［Android］修改Bitmap的Config格式设置及其Config参数源码阅读

关于修改Bitmap的Config设置

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原创博客，转载请注明出处：
http://blog.csdn.net/u011035622/article/details/50277565

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Bitmap.Config介绍

　　首先先介绍一下Config设置里面的参数吧，一方面让读者清楚，另一方面也是给自己加深印象。
　　Bitmap.Config一共有四个参数如下：
（这些参数决定了Bitmap位图的配置，会影响到bitmap的像素如何、色彩、以及是否有透明度的能力）

1. Bitmap.Config ALPHA_8
   这个参数每个像素占用1字节的空间。
   它代表每个像素点被存储为单个透明度的通道，这对于设置遮罩的图片用例十分有用，它不存储颜色信息。

2. Bitmap.Config RGB_565
   这个参数每个像素占用2字节的空间。
   它代表只有RGB通道的编码，其中红色占用5位地址，绿色占用6位地址，蓝色占用5位地址。没有透明度的通道。
   使用不透明的位图时，不要求高的色彩保真度使用此配置是不错的选择。

3. Bitmap.Config ARGB_4444
   这个参数每个像素占用2字节的空间。
   它一共有四个通道，顾名思义，分别是透明度、红、绿、蓝。每个通道分别占用四位地址，所以一共2字节。
   当应用需要节省内存(对色彩质量要求低)，同时又需要存储透明度信息，这个配置可以作为选择，但官方比较推荐用ARGB_8888的设置，因为这个的色彩质量差。

4. Bitmap.Config ARGB_8888
   这个参数每个像素占用4字节的空间。
   这也是一共4个通道，但不一样的是每个通道站8位地址，因而色彩质量比上一个设置高了特别特别多（16倍）。
   能够满足最好的位图质量，在内存充足的情况下，十分推荐使用这个。

   以下附参考的官方源码解释：

    /**
     * Possible bitmap configurations. A bitmap configuration describes
     * how pixels are stored. This affects the quality (color depth) as
     * well as the ability to display transparent/translucent colors.
     */
    public enum Config {
        // these native values must match up with the enum in SkBitmap.h

        /**
         * Each pixel is stored as a single translucency (alpha) channel.
         * This is very useful to efficiently store masks for instance.
         * No color information is stored.
         * With this configuration, each pixel requires 1 byte of memory.
         */
        ALPHA_8     (1),

        /**
         * Each pixel is stored on 2 bytes and only the RGB channels are
         * encoded: red is stored with 5 bits of precision (32 possible
         * values), green is stored with 6 bits of precision (64 possible
         * values) and blue is stored with 5 bits of precision.
         * 
         * This configuration can produce slight visual artifacts depending
         * on the configuration of the source. For instance, without
         * dithering, the result might show a greenish tint. To get better
         * results dithering should be applied.
         * 
         * This configuration may be useful when using opaque bitmaps
         * that do not require high color fidelity.
         */
        RGB_565     (3),

        /**
         * Each pixel is stored on 2 bytes. The three RGB color channels
         * and the alpha channel (translucency) are stored with a 4 bits
         * precision (16 possible values.)
         * 
         * This configuration is mostly useful if the application needs
         * to store translucency information but also needs to save
         * memory.
         * 
         * It is recommended to use {@link #ARGB_8888} instead of this
         * configuration.
         *
         * Note: as of {@link android.os.Build.VERSION_CODES#KITKAT},
         * any bitmap created with this configuration will be created
         * using {@link #ARGB_8888} instead.
         * 
         * @deprecated Because of the poor quality of this configuration,
         *             it is advised to use {@link #ARGB_8888} instead.
         */
        @Deprecated
        ARGB_4444   (4),

        /**
         * Each pixel is stored on 4 bytes. Each channel (RGB and alpha
         * for translucency) is stored with 8 bits of precision (256
         * possible values.)
         * 
         * This configuration is very flexible and offers the best
         * quality. It should be used whenever possible.
         */
        ARGB_8888   (5);

        final int nativeInt;

        @SuppressWarnings({"deprecation"})
        private static Config sConfigs[] = {
            null, ALPHA_8, null, RGB_565, ARGB_4444, ARGB_8888
        };

        Config(int ni) {
            this.nativeInt = ni;
        }

        static Config nativeToConfig(int ni) {
            return sConfigs[ni];
        }
    }

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以上第一部分只是介绍，而接下来的内容才是我想记录的内容。

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修改Bitmap的Config设置

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修改时遇到的问题

　　如果是本地图片路径的话，我们可以直接通过这样的方法得到一张bitmap并通过Options.inPreferredConfig来设置他的Config参数（如果你看一下BitmapFactory的源码，就会发现默认的就是Config.ARGB_8888格式）。

