计算Bitmap内存占用计算
uzsu4968
8年前
<p>densityDpi表示一英寸有多少个像素点</p> <table> <thead> <tr> <th>densityDpi</th> <th>density</th> <th>drawable目录</th> <th>ic_launcher大小</th> </tr> </thead> <tbody> <tr> <td>640</td> <td>4</td> <td>drawable-xxxhdpi</td> <td>192x192</td> </tr> <tr> <td>480</td> <td>3</td> <td>drawable-xxhdpi</td> <td>144x144</td> </tr> <tr> <td>320</td> <td>2</td> <td>drawable-xhdpi</td> <td>96x96</td> </tr> <tr> <td>240</td> <td>1.5</td> <td>drawable-hdpi</td> <td>72x72</td> </tr> <tr> <td>160</td> <td>1</td> <td>drawable-mdpi</td> <td>48x48</td> </tr> <tr> <td>160</td> <td>1</td> <td>drawable</td> <td>-</td> </tr> <tr> <td>-</td> <td>-</td> <td>drawable-nodpi</td> <td>-</td> </tr> </tbody> </table> <p>xml布局使用R.drawable.xx相当于代码</p> <p>BitmapFactory.decodeResource(getResources(), R.drawable.xx)</p> <p>假设当前设备的dpi是560，那么系统匹配drawable目录的顺序是：</p> <p>1) drawable-xxxhdpi</p> <p>2) drawable-nodpi</p> <p>3) drawable-xxhdpi</p> <p>4) drawable-xhdpi</p> <p>5) drawable-hdpi</p> <p>6) drawable-mdpi</p> <p>7) drawable</p> <p>加载图片会经过缩放，内存占用计算公式如下</p> <pre> <code class="language-java">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;</code></pre> <p>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，从下上面的公式和表格，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</p> <p>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</p> <pre> <code class="language-java">int byteCount=imgWidth * imgHeight * 4</code></pre> <p>再说下，为什么说一个像素点是4个字节</p> <p>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</p> <p>ALPHA_8就是Alpha由8位组成=1字节</p> <p>ARGB_4444就是由4个4位组成即16位=2字节</p> <p>ARGB_8888就是由4个8位组成即32位=4字节</p> <p>RGB_565就是R为5位，G为6位，B为5位共16位=2字节</p> <p>ARGB_4444已经不建议使用，质量太差，官方有这样一句注释</p> <p>@deprecated Because of the poor quality of this configuration, it is advised to use {@link #ARGB_8888} instead.</p> <h2><strong>关于jpg和png</strong></h2> <p>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</p> <p>Demo</p> <p><img src="https://simg.open-open.com/show/2622aca071f34c071583487b01f486f7.png"></p> <pre> <code class="language-java">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()); DisplayMetrics displayMetrics = new DisplayMetrics(); getWindowManager().getDefaultDisplay().getMetrics(displayMetrics); float realDensityDpi = displayMetrics.densityDpi; // 根据densityDpi从不同的目录加载图片 float 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 = 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); } }</code></pre> <p> </p> <p> </p> <p> </p> <p>来自：http://www.jianshu.com/p/e1d42bf88d40</p> <p> </p>