Question

In my android application I am working on canvas bitmaps (actually it is SCanvas from Samsung SPen SDK, but it does not matter), as a rule 90-95% of area of such bitmaps is transparent, so I expected to have not too large bitmap size in KB. But seems like bitmap size (in KB) does not depend on whether it is a simple background or complex picture, so for example if I have two images (sorry, I am a new user and I can’t post any images):

1) empty frame (1000×700 px, background is transparent, color border)

2) full frame (1000×700 px, background is transparent, color border, a lot of text inside)

the size of both bitmaps are about 1.3MB.

But if I convert these bitmaps in byte arrays, the size of the first array is about 11 times less than the size of the second array.

I have to store a lot of such images as BLOBs in DB and display them as ImageView bitmaps.

Question 1: if I need to display 20 ImageView objects with such 95% transparent images, is there any way to not use 20 * 1.3MB at the same time? To me it seems like it should be only 1 alpha layer + 20 “data” layers.

Question 2: is there any way to reduce size (in KB) of an image with transparency without losing too much quality? The only way I saw is to decode the image byte array with inSampleSize = 2 and then create a scaled bitmap to keep the original image dimensions, something like:

originalBitmap.compress(Bitmap.CompressFormat.PNG, 0, stream);
byte[] bitmapBytes = stream.toByteArray();
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeByteArray(bitmapBytes, 0, bitmapBytes.length, options);
int bWidth = options.outWidth;
int bHeight = options.outHeight;
options.inSampleSize = 2;
options.inPreferredConfig = Bitmap.Config.ARGB_4444;
options.inJustDecodeBounds = false;
Bitmap scaledBitmap = BitmapFactory.decodeByteArray(bitmapBytes, 0, bitmapBytes.length, options);
Bitmap finalBitmap = Bitmap.createScaledBitmap(scaledBitmap, bWidth, bHeight, true);

But the finalBitmap quality is unacceptable, no matter what options I used (inPreferredConfig, inDither).

Any other thoughts about the optimal way to handle the images with about 90% of simple transparent background and 10% of one-color “data”?

Answer 1

No matter what format you use, it will always end up as raw uncompressed (decoded) data before displaying, if it weren’t decoded, you couldn’t see it on the display. If you’re worried about memory, try not to have all the images decoded all the time, just decode the one you have to show, and always release the others (don’t keep a reference to them).