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Access to raw data in ARGB_8888 Android Bitmap

Posted by: admin May 14, 2020 Leave a comment

Questions:

I am trying to access the raw data of a Bitmap in ARGB_8888 format on Android, using the copyPixelsToBuffer and copyPixelsFromBuffer methods. However, invocation of those calls seems to always apply the alpha channel to the rgb channels. I need the raw data in a byte[] or similar (to pass through JNI; yes, I know about bitmap.h in Android 2.2, cannot use that).

Here is a sample:

    // Create 1x1 Bitmap with alpha channel, 8 bits per channel
    Bitmap one = Bitmap.createBitmap(1,1,Bitmap.Config.ARGB_8888);
    one.setPixel(0,0,0xef234567);
    Log.v("?","hasAlpha() = "+Boolean.toString(one.hasAlpha()));
    Log.v("?","pixel before = "+Integer.toHexString(one.getPixel(0,0)));

    // Copy Bitmap to buffer
    byte[] store = new byte[4];
    ByteBuffer buffer  = ByteBuffer.wrap(store);
    one.copyPixelsToBuffer(buffer);

    // Change value of the pixel
    int value=buffer.getInt(0);
    Log.v("?", "value before = "+Integer.toHexString(value));
    value = (value >> 8) | 0xffffff00;
    buffer.putInt(0, value);
    value=buffer.getInt(0);
    Log.v("?", "value after = "+Integer.toHexString(value));

    // Copy buffer back to Bitmap
    buffer.position(0);
    one.copyPixelsFromBuffer(buffer);
    Log.v("?","pixel after = "+Integer.toHexString(one.getPixel(0,0)));

The log then shows

hasAlpha() = true
pixel before = ef234567
value before = 214161ef
value after = ffffff61
pixel after = 619e9e9e

I understand that the order of the argb channels is different; that’s fine. But I don’t
want the alpha channel to be applied upon every copy (which is what it seems to be doing).

Is this how copyPixelsToBuffer and copyPixelsFromBuffer are supposed to work? Is there any way to get the raw data in a byte[]?

Added in response to answer below:

Putting in buffer.order(ByteOrder.nativeOrder()); before the copyPixelsToBuffer does change the result, but still not in the way I want it:

pixel before = ef234567
value before = ef614121
value after = ffffff41
pixel after = ff41ffff

Seems to suffer from essentially the same problem (alpha being applied upon each copyPixelsFrom/ToBuffer).

How to&Answers:

I realize this is very stale and probably won’t help you now, but I came across this recently in trying to get copyPixelsFromBuffer to work in my app. (Thank you for asking this question, btw! You saved me tons of time in debugging.) I’m adding this answer in the hopes it helps others like me going forward…

Although I haven’t used this yet to ensure that it works, it looks like that, as of API Level 19, we’ll finally have a way to specify not to “apply the alpha” (a.k.a. premultiply) within Bitmap. They’re adding a setPremultiplied(boolean) method that should help in situations like this going forward by allowing us to specify false.

I hope this helps!

Answer:

One way to access data in Bitmap is to use getPixels() method. Below you can find an example I used to get grayscale image from argb data and then back from byte array to Bitmap (of course if you need rgb you reserve 3x bytes and save them all…):

/*Free to use licence by Sami Varjo (but nice if you retain this line)*/

public final class BitmapConverter {

    private BitmapConverter(){};

   /**
    * Get grayscale data from argb image to byte array
    */
   public static byte[] ARGB2Gray(Bitmap img)
   {

       int width = img.getWidth();
       int height = img.getHeight();

       int[] pixels = new int[height*width];
       byte grayIm[] = new byte[height*width];

       img.getPixels(pixels,0,width,0,0,width,height);

       int pixel=0;
       int count=width*height;

       while(count-->0){
           int inVal = pixels[pixel];

           //Get the pixel channel values from int 
           double r = (double)( (inVal & 0x00ff0000)>>16 );
           double g = (double)( (inVal & 0x0000ff00)>>8  );
           double b = (double)(  inVal & 0x000000ff)      ;

           grayIm[pixel++] = (byte)( 0.2989*r + 0.5870*g + 0.1140*b );
       }

       return grayIm;
   }

   /**
    * Create a gray scale bitmap from byte array
    */
   public static Bitmap gray2ARGB(byte[] data, int width, int height)
   {
       int count = height*width;
       int[] outPix = new int[count];
       int pixel=0;
       while(count-->0){
           int val = data[pixel] & 0xff; //convert byte to unsigned
           outPix[pixel++] = 0xff000000 | val << 16 | val << 8 | val ;
       }

       Bitmap out =  Bitmap.createBitmap(outPix,0,width,width, height, Bitmap.Config.ARGB_8888);
       return out;
   }

}

Answer:

My guess is that this might have to do with the byte order of the ByteBuffer you are using. ByteBuffer uses big endian by default.
Set endianess on the buffer with

buffer.order(ByteOrder.nativeOrder());

See if it helps.

Moreover, copyPixelsFromBuffer/copyPixelsToBuffer does not change the pixel data in any way. They are copied raw.

Answer:

This is an old question, but i got to the same issue, and just figured out that the bitmap byte are pre-multiplied, you can set the bitmap (as of API 19) to not pre-multiply the buffer, but in the API they make no guarantee.

From the docs:

public final void setPremultiplied(boolean premultiplied)

Sets whether the bitmap should treat its data as pre-multiplied.
Bitmaps are always treated as pre-multiplied by the view system and Canvas for performance reasons. Storing un-pre-multiplied data in a Bitmap (through setPixel, setPixels, or BitmapFactory.Options.inPremultiplied) can lead to incorrect blending if drawn by the framework.

This method will not affect the behaviour of a bitmap without an alpha channel, or if hasAlpha() returns false.

Calling createBitmap or createScaledBitmap with a source Bitmap whose colors are not pre-multiplied may result in a RuntimeException, since those functions require drawing the source, which is not supported for un-pre-multiplied Bitmaps.