Resize a large bitmap file to scaled output file on Android

Question

I have a large bitmap (say 3888x2592) in a file. Now, I want to resize that bitmap to 800x533 and save it to another file. I normally would scale the bitmap by calling

Answer 1

Taking into account that you want to resize to exact size and want to keep as much quality as needed I think you should try this.

1. Find out the size of the resized image with call of BitmapFactory.decodeFile and providing the checkSizeOptions.inJustDecodeBounds
2. Calculate the maximum possible inSampleSize you can use on your device to not exceed the memory. bitmapSizeInBytes = 2*width*height; Generally for your picture inSampleSize=2 would be fine since you will need only 2*1944x1296)=4.8Mbб which should feet in memory
3. Use BitmapFactory.decodeFile with inSampleSize to load the Bitmap
4. Scale the bitmap to exact size.

Motivation: multiple-steps scaling could give you higher quality picture, however there is no guarantee that it will work better than using high inSampleSize. Actually, I think that you also can use inSampleSize like 5 (not pow of 2) to have direct scaling in one operation. Or just use 4 and then you can just use that image in UI. if you send it to server - than you can do scaling to exact size on server side which allow you to use advanced scaling techniques.

Notes: if the Bitmap loaded in step-3 is at least 4 times larger (so the 4*targetWidth < width) you probably can use several resizing to achieve better quality. at least that works in generic java, in android you don't have the option to specify the interpolation used for scaling http://today.java.net/pub/a/today/2007/04/03/perils-of-image-getscaledinstance.html