Bitmap load memory analysis

One 、Bitmap Memory

1.Bitmap How memory is calculated

1.decodeResource from drawable、mipmap The corresponding file directory is loaded with pictures
Bitamp Occupied memory size = Width pixels x （inTargetDensity / inDensity） x Height pixel x （inTargetDensity / inDensity）x Memory occupied by one pixel
inTargetDensity ： The equipment dpi
inDensity : Current file location drawable Folder Directory Corresponding dpi

Memory occupied by one pixel , There are several pixel formats :

ALPHA_8 1byte
ARGB_4444 2byte
ARGB_8888 4byte
RGB_565 2byte

2. From local disk 、asset、 And so on
Bitamp Occupied memory size = Width pixels x Height pixel x Memory occupied by one pixel

Be careful ： When using third-party libraries , These formulas cannot be used to calculate , Because the internal memory of the third-party library will be re optimized .
eg:
fresco： No matter where the picture comes from , Even if it's res, The memory size occupied by the image is still calculated according to the resolution of the original image .
Glide： If there is a control to set the picture display , Then it will be automatically according to the size of the control , Reduce the resolution of the picture . The source of the picture is res The resolution conversion rule of is also invalid for it .

Be careful ：Bitmap The memory size of And the pixel size is different .
Memory size = Pixel size （ I.e. picture width x high ）x Memory occupied by one pixel

Two 、 Two factors that affect memory

1、 Image resolution , That is, the size of the picture
2、 The amount of memory occupied by pixels , Corresponding 4 Two pixel formats

So when I do memory optimization , We can start from these two aspects ：
1. Reduce the size of the picture
2. Adjust to pixel format that takes up less memory .
In development , We prefer the first , Because changing the pixel format , It will cause the picture quality to decline , Cause distortion .
And for the first , You can refer to Luban

3、 ... and 、 Conclusion

1. The source of the picture is res When different resource directories within , The system will be based on the current status of the equipment dpi Value and the corresponding resource directory dpi value , Do a resolution conversion , The following rules ： New resolution = The horizontal resolution of the original image * ( The equipment dpi / The directory corresponds to dpi ) * The vertical resolution of the original image * ( The equipment dpi / The directory corresponds to dpi ).
2. Source of other pictures , Disk , file , Flow, etc. , The memory size of the image is calculated according to the resolution of the original image .
3. jpg、png Just a container for pictures , The storage size of the image file itself has nothing to do with the memory size it occupies .

Reference resources

1.Bitmap analysis
2.https://www.cnblogs.com/dasusu/p/9789389.html
3.Bitmap analysis