Bitmap.CompressFormat.WEBP on Android API 19 alpha channel becomes black

Question

I wrote a service that retrieves image from web (JPEG, PNG, ...), which I then, save to disk in webp format.

I save the image using the following code:

try (FileOutputStream fos = new FileOutputStream(imgFile)) {
    bitmap.compress(Bitmap.CompressFormat.WEBP, 100, fos);
} catch (IOException e) {
    Log.e(TAG, "IOException writing file");
} catch (SecurityException e) {
    Log.e(TAG, "SecurityException writing file");
}

I have not got any warnings.

If the App runs in a device with API 22 or 23 the decoded image files are shown with the alpha channel when is present.

If I run the app in an API19 Emulator the decoded image is not shown with the alpha channel (it is shown black).

I preferred WEBP because is lighter than PNG and it should have alpha channel.

PS: I tried with quality 80 and 100 too

Comment 1

The retrieved image is a PNG with alpha channel.

The same image in other devices (and emulators too) with API > 22 when converted in WEBP it shows the transparency.

I tried with other PNG's too. If I use the retrieved bitmap and I save it in PNG in all devices it is shown correctly with transparency, in API 19 too.

The issue is verified only when I save it in webp format. I tried to pull the webp encoded file from the emulator API 19 and it hasn't the alpha channel anymore.

I retrieve the bitmap in this way:

        try (InputStream is = new URL(mImageUrl).openStream()) {
            bitmap = BitmapFactory.decodeStream(is);
        } catch (MalformedURLException e) {
            Log.e(TAG, "MalformedURLException " + e.getMessage());
        } catch (IOException e) {
            Log.e(TAG, "IOException decoding url " + e.getMessage());
        }

COMMENT 2

If I tried to replace the WEBP file in the emulator with one with transparency from my computer, then... the App will show the image with transparency.

So, it is a problem of how the file is saved!