Question

i implement some code for water color effect on image in android but it was to slow(it’s take more then 2 minute) now i try to implement this in JNI for batter speed ,
hear is my java code for

the inPixels is pixel of Bitmap .

 protected int[] filterPixels( int width, int height, int[] inPixels ) 
    {
        int levels = 256;
        int index = 0;

        int[] rHistogram = new int[levels];
        int[] gHistogram = new int[levels];
        int[] bHistogram = new int[levels];
        int[] rTotal = new int[levels];
        int[] gTotal = new int[levels];
        int[] bTotal = new int[levels];
        int[] outPixels = new int[width * height];

        for (int y = 0; y < height; y++) 
        {
            for (int x = 0; x < width; x++) 
            {
                for (int i = 0; i < levels; i++)
                    rHistogram[i] = gHistogram[i] = bHistogram[i] = rTotal[i] = gTotal[i] = bTotal[i] = 0;

                for (int row = -range; row <= range; row++) 
                {
                    int iy = y+row;
                    int ioffset;
                    if (0 <= iy && iy < height) 
                    {
                        ioffset = iy*width;
                        for (int col = -range; col <= range; col++) 
                        {
                            int ix = x+col;
                            if (0 <= ix && ix < width) {
                                int rgb = inPixels[ioffset+ix];
                                int r = (rgb >> 16) & 0xff;
                                int g = (rgb >> 8) & 0xff;
                                int b = rgb & 0xff;
                                int ri = r*levels/256;
                                int gi = g*levels/256;
                                int bi = b*levels/256;
                                rTotal[ri] += r;
                                gTotal[gi] += g;
                                bTotal[bi] += b;
                                rHistogram[ri]++;
                                gHistogram[gi]++;
                                bHistogram[bi]++;
                            }
                        }
                    }
                }

                int r = 0, g = 0, b = 0;
                for (int i = 1; i < levels; i++) 
                {
                    if (rHistogram[i] > rHistogram[r])
                        r = i;
                    if (gHistogram[i] > gHistogram[g])
                        g = i;
                    if (bHistogram[i] > bHistogram[b])
                        b = i;
                }
                r = rTotal[r] / rHistogram[r];
                g = gTotal[g] / gHistogram[g];
                b = bTotal[b] / bHistogram[b];
                outPixels[index] = (inPixels[index] & 0xff000000) | ( r << 16 ) | ( g << 8 ) | b;
                index++;
            }
        }

        return outPixels;
    }

**OUTPUT image **

and i try to convert this java code to c code but i don’t what is the wrong ,
hear the code for C

 void filterPixels( int width, int height, int inPixels[] )
    {



        int levels = 256;
        int index = 0;

        int rHistogram [levels];
        int gHistogram [levels];
        int bHistogram [levels];
        int rTotal   [levels];
        int gTotal [levels];
        int bTotal [levels];
        int outPixels [width * height];

        //Loop Variables
        int y ;
        int x ;
        int i ;
        int row ;
        int col ;
        int j ;

        int range = 5 ;

        for ( y = 0; y < height; y++)
        {
            for ( x = 0; x < width; x++)
            {
                for ( i = 0; i < levels; i++)
                    rHistogram[i] = gHistogram[i] = bHistogram[i] = rTotal[i] = gTotal[i] = bTotal[i] = 0;

                for ( row = -range; row <= range; row++)
                {
                    int iy = y+row;
                    int ioffset;
                    if (0 <= iy && iy < height)
                    {
                        ioffset = iy*width;
                        for ( col = -range; col <= range; col++)
                        {
                            int ix = x+col;
                            if (0 <= ix && ix < width) {
                                int rgb = inPixels[ioffset+ix];
                                int r = (rgb >> 16) & 0xff;
                                int g = (rgb >> 8) & 0xff;
                                int b = rgb & 0xff;
                                int ri = r*levels/256;
                                int gi = g*levels/256;
                                int bi = b*levels/256;
                                rTotal[ri] += r;
                                gTotal[gi] += g;
                                bTotal[bi] += b;
                                rHistogram[ri]++;
                                gHistogram[gi]++;
                                bHistogram[bi]++;
                            }
                        }
                    }
                }

                int r = 0, g = 0, b = 0;
                for ( j = 1; j < levels; j++)
                {
                    if (rHistogram[j] > rHistogram[r])
                        r = j;
                    if (gHistogram[j] > gHistogram[g])
                        g = j;
                    if (bHistogram[j] > bHistogram[b])
                        b = j;
                }
                r = rTotal[r] / rHistogram[r];
                g = gTotal[g] / gHistogram[g];
                b = bTotal[b] / bHistogram[b];
                outPixels[index] = (inPixels[index] & 0xff000000) | ( r << 16 ) | ( g << 8 ) | b;
                index++;
            }
        }
    }

i check the pixel value of java code and c code both are same(for same image)

code for call native function from my android activity .

int[] pix = new int[oraginal.getWidth() * oraginal.getHeight()];

                Bitmap bitmap = oraginal.copy(oraginal.getConfig(), true);
                bitmap.getPixels(pix, 0, bitmap.getWidth(), 0, 0,bitmap.getWidth(), bitmap.getHeight());
                filterPixelsJNI(bitmap.getWidth(), bitmap.getHeight(), pix);

                 bitmap.setPixels(pix, 0, bitmap.getWidth(), 0, 0,bitmap.getWidth(), bitmap.getHeight());
                 myView.setImageBitmap(bitmap);

this is my first try for JNI so plz help me in this .

UPDATE

public native void filterPixelsJNI( int width, int height, int inPixels[] );

JNI

 JNIEXPORT void JNICALL Java_com_testndk_HelloWorldActivity_filterPixelsJNI (JNIEnv * env, jobject obj , jint width,jint height,jint inPixels[]){

     filterPixels( width, height, inPixels);
 }

filterPixels method witch is call from c code .

Answer 1

There are several problems with your JNI code. The algorithmic part is probably correct, but you’re not dealing with the Java array to C array conversion correctly.

First of all, the last argument of Java_com_testndk_HelloWorldActivity_filterPixelsJNI should be of type jintArray, and not jint []. This is how you pass a Java array to C code.

Once you get this array, you can’t process it directly, you’ll have to convert it to a C array:

JNIEXPORT void JNICALL Java_com_testndk_HelloWorldActivity_filterPixelsJNI (JNIEnv * env, jobject obj , jint width, jint height, jintArray inPixels) {

    int *c_inPixels = (*env)->GetIntArrayElements(env, inPixels, NULL);
    filterPixels( width, height, c_inPixels);
    // passing 0 as the last argument should copy native array to Java array
    (*env)->ReleaseIntArrayElements(env, inPixels, c_inPixels, 0);
}

I advise you to look at the JNI documentation, which explains how to deal with arrays: http://docs.oracle.com/javase/1.5.0/docs/guide/jni/spec/functions.html

Note that there are now easier ways of processing Java Bitmap objects using android NDK. See an other of my answers here for details.