be used for instance to programmatically apply

        * effects to a Bitmap loaded through BitmapFactory.

        * <p>Can not be set simultaneously with inPreferredConfig =

        * {@link android.graphics.Bitmap.Config#HARDWARE},

        * because hardware bitmaps are always immutable.

        */

        @SuppressWarnings({"UnusedDeclaration"}) // used in native code

        public boolean inMutable;

        /**

        * If set to true, the decoder will return null (no bitmap), but

        * the <code>out...</code> fields will still be set, allowing the caller to

        * query the bitmap without having to allocate the memory for its pixels.

        */

        public boolean inJustDecodeBounds;

        /**

        * If set to a value > 1, requests the decoder to subsample the original

        * image, returning a smaller image to save memory. The sample size is

        * the number of pixels in either dimension that correspond to a single

        * pixel in the decoded bitmap. For example, inSampleSize == 4 returns

        * an image that is 1/4 the width/height of the original, and 1/16 the

        * number of pixels. Any value <= 1 is treated the same as 1. Note: the

        * decoder uses a final value based on powers of 2, any other value will

        * be rounded down to the nearest power of 2.

        */

        public int inSampleSize;

        /**

        * If this is non-null, the decoder will try to decode into this

        * internal configuration. If it is null, or the request cannot be met,

        * the decoder will try to pick the best matching config based on the

        * system's screen depth, and characteristics of the original image such

        * as if it has per-pixel alpha (requiring a config that also does).

        *

        * Image are loaded with the {@link Bitmap.Config#ARGB_8888} config by

        * default.

        */

        public Bitmap.Config inPreferredConfig = Bitmap.Config.ARGB_8888;

```

## 2.createScaledBitmap简便地得到目标大小的Bitmap

```

  /**

    * Creates a new bitmap, scaled from an existing bitmap, when possible. If the

    * specified width and height are the same as the current width and height of

    * the source bitmap, the source bitmap is returned and no new bitmap is

    * created.

    *

    * @param src      The source bitmap.

    * @param dstWidth  The new bitmap's desired width.

    * @param dstHeight The new bitmap's desired height.

    * @param filter    true if the source should be filtered.

    * @return The new scaled bitmap or the source bitmap if no scaling is required.

    * @throws IllegalArgumentException if width is <= 0, or height is <= 0

    */

    public static Bitmap createScaledBitmap(@NonNull Bitmap src, int dstWidth, int dstHeight,

            boolean filter) {

        Matrix m = new Matrix();

        final int width = src.getWidth();

        final int height = src.getHeight();

        if (width != dstWidth || height != dstHeight) {

            final float sx = dstWidth / (float) width;

            final float sy = dstHeight / (float) height;

            m.setScale(sx, sy);

        }

        return Bitmap.createBitmap(src, 0, 0, width, height, m, filter);

    }

```