Android - Read PNG image without alpha and decode as ARGB_8888

Question

I try to read an image from sdcard (in emulator) and then create a Bitmap image with the

BitmapFactory.decodeByteArray

method. I set the options:

options.inPrefferedConfig = Bitmap.Config.ARGB_8888
options.inDither = false

Then I extract the pixels into a ByteBuffer.

ByteBuffer buffer = ByteBuffer.allocateDirect(width*height*4)
bitmap.copyPixelsToBuffer(buffer)

I use this ByteBuffer then in the JNI to convert it into RGB format and want to calculate on it.

But always I get false data - I test without modifying the ByteBuffer. Only thing I do is to put it into the native method into JNI. Then cast it into a unsigned char* and convert it back into a ByteBuffer before returning it back to Java.

unsigned char* buffer = (unsinged char*)(env->GetDirectBufferAddress(byteBuffer))
jobject returnByteBuffer = env->NewDirectByteBuffer(buffer, length)

Before displaying the image I get data back with

bitmap.copyPixelsFromBuffer( buffer )

But then it has wrong data in it.

My Question is if this is because the image is internally converted into RGB 565 or what is wrong here?

.....

Have an answer for it:

->>> yes, it is converted internally to RGB565.

Does anybody know how to create such an bitmap image from PNG with ARGB8888 pixel format?

If anybody has an idea, it would be great!

Answer 1

An ARGB_8888 Bitmap (on pre Honeycomb versions) is natively stored in the RGBA format. So the alpha channel is moved at the end. You should take this into account when accessing a Bitmap's pixels natively.

I assume you are writing code for a version of Android lower than 3.2 (API level < 12), because since then the behavior of the methods

BitmapFactory.decodeFile(pathToImage);
BitmapFactory.decodeFile(pathToImage, opt);
bitmapObject.createScaledBitmap(bitmap, desiredWidth, desiredHeight, false /*filter?*/);

has changed.

On older platforms (API level < 12) the BitmapFactory.decodeFile(..) methods try to return a Bitmap with RGB_565 config by default, if they can't find any alpha, which lowers the quality of an iamge. This is still ok, because you can enforce an ARGB_8888 bitmap using

options.inPrefferedConfig = Bitmap.Config.ARGB_8888
options.inDither = false 

The real problem comes when each pixel of your image has an alpha value of 255 (i.e. completely opaque). In that case the Bitmap's flag 'hasAlpha' is set to false, even though your Bitmap has ARGB_8888 config. If your *.png-file had at least one real transparent pixel, this flag would have been set to true and you wouldn't have to worry about anything.

So when you want to create a scaled Bitmap using

bitmapObject.createScaledBitmap(bitmap, desiredWidth, desiredHeight, false /*filter?*/);

the method checks whether the 'hasAlpha' flag is set to true or false, and in your case it is set to false, which results in obtaining a scaled Bitmap, which was automatically converted to the RGB_565 format.

Therefore on API level >= 12 there is a public method called

public void setHasAlpha (boolean hasAlpha);

which would have solved this issue. So far this was just an explanation of the problem. I did some research and noticed that the setHasAlpha method has existed for a long time and it's public, but has been hidden (@hide annotation). Here is how it is defined on Android 2.3:

/**
 * Tell the bitmap if all of the pixels are known to be opaque (false)
 * or if some of the pixels may contain non-opaque alpha values (true).
 * Note, for some configs (e.g. RGB_565) this call is ignore, since it does
 * not support per-pixel alpha values.
 *
 * This is meant as a drawing hint, as in some cases a bitmap that is known
 * to be opaque can take a faster drawing case than one that may have
 * non-opaque per-pixel alpha values.
 *
 * @hide
 */
public void setHasAlpha(boolean hasAlpha) {
    nativeSetHasAlpha(mNativeBitmap, hasAlpha);
}

Now here is my solution proposal. It does not involve any copying of bitmap data:

1. Checked at runtime using java.lang.Reflect if the current Bitmap implementation has a public 'setHasAplha' method. (According to my tests it works perfectly since API level 3, and i haven't tested lower versions, because JNI wouldn't work). You may have problems if a manufacturer has explicitly made it private, protected or deleted it.

2. Call the 'setHasAlpha' method for a given Bitmap object using JNI. This works perfectly, even for private methods or fields. It is official that JNI does not check whether you are violating the access control rules or not. Source: http://java.sun.com/docs/books/jni/html/pitfalls.html (10.9) This gives us great power, which should be used wisely. I wouldn't try modifying a final field, even if it would work (just to give an example). And please note this is just a workaround...

Here is my implementation of all necessary methods:

JAVA PART:

// NOTE: this cannot be used in switch statements
    private static final boolean SETHASALPHA_EXISTS = setHasAlphaExists();

    private static boolean setHasAlphaExists() {
        // get all puplic Methods of the class Bitmap
        java.lang.reflect.Method[] methods = Bitmap.class.getMethods();
        // search for a method called 'setHasAlpha'
        for(int i=0; i<methods.length; i++) {
            if(methods[i].getName().contains("setHasAlpha")) {
                Log.i(TAG, "method setHasAlpha was found");
                return true;
            }
        }
        Log.i(TAG, "couldn't find method setHasAlpha");
        return false;
    }

    private static void setHasAlpha(Bitmap bitmap, boolean value) {
        if(bitmap.hasAlpha() == value) {
            Log.i(TAG, "bitmap.hasAlpha() == value -> do nothing");
            return;
        }

        if(!SETHASALPHA_EXISTS) {   // if we can't find it then API level MUST be lower than 12
            // couldn't find the setHasAlpha-method
            // <-- provide alternative here...
            return;
        }

