Bitmap详解

1.bitmap占多少内存

getByteCount()方法是在API12加入的,代表存储Bitmap的色素需要的最少内存。API19开始getAllocationByteCount()方法代替了getByteCount()。 这是API26的

    public final int getByteCount() {
        if (mRecycled) {
            Log.w(TAG, "Called getByteCount() on a recycle()'d bitmap! "
                    + "This is undefined behavior!");
            return 0;
        }
        // int result permits bitmaps up to 46,340 x 46,340
        return getRowBytes() * getHeight();
    }

    public final int getAllocationByteCount() {
        if (mRecycled) {
            Log.w(TAG, "Called getAllocationByteCount() on a recycle()'d bitmap! "
                    + "This is undefined behavior!");
            return 0;
        }
        return nativeGetAllocationByteCount(mNativePtr);
    }

getByteCount()与getAllocationByteCount()的区别 一般情况下两者是相等的;如果通过复用Bitmap来解码图片,如果被复用的Bitmap的内存比待分配内存的Bitmap大,那么getByteCount()表示新解码图片占用内存的大小(并非实际内存大小,实际大小是复用的那个Bitmap的大小),getAllocationByteCount()表示被复用Bitmap真实占用的内存大小

2.如何计算Bitmap占用的内存

通常情况下认为 bitmap占用的内存 = width * height * 一个像素所占的内存。 下面是API26里面的一个像素所占的内存

public enum Config {
        ALPHA_8     (1),//With this configuration, each pixel requires 1 byte of memory.
        RGB_565     (3),//Each pixel is stored on 2 bytes
        @Deprecated
        ARGB_4444   (4),//Each pixel is stored on 2 bytes
        ARGB_8888   (5),//Each pixel is stored on 4 bytes   如果没有指明,默认这个
        RGBA_F16    (6),//Each pixels is stored on 8 bytes   高色域高动态范围图像
        HARDWARE    (7);//stored only in graphic memory
    }

通常用的是RGB_565 和 ARGB_8888(默认) 其实这个说法不全对,没有说明场景,同时也忽略了一个影响项:Density。

public static Bitmap decodeResourceStream(Resources res, TypedValue value,
            InputStream is, Rect pad, Options opts) {
        validate(opts);
        if (opts == null) {
            opts = new Options();
        }

        if (opts.inDensity == 0 && value != null) {
            final int density = value.density;
            if (density == TypedValue.DENSITY_DEFAULT) {
                opts.inDensity = DisplayMetrics.DENSITY_DEFAULT;
            } else if (density != TypedValue.DENSITY_NONE) {
                opts.inDensity = density;
            }
        }

        if (opts.inTargetDensity == 0 && res != null) {
            opts.inTargetDensity = res.getDisplayMetrics().densityDpi;
        }

        return decodeStream(is, pad, opts);
    }

因此,加载一张本地资源图片,那么它占用的内存 = width * height * nTargetDensity/inDensity * nTargetDensity/inDensity * 一个像素所占的内存。

        // 不做处理,默认缩放。
        BitmapFactory.Options options = new BitmapFactory.Options();
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);
        Log.i(TAG, "bitmap:ByteCount = " + bitmap.getByteCount() + ":::bitmap:AllocationByteCount = " + bitmap.getAllocationByteCount());
        Log.i(TAG, "width:" + bitmap.getWidth() + ":::height:" + bitmap.getHeight());
        Log.i(TAG, "inDensity:" + options.inDensity + ":::inTargetDensity:" + options.inTargetDensity);

        Log.i(TAG,"===========================================================================");

        // 手动设置inDensity与inTargetDensity,影响缩放比例。
        BitmapFactory.Options options_setParams = new BitmapFactory.Options();
        options_setParams.inDensity = 320;
        options_setParams.inTargetDensity = 160;
        Bitmap bitmap_setParams = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options_setParams);
        Log.i(TAG, "bitmap_setParams:ByteCount = " + bitmap_setParams.getByteCount() + ":::bitmap_setParams:AllocationByteCount = " + bitmap_setParams.getAllocationByteCount());
        Log.i(TAG, "width:" + bitmap_setParams.getWidth() + ":::height:" + bitmap_setParams.getHeight());
        Log.i(TAG, "inDensity:" + options_setParams.inDensity + ":::inTargetDensity:" + options_setParams.inTargetDensity);


07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: bitmap:ByteCount = 538800:::bitmap:AllocationByteCount = 538800
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: width:449:::height:300
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: inDensity:320:::inTargetDensity:320
07-15 14:49:06.298 8496-8496/com.company.glidetest I/MainActivity: ===========================================================================
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: bitmap_setParams:ByteCount = 135000:::bitmap_setParams:AllocationByteCount = 135000
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: width:225:::height:150
07-15 14:49:06.318 8496-8496/com.company.glidetest I/MainActivity: inDensity:320:::inTargetDensity:160

