image - Android - how to get or set (print) DPI ( dots per inch ) of JPEG file while loading or saving it programmatically?

Question

I have an app developed on Android versions 4.0 and above. ( The app does not support Android versions below 4.0 [Ice Cream Sandwich] ).

The question is related to (print) DPI of various images ( for eg. of jpeg or png ) format.

This question does NOT relate to SCREEN DPI or sizes of various Android devices. It is also NOT related to showing the Bitmap on the device in screen size.

I am using the following code to load the image file in 'Bitmap'. Then I have been cropping it and saving it to another file in JPEG format with jpegCompression. I have been able to do this by the following code, but I am unable to get DPI of loaded or set the DPI of saved Image file.

So I have two questions.

1) How can I get the (print) DPI from the JPEG file, after or while loading it in 'Bitmap'?

2) While saving the new generated 'Bitmap', how can I set the DPI again in the JPEG file?

Following is the part of code for reference.

    FileInputStream inputStream = new FileInputStream(theSourcePhotoFilePathName);

    Bitmap bitmap = null;
    BitmapRegionDecoder decoder = null;

    BitmapFactory.Options options = new BitmapFactory.Options();        
    options.inSampleSize = 1;
    options.inDensity = 300;   // Tried this but not working.

    try {
        decoder = BitmapRegionDecoder.newInstance(in, false);
        bitmap = decoder.decodeRegion(region, options);      // the region has cropping coordinates.
    } catch (IllegalArgumentException e){
        Log.d("First Activity", "Failed to recycle bitmap for rect=" + region, e);
    } catch (IOException e) {
        Log.d("First Activity", "Failed to decode into rect=" + region, e);
    } finally {
        if (decoder != null) decoder.recycle();
    }

    inputStream.close();
    inputStream = null;

    FileOutputStream fos = new FileOutputStream( theTargetTempFolderDestFilePath );
    bitmap.compress(CompressFormat.JPEG, jpegCompressionRatio, fos);
    fos.flush();
    fos.close();
    fos = null;

I have tried to find from stackoverflow and from other sites by googling, but could not get the proper related answer. So I have decided to ask it in this forum.

Your hints and suggestions are welcome.

Sanjay.

Answer 1

Just for convenience here is ready to copy and paste method:

public static void saveBitmapToJpg(Bitmap bitmap, File file, int dpi) throws IOException {
    ByteArrayOutputStream imageByteArray = new ByteArrayOutputStream();
    bitmap.compress(Bitmap.CompressFormat.JPEG, 100, imageByteArray);
    byte[] imageData = imageByteArray.toByteArray();

    setDpi(imageData, dpi);

    FileOutputStream fileOutputStream = new FileOutputStream(file);
    fileOutputStream.write(imageData);
    fileOutputStream.close();
}

private static void setDpi(byte[] imageData, int dpi) {
    imageData[13] = 1;
    imageData[14] = (byte) (dpi >> 8);
    imageData[15] = (byte) (dpi & 0xff);
    imageData[16] = (byte) (dpi >> 8);
    imageData[17] = (byte) (dpi & 0xff);
}

saved file will have properly set Image DPI value: