Bitmap setPixels will lose alpha channel if background is black

I am trying to create a custom view in Android using the method canvas.drawBitmap()

. However, I found that the alpha channel will be lost if I do it in native JNI code and the background is black. To the summary, case:

• Call java bitmap.setPixels()
  
  and set the color of the bitmaps to the NDK, when the background is white , both bitmaps display correctly
• Call java bitmap.setPixels()
  
  and set the color of the bitmap pixels to the NDK, when the background is black , only the bitmap drawn by the java API is displayed correctly, the one that was output with the NDK lost alpha channel

The question is, why is the result on a white background okay but not on a black background? Am I missing something or have I done it wrong?

Layout XML file:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="16dp" >

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@android:color/black"
        android:orientation="horizontal"
        android:padding="16dp" >

        <com.example.android.TestView
            android:id="@+id/testview1"
            android:layout_width="320px"
            android:layout_height="320px"
            android:layout_margin="16dp" />

        <com.example.android.TestView
            android:id="@+id/testview2"
            android:layout_width="320px"
            android:layout_height="320px"
            android:layout_margin="16dp" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="@android:color/white"
        android:orientation="horizontal"
        android:padding="16dp" >

        <com.example.android.TestView
            android:id="@+id/testview3"
            android:layout_width="320px"
            android:layout_height="320px"
            android:layout_margin="16dp" />

        <com.example.android.TestView
            android:id="@+id/testview4"
            android:layout_width="320px"
            android:layout_height="320px"
            android:layout_margin="16dp" />
    </LinearLayout>

</LinearLayout>

      

        
        
        
      

    

MainActivity.java:

package com.example.android;
import com.example.android.R;

import android.app.Activity;
import android.os.Bundle;

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        TestView tv2 = (TestView) findViewById(R.id.testview2);
        TestView tv4 = (TestView) findViewById(R.id.testview4);
        tv2.setDrawFromNative();
        tv4.setDrawFromNative();
    }
}

      

        
        
        
      

    

TestView.java:

package com.example.android;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.view.View;

public class TestView extends View {
    private Bitmap mBitmap;
    private boolean mDrawFromNative;
    private static final int WIDTH = 320;
    private static final int HEIGHT = 320;

    static {
        System.loadLibrary("bitmaptest");
    }
    private native void nativeDrawBitmap(Object bitmap);

    private static void javaDrawBitmap(Bitmap bitmap) {
        int pixels[] = new int[WIDTH * HEIGHT];
        for (int i = 0; i < pixels.length; i++) {
            pixels[i] = 0x88FF0000;
        }
        bitmap.setPixels(pixels, 0, WIDTH, 0, 0, WIDTH, HEIGHT);
    }

    public TestView(Context context, AttributeSet attrs) {
        super(context, attrs);
        mBitmap = Bitmap.createBitmap(WIDTH, HEIGHT, Bitmap.Config.ARGB_8888);
    }

    public void setDrawFromNative() {
        mDrawFromNative = true;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        if(mDrawFromNative) {
            nativeDrawBitmap(mBitmap);
        } else {
            javaDrawBitmap(mBitmap);
        }
        canvas.drawBitmap(mBitmap, 0, 0, null);
    }
}

      

        
        
        
      

    

TestNative.cpp:

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

#define LOGD(...)  __android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)
#define LOGW(...)  __android_log_print(ANDROID_LOG_WARN,LOG_TAG,__VA_ARGS__)
#define LOGE(...)  __android_log_print(ANDROID_LOG_ERROR,LOG_TAG,__VA_ARGS__)

#define LOG_TAG "BMPTEST"

extern "C" {
void Java_com_example_android_TestView_nativeDrawBitmap(JNIEnv* env, jobject thiz, jobject bitmap) {

    AndroidBitmapInfo info;
    void* dst_pixels;
    int   ret;

    if((ret = AndroidBitmap_getInfo(env, bitmap, &info)) < 0) {
        LOGE("AndroidBitmap_getInfo() failed ! error=%d", ret);
        return;
    }
    if ((ret = AndroidBitmap_lockPixels(env, bitmap, &dst_pixels)) < 0) {
        LOGE("AndroidBitmap_lockPixels() failed ! error=%d", ret);
        return;
    }

    unsigned int *dst = (unsigned int *)dst_pixels;
    for(int i=0; i< info.width * info.height; i++) {
            *(dst+i) = (0x88<<24 | 0xff | 0x00<<8 | 0x00<<16); //(ARGB->ABGR)
    }
    AndroidBitmap_unlockPixels(env, bitmap);
}
}

      

        
        
        
      

    

Android.mk for native code:

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := libbitmaptest
LOCAL_SRC_FILES := \
    TestNative.cpp
LOCAL_LDLIBS += -llog -ljnigraphics
include $(BUILD_SHARED_LIBRARY)

      

        
        
        
      

    

ScreenShot of the result: