Downsizing Android Bitmap always results in jagged edges

Question

I'm seeing very ugly artifacting / jagged edges when I downsize an image on an Android device no matter what I try. I've gone through several potential solutions I found on StackOverflow and blogs, and everything seems to give me similar results.

Original Image (4096 x 4096):

Scaled Image (214 x 214) (notice the jagged edges):

What I have tried:

1. Drawing the image to a Canvas using a Paint with anti-aliasing, and filtering enabled
2. Multiple variations of BitmapFactory.decode
3. bitmap.scale()
4. Compressor - an Android Image Scaling Library

All of the above trials have yielded almost the exact same result. This is such a common problem though, that surely I'm overlooking something, or not doing something properly.

If I use a web-based image-resizer, here is the result: What it should look like:

What can I do to get the same results as the above image?

Answer 1

The lack of smoothness in the trial image indicate some anti-aliasing is missing.

A relatively quick search show that Sub-pixel antialiasing rules can get complicated.

The next thing is to figure out which tool the "web-based image resizer" used to generate the trial image.

Nine times out of ten (or more) the website would be using FOSS software, which would lead to ImageMagick

A quick run of your source image through ImageMagick with a command like:

convert SO_source_image.png -resize 5% SO_result.png

Results in this similar but not exact (205x205 pixels) image:

At this point you could either dive into the algorithms used by ImageMagick (see the command line options for the variety of methods) or see if you can get a similar result with an existing port like: cherryleafroad/Android-ImageMagick7 or paulasiimwe/Android-ImageMagick

Answer 2

There is a tricky way to (manage to) achieve that only with standard APIs. Just avoid scaling down the bitmap at once.

Bitmap bitmap = BitmapFactory.decodeStream(stream);

:
    
Matrix m = new Matrix();
m.setScale(0.5F, 0.5F);
while (bitmap.getWidth() > 256)
{
    Bitmap bitmap_half = Bitmap.createBitmap(bitmap, 0, 0, bitmap.getWidth(), bitmap.getHeight(), m, true);
    bitmap.recycle();
    bitmap = bitmap_half;
}

imageView.setImageBitmap(bitmap);

Result:

How this works:

Since the filter is short-ranged (primarily designed for scaling up e.g. bilinear or bicubic), it's useless for scaling down (=discrete resampling) in general cases. However it does refer neighboring pixels for calculating new pixel colors. So by avoiding too sparse resampling, it's possible to make it work as a smoothing filter for scaling down. maybe.

Answer 3

BitmapCompat.createScaledBitmap() will downscale at much higher quality. The documentation for the function states: "This algorithm is intended for downscaling by large ratios when high quality is desired"