可以看出: 1.不使用Bitmap复用时,getByteCount()与getAllocationByteCount()的值是一致的; 2.默认情况下,华为手机在xhdpi的文件夹下,inDensity为320,inTargetDensity为320,内存大小为538800;而538800= 449* 300* 4。 3.手动设置inDensity与inTargetDensity,使其比例为2,内存大小为135000,而 449* 300* 0.5 * 0.5 * 4 = 134700。

3.Bitmap如何压缩

inSampleSize 设置inSampleSize之后,Bitmap的宽、高都会缩小inSampleSize倍。例如:一张宽高为2048x1536的图片,设置inSampleSize为4之后,实际加载到内存中的图片宽高是512x384。占有的内存就是0.75M而不是12M,足足节省了15倍。

    public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight) {
        // 设置inJustDecodeBounds属性为true,只获取Bitmap原始宽高,不分配内存;
        final BitmapFactory.Options options = new BitmapFactory.Options();
        options.inJustDecodeBounds = true;
        BitmapFactory.decodeResource(res, resId, options);// 这里会找到最适合的inTargetDensity和inDensity
        Log.i(TAG, "inTargetDensity = " + options.inTargetDensity);
        Log.i(TAG, "inDensity = " + options.inDensity);
        Log.i(TAG, "inPreferredConfig = " + options.inPreferredConfig);
        // 计算inSampleSize值;
        options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
        // 真实加载Bitmap;
        options.inJustDecodeBounds = false;
        return BitmapFactory.decodeResource(res, resId, options);
    }

    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;
            // 宽和高比需要的宽高大的前提下最大的inSampleSize
            while ((halfHeight / inSampleSize) >= reqHeight
                    && (halfWidth / inSampleSize) >= reqWidth) {
                inSampleSize *= 2;
            }
        }
        Log.i(TAG, "inSampleSize = " + inSampleSize);
        return inSampleSize;
    }

测试代码及打印结果

Bitmap bitmap1 = decodeSampledBitmapFromResource(getResources(), R.mipmap.aaa, 100, 100);
Log.i(TAG, "bitmap1 = " + bitmap1.getAllocationByteCount());

07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inTargetDensity = 320
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inDensity = 320
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inPreferredConfig = ARGB_8888
07-15 15:06:28.988 23977-23977/com.company.glidetest I/MainActivity: inSampleSize = 2
07-15 15:06:28.998 23977-23977/com.company.glidetest I/MainActivity: bitmap1 = 135000

还是之前的图片(449* 300)正式算的时候应该是把449向上取整数来算的 450/2* 300/2* 1 * 1 * 4 = 135000 (由此可见其实还和inSampleSize有关)

4.Bitmap如何复用

(1)使用LruCache和DiskLruCache做内存和磁盘缓存; (2)使用Bitmap复用,同时针对版本进行兼容。

        BitmapFactory.Options options = new BitmapFactory.Options();
        // 图片复用,这个属性必须设置;
        options.inMutable = true;
        // 手动设置缩放比例,使其取整数,方便计算、观察数据;
        options.inDensity = 320;
        options.inTargetDensity = 320;
        Bitmap bitmap = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);

        // 使用inBitmap属性,这个属性必须设置;
        options.inBitmap = bitmap;
        options.inDensity = 320;
        // 设置缩放宽高为原始宽高一半;
        options.inTargetDensity = 160;
        options.inMutable = true;
        Bitmap bitmapReuse = BitmapFactory.decodeResource(getResources(), R.mipmap.aaa, options);

        // 复用对象的内存地址;
        Log.i(TAG, "bitmap = " + bitmap);
        Log.i(TAG, "bitmapReuse = " + bitmapReuse);
        Log.i(TAG, "bitmap:AllocationByteCount = " + bitmap.getAllocationByteCount());
        Log.i(TAG, "bitmapReuse:ByteCount = " + bitmapReuse.getByteCount() + ":::bitmapReuse:AllocationByteCount = " + bitmapReuse.getAllocationByteCount());

运行结果 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmap = android.graphics.Bitmap@20cffbbc 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmapReuse = android.graphics.Bitmap@20cffbbc 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmap:AllocationByteCount = 538800 07-15 15:24:40.038 8463-8463/com.company.glidetest I/MainActivity: bitmapReuse:ByteCount = 135000:::bitmapReuse:AllocationByteCount = 538800

可以看到,共用同一块内存,复用的bitmap因为inTargetDensity 和 inDensity的原因大约占到了四分之一的内存(全部分配给了它,但是只用了四分之一)

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