BitmapFactory.decodeStream() returns a Bitmap ~100 times more than original

问题

I have a PNG image of 168.2KB and 1991x1756 and tried to import it into a Bitmap using BitmapFactory.decodeStream(). The problem is that I run into OutOfMemoryError where the Bitmap size ends up being 13,657KB.
What I don't understand is why is the file so big after the import and how to mitigate this.
Seems that this OutOfMemoryError issue is very common with decoded Bitmap images but can't find a way to fix it. Any ideas?

Bitmap bitmap = BitmapFactory.decodeStream(inputStream, null, bitmapOptions);

E/AndroidRuntime(10744): java.lang.OutOfMemoryError: bitmap size exceeds VM budget(Heap Size=7815KB, Allocated=3279KB, Bitmap Size=13657KB)

回答1:

A png-file is highly compressed, meaning that the file size on disk is much smaller than it would be when all the data it contains is loaded into memory.
A bitmap is a very simple representation of an image, where every pixel is represented by a value. The size of this value depends on the current settings (you may have heard of 32-bit images, 16 bit images or 8 bit images which are some common sizes).

To calculate the true size of an image represented as a bitmap you have to calculate the number of pixels, and multiply this with how many bits you have for each pixel.
Your image is 1991x1756 pixels (3 496 196 pixels in total). If you load the bitmap using ARGB_8888 format (one byte for each color/alpha channel = 4 bytes per pixel) you end up with 3 496 196 * 4 = 13 984 784 bytes.

You can reduce the total size by using fewer bytes/channel (RGB_565 for example) although this also reduces the quality of your image. A better option would most likely be to reduce the size of your images, or import them using the inSampleSize parameter in your BitmapFactory.Options to load a smaller image.