android 计算图片占用内存大小

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我是靠谱客的博主深情橘子，最近开发中收集的这篇文章主要介绍android 计算图片占用内存大小，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

android 中计算图片占用内存大小公式：

占用内存大小 = 横向显示像素 * 竖向显示像素 * 单个像素占用字节数 ;

显示像素 = 图片实际像素 * 手机density / 图片density ;

图片density根据图片放在哪个目录来决定，参考下图

如果放在drawable目录，与放在mdpi目录相同。如果放在drawable-nodpi目录，图片density取手机density

手机density由硬件决定

单个像素占用字节数，根据图片的显示配置决定。

如果是在xml为ImageView指定图片，使用的是ARGB-8888

Enum Values
Bitmap.Config	ALPHA_8	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. 此时图片只有alpha值，没有RGB值，一个像素占用一个字节
Bitmap.Config	ARGB_4444	This field is deprecated. Because of the poor quality of this configuration, it is advised to use `ARGB_8888`instead. 这种格式的图片，看起来质量太差，已经不推荐使用。 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 ARGB_8888 instead of this configuration. 一个像素占用2个字节，alpha(A)值，Red（R）值，Green(G)值，Blue（B）值各占4个bites,共16bites,即2个字节
Bitmap.Config	ARGB_8888	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 一个像素占用4个字节，alpha(A)值，Red（R）值，Green(G)值，Blue（B）值各占8个bites,共32bites,即4个字节这是一种高质量的图片格式，电脑上普通采用的格式。它也是Android手机上一个BitMap的默认格式。
Bitmap.Config	RGB_565	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. 一个像素占用2个字节，没有alpha(A)值，即不支持透明和半透明，Red（R）值占5个bites ，Green(G)值占6个bites ，Blue（B）值占5个bites,共16bites,即2个字节.对于没有透明和半透明颜色的图片来说，该格式的图片能够达到比较的呈现效果，相对于ARGB_8888来说也能减少一半的内存开销。因此它是一个不错的选择。另外我们通过android.content.res.Resources来取得一个张图片时，它也是以该格式来构建BitMap的. 从Android4.0开始，该选项无效。即使设置为该值，系统任然会采用 ARGB_8888来构造图片