     Bitmap bitmap = null;
     File file = new File(path);
     if(file.exists()) {
         BitmapFactory.Options opts = new BitmapFactory.Options();
         opts.inPreferredConfig = Bitmap.Config.ARGB_8888;
         try {
             bitmap = BitmapFactory.decodeFile(path, opts);
         } catch (OutOfMemoryError error) {
             bitmap = null;
         }
     }

　　这并没有什么问题。
　　但是当我们已经有一张Bitmap了，如何修改他的Config设置呢，大家去看Bitmap里面的方法会发现有类似于setConfig或这reconfigure的方法（事实上前者调用的是后者的方法，后者调用的是native方法－nativeReconfigure）。
　　当看到这个方法时我直接的调用了它。

bitmap.setConfig(Bitmap.Config.ARGB_8888);

　　当然跪了。。
***AndroidRuntime: FATAL EXCEPTION:
main java.lang.NoSuchMethodError:android.graphics.Bitmap.reconfigure***

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问题解决方法

折腾好一会不知道错哪里，点看源码看下才知道：
　　

setConfig、reconfigure方法不能用于目前使用的位图！！！＝。＝
　　（最后贴官方源码读者可以参考看看）

　　思考了一下，我们就只能先将当前的bitmap转位字节流，然后通过Bitmapfactory中的方法将这个字节流根据设置重新生成bitmap。
　　果然，问题解决：

      /*传入bitmap参数，返回bitmap。*/
      ByteArrayOutputStream dataByte = new ByteArrayOutputStream();
      bitmap.compress(Bitmap.CompressFormat.PNG, 100, dataByte);
      BitmapFactory.Options opts = new BitmapFactory.Options();
      opts.inSampleSize = OPT_INSAMPLE_SIZE;
      opts.inPreferredConfig = Bitmap.Config.ARGB_8888;
      bitmap = BitmapFactory.decodeByteArray(dataByte.toByteArray(), 0, dataByte.size(), opts);

　　多次经历告诉我，学习时常常点进去看看源码和介绍真的很重要。决定找个时间把Bitmap和BitmapFactory的源码走一遍！

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最后贴上setConfig的官方源码部分：

    /**
     * 
Convenience method for calling {@link #reconfigure(int, int, Config)} with the current height and width.
     *(警告不能用于当前bitmap＝。＝)
     * 
WARNING: this method should not be used on bitmaps currently used by the view system, see {@link #reconfigure(int, int, Config)} for more details. */

    public void setConfig(Config config) {
        reconfigure(getWidth(), getHeight(), config);
    }

reconfigure:

这里还讲到：
1、修改Bitmap可能会影响到hasalpha，即格式转变可能有或没有透明通道。
2、如果位图的配置不够大，支持新的配置，IllegalArgumentException将抛出，位图将不被修改。

    /**
     * 
Modifies the bitmap to have a specified width, height, and {@link
     * Config}, without affecting the underlying allocation backing the bitmap.
     * Bitmap pixel data is not re-initialized for the new configuration.
     *
     * 
This method can be used to avoid allocating a new bitmap, instead
     * reusing an existing bitmap's allocation for a new configuration of equal
     * or lesser size. If the Bitmap's allocation isn't large enough to support
     * the new configuration, an IllegalArgumentException will be thrown and the
     * bitmap will not be modified.
     *
     * 
The result of {@link #getByteCount()} will reflect the new configuration,
     * while {@link #getAllocationByteCount()} will reflect that of the initial
     * configuration.
     *
     * 
Note: This may change this result of hasAlpha(). When converting to 565,
     * the new bitmap will always be considered opaque. When converting from 565,
     * the new bitmap will be considered non-opaque, and will respect the value
     * set by setPremultiplied().
     *
     * 
WARNING: This method should NOT be called on a bitmap currently used
     * by the view system. It does not make guarantees about how the underlying
     * pixel buffer is remapped to the new config, just that the allocation is
     * reused. Additionally, the view system does not account for bitmap
     * properties being modifying during use, e.g. while attached to
     * drawables.
     *
     * @see #setWidth(int)
     * @see #setHeight(int)
     * @see #setConfig(Config)
     */
    public void reconfigure(int width, int height, Config config) {
        checkRecycled("Can't call reconfigure() on a recycled bitmap");
        if (width <= 0 || height <= 0) {
            throw new IllegalArgumentException("width and height must be > 0");
        }
        if (!isMutable()) {
            throw new IllegalStateException("only mutable bitmaps may be reconfigured");
        }
        if (mBuffer == null) {
            throw new IllegalStateException("native-backed bitmaps may not be reconfigured");
        }

        nativeReconfigure(mNativeBitmap, width, height, config.nativeInt, mBuffer.length,
                mRequestPremultiplied);
        mWidth = width;
        mHeight = height;
    }