        // using android.os.Build.VERSION.SDK to support API level 3 and above
        // use android.os.Build.VERSION.SDK_INT to support API level 4 and above
        if(Integer.valueOf(android.os.Build.VERSION.SDK) <= 11) {
            Log.i(TAG, "BEFORE: bitmap.hasAlpha() == " + bitmap.hasAlpha());
            Log.i(TAG, "trying to set hasAplha to true");
            int result = setHasAlphaNative(bitmap, value);
            Log.i(TAG, "AFTER: bitmap.hasAlpha() == " + bitmap.hasAlpha());

            if(result == -1) {
                Log.e(TAG, "Unable to access bitmap."); // usually due to a bug in the own code
                return;
            }
        } else {    //API level >= 12
            bitmap.setHasAlpha(true);
        }
    }

    /**
     * Decodes a Bitmap from the SD card
     * and scales it if necessary
     */
    public Bitmap decodeBitmapFromFile(String pathToImage, int pixels_limit) {
        Bitmap bitmap;

        Options opt = new Options();
        opt.inDither = false;   //important
        opt.inPreferredConfig = Bitmap.Config.ARGB_8888;
        bitmap = BitmapFactory.decodeFile(pathToImage, opt);

        if(bitmap == null) {
            Log.e(TAG, "unable to decode bitmap");
            return null;
        }

        setHasAlpha(bitmap, true);  // if necessary

        int numOfPixels = bitmap.getWidth() * bitmap.getHeight();

        if(numOfPixels > pixels_limit) {    //image needs to be scaled down 
            // ensures that the scaled image uses the maximum of the pixel_limit while keeping the original aspect ratio
            // i use: private static final int pixels_limit = 1280*960; //1,3 Megapixel
            imageScaleFactor = Math.sqrt((double) pixels_limit / (double) numOfPixels);
            Bitmap scaledBitmap = Bitmap.createScaledBitmap(bitmap,
                    (int) (imageScaleFactor * bitmap.getWidth()), (int) (imageScaleFactor * bitmap.getHeight()), false);

            bitmap.recycle();
            bitmap = scaledBitmap;

            Log.i(TAG, "scaled bitmap config: " + bitmap.getConfig().toString());
            Log.i(TAG, "pixels_limit = " + pixels_limit);
            Log.i(TAG, "scaled_numOfpixels = " + scaledBitmap.getWidth()*scaledBitmap.getHeight());

            setHasAlpha(bitmap, true); // if necessary
        }

        return bitmap;
    }

Load your lib and declare the native method:

static {
    System.loadLibrary("bitmaputils");
}

private static native int setHasAlphaNative(Bitmap bitmap, boolean value);

Native section ('jni' folder)

Android.mk:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE    := bitmaputils
LOCAL_SRC_FILES := bitmap_utils.c
LOCAL_LDLIBS := -llog -ljnigraphics -lz -ldl -lgcc
include $(BUILD_SHARED_LIBRARY)

bitmapUtils.c:

#include <jni.h>
#include <android/bitmap.h>
#include <android/log.h>

#define  LOG_TAG    "BitmapTest"
#define  Log_i(...)  __android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
#define  Log_e(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)


// caching class and method IDs for a faster subsequent access
static jclass bitmap_class = 0;
static jmethodID setHasAlphaMethodID = 0;

jint Java_com_example_bitmaptest_MainActivity_setHasAlphaNative(JNIEnv * env, jclass clazz, jobject bitmap, jboolean value) {
    AndroidBitmapInfo info;
    void* pixels;


    if (AndroidBitmap_getInfo(env, bitmap, &info) < 0) {
        Log_e("Failed to get Bitmap info");
        return -1;
    }

    if (info.format != ANDROID_BITMAP_FORMAT_RGBA_8888) {
        Log_e("Incompatible Bitmap format");
        return -1;
    }

    if (AndroidBitmap_lockPixels(env, bitmap, &pixels) < 0) {
        Log_e("Failed to lock the pixels of the Bitmap");
        return -1;
    }


    // get class
    if(bitmap_class == NULL) {  //initializing jclass
        // NOTE: The class Bitmap exists since API level 1, so it just must be found.
        bitmap_class = (*env)->GetObjectClass(env, bitmap);
        if(bitmap_class == NULL) {
            Log_e("bitmap_class == NULL");
            return -2;
        }
    }

    // get methodID
    if(setHasAlphaMethodID == NULL) { //initializing jmethodID
        // NOTE: If this fails, because the method could not be found the App will crash.
        // But we only call this part of the code if the method was found using java.lang.Reflect
        setHasAlphaMethodID = (*env)->GetMethodID(env, bitmap_class, "setHasAlpha", "(Z)V");
        if(setHasAlphaMethodID == NULL) {
            Log_e("methodID == NULL");
            return -2;
        }
    }

    // call java instance method
    (*env)->CallVoidMethod(env, bitmap, setHasAlphaMethodID, value);

    // if an exception was thrown we could handle it here
    if ((*env)->ExceptionOccurred(env)) {
        (*env)->ExceptionDescribe(env);
        (*env)->ExceptionClear(env);
        Log_e("calling setHasAlpha threw an exception");
        return -2;
    }

    if(AndroidBitmap_unlockPixels(env, bitmap) < 0) {
        Log_e("Failed to unlock the pixels of the Bitmap");
        return -1;
    }

    return 0;   // success
}

That's it. We are done. I've posted the whole code for copy-and-paste purposes. The actual code isn't that big, but making all these paranoid error checks makes it a lot bigger. I hope this could be helpful to anyone.