densityDpi表示一英寸有多少个像素点
densityDpi | density | drawable目录 | ic_launcher大小 |
---|---|---|---|
640 | 4 | drawable-xxxhdpi | 192x192 |
480 | 3 | drawable-xxhdpi | 144x144 |
320 | 2 | drawable-xhdpi | 96x96 |
240 | 1.5 | drawable-hdpi | 72x72 |
160 | 1 | drawable-mdpi | 48x48 |
160 | 1 | drawable | - |
- | - | drawable-nodpi | - |
xml布局使用R.drawable.xx相当于代码
BitmapFactory.decodeResource(getResources(), R.drawable.xx)
假设当前设备的dpi是560,那么系统匹配drawable目录的顺序是:
- drawable-xxxhdpi
- drawable-nodpi
- drawable-xxhdpi
- drawable-xhdpi
- drawable-hdpi
- drawable-mdpi
- drawable
加载图片会经过缩放,内存占用计算公式如下
float scale = (float) deviceDensityDpi / drawableDirDensityDpi;
int scaledWidth = (int) (imgWidth * scale + 0.5f);
int scaledHeight = (int) (imgHeight * scale + 0.5f);
// 这里为什么是一个像素点占4字节,下面再说
int byteCount = scaledWidth * scaledHeight * 4;
如果最终选择的是drawable目录,经过测试,其实就和放在drawable-mdpi是一样的,从上面的公式和表格,可以得出drawableDirDensityDpi越小,最终byteCount越大。所以最坏的情况就是本该放在drawable-xxxhdpi目录的图片放到了drawable目录
如果最终选择的是drawable-nodpi目录,那么是不会经过缩放的,也就是
int byteCount = imgWidth * imgHeight * 4
再说下,为什么说一个像素点是4个字节
其实加载图片的时候还要根据BitmapFactory.Options,如果options.inPreferredConfig = Bitmap.Config.ARGB_8888那么一个像素就是占4字节,而默认就是ARGB_8888
ALPHA_8就是Alpha由8位组成=1字节
ARGB_4444就是由4个4位组成即16位=2字节
ARGB_8888就是由4个8位组成即32位=4字节
RGB_565就是R为5位,G为6位,B为5位共16位=2字节
ARGB_4444已经不建议使用,质量太差,官方有这样一句注释
@deprecated Because of the poor quality of this configuration, it is advised to use {@link #ARGB_8888} instead.
关于jpg和png
对于Android来说并没有区别,比如一张100x100的jpg,大小10k,一张100x100的png,大小15k,如果放在同一个drawable目录下,它们加载进来占用的内存是一摸一样的,只跟分辨率有关。通常jpg图片的文件大小会比png图片小,是因为jpg是有损压缩,而png是无损压缩,而且jpg没有alpha通道
Demo
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
BitmapFactory.Options options = new BitmapFactory.Options();
options.inPreferredConfig = Bitmap.Config.ARGB_8888;
// 同一张图片,每个目录都存一份
// drawable-nodpi
Bitmap nodpi = BitmapFactory.decodeResource(getResources(), R.drawable.nodpi);
// drawable
Bitmap _default = BitmapFactory.decodeResource(getResources(), R.drawable._default);
// drawable-mhdpi
Bitmap mhdpi = BitmapFactory.decodeResource(getResources(), R.drawable.mhdpi);
// drawable-hdpi
Bitmap hdpi = BitmapFactory.decodeResource(getResources(), R.drawable.hdpi);
// drawable-xhdpi
Bitmap xhdpi = BitmapFactory.decodeResource(getResources(), R.drawable.xhdpi);
// drawable-xxhdpi
Bitmap xxhdpi = BitmapFactory.decodeResource(getResources(), R.drawable.xxhdpi);
// drawable-xxxhdpi
Bitmap xxxhdpi = BitmapFactory.decodeResource(getResources(), R.drawable.xxxhdpi);
Log.d("TEST", "nodpi=" + nodpi.getByteCount());
Log.d("TEST", "default=" + _default.getByteCount());
Log.d("TEST", "mhdpi=" + mhdpi.getByteCount());
Log.d("TEST", "hdpi=" + hdpi.getByteCount());
Log.d("TEST", "xhdpi=" + xhdpi.getByteCount());
Log.d("TEST", "xxhdpi=" + xxhdpi.getByteCount());
Log.d("TEST", "xxxhdpi=" + xxxhdpi.getByteCount());
int realDensityDpi = getResources().getDisplayMetrics().densityDpi;
// 根据densityDpi从不同的目录加载图片
int drawableDirDensityDpi;
if (realDensityDpi < 240) {
drawableDirDensityDpi = 160;
} else if (realDensityDpi < 320) {
drawableDirDensityDpi = 240;
} else if (realDensityDpi < 480) {
drawableDirDensityDpi = 320;
} else if (realDensityDpi < 560) {
drawableDirDensityDpi = 480;
} else {
drawableDirDensityDpi = 640;
}
float scale = (float) realDensityDpi / drawableDirDensityDpi;
// 默认是Bitmap.Config.ARGB_8888,所以是4字节
// 我的设备dpi是560,所以是在xxxhdpi目录下
int byteCount = (int) (192 * scale + 0.5) * (int) (192 * scale + 0.5) * 4;
Bitmap ic_launcher = BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher);
Log.d("TEST", ic_launcher.getByteCount() + " " + byteCount);
}
}
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