How to convert Android code to C in Android NDK

Issue

I am trying to convert this Android code in C. I want to do this part in C NDK. Here is what I have tried so far. I am not able to convert getPixel() and setPixel() method of android. Please see the code below.

JNIEXPORT jobject JNICALL
Java_com_test_mypic_StylePreviewActivity_mergeBitmaps(JNIEnv *env, jobject instance,
                                                                      jobject bm, jobject filter) {
    AndroidBitmapInfo bm1;
    AndroidBitmap_getInfo(env, bm, &bm1);

    AndroidBitmapInfo filter1;
    AndroidBitmap_getInfo(env, filter, &filter1);

    int width = bm1.width;
    int height = bm1.height;
    int w2 = filter1.width;
    int h2 = filter1.height;
    float scaleX = (float) w2 / (float) width;

    Bitmap result = Bitmap.createBitmap(w2, h2, ANDROID_BITMAP_FORMAT_RGB_565);

    void* bitmapPixels;
    AndroidBitmap_lockPixels(env, bm, &bitmapPixels);

    for (int x = 0; x < w2; x++) {
        for (int y = 0; y < h2 && y < height; y++) {
            int xx =(int) ((float) x / scaleX);
            int yy = (int) ((float) y / scaleX);
            int pixel = bm1.getPixel(xx, yy);
            int fp = filter1.getPixel(x, y);

            int alpha = (fp & 0xFF000000) >> 24;

            if (alpha == 0) {
                result.setPixel(x, y, pixel);
            }
        }
    }
    result = Bitmap.createBitmap(result, 0, 0, width, height);
    return result;
}

With the help of @Serhio I wrote this method. But it seems something wrong in here.

   JNIEXPORT jobject JNICALL
Java_com_test_mypic_StylePreviewActivity_mergeBitmaps(JNIEnv *env, jobject instance,
                                                                      jobject bm, jobject filter, jobject result) {
    AndroidBitmapInfo bm1;
    AndroidBitmap_getInfo(env, bm, &bm1);

    AndroidBitmapInfo filter1;
    AndroidBitmap_getInfo(env, filter, &filter1);

//    AndroidBitmapInfo result1;
//    AndroidBitmap_getInfo(env, result, &result1);

    int width = bm1.width;
    int height = bm1.height;
    int w2 = filter1.width;
    int h2 = filter1.height;
    float scaleX = (float) w2 / (float) width;

    for (int x = 0; x < w2; x++) {
        for (int y = 0; y < h2 && y < height; y++) {
            int xx =(int) ((float) x / scaleX);
            int yy = (int) ((float) y / scaleX);
            int pixel = getPixel(env,bm, xx, yy);
            int fp = getPixel(env,filter,x, y);

            int alpha = (fp & 0xFF000000) >> 24;

            if (alpha == 0) {
                setPixel(env, result, x, y, pixel);
            }
        }
    }

    return result;
}

Solution

There is no straight analogue for getPixel()/setPixel() in native code. Instead of this, you obtain raw pointer to pixel buffer via AndroidBitmap_lockPixels(), and then you can freely modify data in this buffer. To get/set single pixel you can use next functions:

#include <stdint.h>
#include <assert.h>
#include <jni.h>
#include <android/bitmap.h>

uint32_t getPixel(JNIEnv *env, jobject bm, int x, int y)
{
    AndroidBitmapInfo bi = {0};
    uint8_t *pixelBuf;
    uint8_t a, r, g, b;

    AndroidBitmap_getInfo(env, bm, &bi);
    /* ensure that we fit into bounds */
    assert(x >= 0 && x < bi.width
            && y >= 0 && y < bi.height);
    /* we support only one format at the moment */
    assert(ANDROID_BITMAP_FORMAT_RGBA_8888 == bi.format);
    /* read pixel components */
    AndroidBitmap_lockPixels(env, bm, (void **)&pixelBuf);
    r = pixelBuf[y * bi.stride + x * 4 + 0];
    g = pixelBuf[y * bi.stride + x * 4 + 1];
    b = pixelBuf[y * bi.stride + x * 4 + 2];
    a = pixelBuf[y * bi.stride + x * 4 + 3];
    AndroidBitmap_unlockPixels(env, bm);
    return a << 24 | r << 16 | g << 8 | b;
}

void setPixel(JNIEnv *env, jobject bm, int x, int y, uint32_t val)
{
    AndroidBitmapInfo bi = {0};
    uint8_t *pixelBuf;

    AndroidBitmap_getInfo(env, bm, &bi);
    /* ensure that we fit into bounds */
    assert(x >= 0 && x < bi.width
            && y >= 0 && y < bi.height);
    /* we support only one format at the moment */
    assert(ANDROID_BITMAP_FORMAT_RGBA_8888 == bi.format);
    /* read pixel components */
    AndroidBitmap_lockPixels(env, bm, (void **)&pixelBuf);
    pixelBuf[y * bi.stride + x * 4 + 0] = (val >> 16) & 0xff;
    pixelBuf[y * bi.stride + x * 4 + 1] = (val >> 8) & 0xff;
    pixelBuf[y * bi.stride + x * 4 + 2] = (val >> 0) & 0xff;
    pixelBuf[y * bi.stride + x * 4 + 3] = (val >> 24) & 0xff;
    AndroidBitmap_unlockPixels(env, bm);
}

Of course you can lock buffer just once and iterate through pixels in a loop.

Nonetheless, seems like your code should perform mask applying to bitmap. Probably it is possible to do in java code only via drawing on bitmap-powered canvas with proper PorterDuffXfermode. See here. It will be done in native code, so performance will be nice.

Answered By - Sergio