My Android application loads images, does some processing and saves the processed images on the SD card. I save temporary files to the SD card instead of using buffers. For example, reading a background, scrolling the image, drawing an annotation, merging background and annotation in a saved temp file to use as next background, and so forth. Typical snippets:
bitmap = Bitmap.createBitmap(imageSizeX, imageSizeY, Bitmap.Config.ARGB_8888);
bitmap = BitmapFactory.decodeFile(path, options);
bitmap.compress(Bitmap.CompressFormat.JPEG, 100, output);
Everything works fine but in some cases the saved images have added noise similar to that described in this post: Bitmap resizing and rotating: linear noise
The author of the post cited solved the problem by subsampling, but I don't wish to do that.
I have tried the solutions suggested here: http://www.curious-creature.org/2010/12/08/bitmap-quality-banding-and-dithering/#more-1218 which if I understand correctly should be achieved by setting: getWindow().setFormat(PixelFormat.TRANSLUCENT); to force 32 bit.
which does not change much. Setting or not setting the dither flag does not seem to change much either. Any other ideas?
In answer to my own question: The noise disappears, or at least the result is way better, by compressing to .png instead of .jpg
bitmap.compress(Bitmap.CompressFormat.PNG, 100, output);
The same sky which looks like a carpet texture in .jpg looks smooth and clean in .png. Since the difference is much more evident than the usual difference between the same image compressed to .png and .jpg, I guess it depends on the Android implementation. Setting explicitly TRANSLUCENT and DITHER does not make much difference one way or another.