看完本文的标题，可能有人要打我。你说黑白的老照片不好吗？非要说什么遗像，我现在就把你变成遗像！好了，言归正传。我想大部分人都用过美颜相机或者剪映等软件吧，它们的滤镜功能是如何实现的，有人想过吗？难道要使用Java/Kotlin遍历Bitmap，然后处理到手机发烫吗？学过计算机图形学的应该知道，图片或视频的图像数据的最小单位是像素px。视频编码H.264的YUV数据不也是视频像素数据嘛，只不过有很多帧。

代码实现

本篇文章，我来教大家简单的处理图像中一个个的像素点的色值，从而达到改变图片风格的效果。首先我们创建一个Native C++项目。

然后我们需要写个简单的xml布局。

<?xml version="1.0" encoding="utf-8"?>

<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ImageView
        android:id="@+id/iv_display"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />
    <Button
        android:id="@+id/btnPicProcess"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintBottom_toBottomOf="parent"
        android:text="黑白"/>

</androidx.constraintlayout.widget.ConstraintLayout>

接下来我们需要实现Activity的代码了。注意这里有个external方法，它相当于java中的带native关键字的方法，只不过这是kotlin的写法。在handleBitmap()方法中调用底层方法处理图像。就这么一个简单的流程，因为我们把图像处理的逻辑全部都放到c++底层代码中了。之所以把图像处理的逻辑全权交给C++的原因我想大家应该都知道了，IO是非常耗时的，而两层循环的时间复杂度是O(n²)。那就是java使用jni跟c++频繁通信会大量调输入输出流，时间都损耗在bitmap的setPixels方法上了。那还不如你处理好了给我结果就好了，我不需要关心过程。

package com.dorachat.myapplication

import android.graphics.Bitmap
import android.graphics.BitmapFactory
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import com.dorachat.myapplication.databinding.ActivityMainBinding

class MainActivity : AppCompatActivity() {

    private lateinit var binding: ActivityMainBinding

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        binding = ActivityMainBinding.inflate(layoutInflater)
        setContentView(binding.root)

        val bitmap = BitmapFactory.decodeResource(resources, R.drawable.pic)
        binding.ivDisplay.setImageBitmap(bitmap)
        binding.btnPicProcess.setOnClickListener {
            binding.ivDisplay.setImageBitmap(handleBitmap(bitmap))
        }
    }
    
    /**
     * jni处理图片。
     */
    private fun handleBitmap(bitmap: Bitmap) : Bitmap {
        val bmp = bitmap.copy(Bitmap.Config.ARGB_8888, true);
        nativeProcessBitmap(bmp);
        return bmp;
    }

    /**
     * 黑白特效，调用native底层方法。
     */
    external fun nativeProcessBitmap(bitmap: Bitmap)

    companion object {
        // Used to load the 'myapplication' library on application startup.
        init {
            System.loadLibrary("myapplication")
        }
    }
}

我们再来看下CMakeLists.txt的代码，注意target_link_libraries中要链接jnigraphics。

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html.
# For more examples on how to use CMake, see https://github.com/android/ndk-samples.

# Sets the minimum CMake version required for this project.
cmake_minimum_required(VERSION 3.22.1)

# Declares the project name. The project name can be accessed via ${ PROJECT_NAME},
# Since this is the top level CMakeLists.txt, the project name is also accessible
# with ${CMAKE_PROJECT_NAME} (both CMake variables are in-sync within the top level
# build script scope).
project("myapplication")

# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.
#
# In this top level CMakeLists.txt, ${CMAKE_PROJECT_NAME} is used to define
# the target library name; in the sub-module's CMakeLists.txt, ${PROJECT_NAME}
# is preferred for the same purpose.
#
# In order to load a library into your app from Java/Kotlin, you must call
# System.loadLibrary() and pass the name of the library defined here;
# for GameActivity/NativeActivity derived applications, the same library name must be
# used in the AndroidManifest.xml file.
add_library(${CMAKE_PROJECT_NAME} SHARED
        # List C/C++ source files with relative paths to this CMakeLists.txt.
        native-lib.cpp)

# Specifies libraries CMake should link to your target library. You
# can link libraries from various origins, such as libraries defined in this
# build script, prebuilt third-party libraries, or Android system libraries.
target_link_libraries(${CMAKE_PROJECT_NAME}
        # List libraries link to the target library
        android
        log
        jnigraphics)

最后就是我们最最重要的底层代码了。通过调用AndroidBitmap_getInfo来读取图片像素数据，并将其保存在AndroidBitmapInfo中。AndroidBitmap_lockPixels 是 Android Native Development Kit (NDK) 中的一个函数，用于锁定一个 android.graphics.Bitmap 对象的像素数据，以便在 Native 代码中进行像素级别的操作。这个函数允许在 C 或 C++ 代码中直接访问 Android 应用中的位图像素数据，以便进行图像处理、渲染或其他图像相关的操作。最后记得调用AndroidBitmap_unlockPixels解锁。至于为什么是(R+G+B)/3，这个问题要问我为什么这么算，我能力有限教不了你们，你得问计算机图像处理学的专家，哈哈。当将彩色图像转换为黑白图像时，通常采用灰度化的方法，即将红、绿、蓝三个通道的数值取平均值，以获得灰度图像。因此，将RGB图像转换为黑白图像时，每个像素的灰度值通常是红、绿、蓝通道值的平均值，即(R+G+B)/3。

#include <jni.h>
#include <string>
#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <android/bitmap.h>

#define MAKE_RGB565(r, g, b) ((((r) >> 3) << 11) | (((g) >> 2) << 5) | ((b) >> 3))
#define MAKE_ARGB(a, r, g, b) ((a&0xff)<<24) | ((r&0xff)<<16) | ((g&0xff)<<8) | (b&0xff)

#define RGB565_R(p) ((((p) & 0xF800) >> 11) << 3)
#define RGB565_G(p) ((((p) & 0x7E0 ) >> 5)  << 2)
#define RGB565_B(p) ( ((p) & 0x1F  )        << 3)

#define RGB8888_A(p) (p & (0xff<<24) >> 24 )
#define RGB8888_R(p) (p & (0xff<<16) >> 16 )
#define RGB8888_G(p) (p & (0xff<<8)  >> 8 )
#define RGB8888_B(p) (p & (0xff) )

#define RGBA_A(p) (((p) & 0xFF000000) >> 24)
#define RGBA_R(p) (((p) & 0x00FF0000) >> 16)
#define RGBA_G(p) (((p) & 0x0000FF00) >>  8)
#define RGBA_B(p)  ((p) & 0x000000FF)
#define MAKE_RGBA(r, g, b, a) (((a) << 24) | ((r) << 16) | ((g) << 8) | (b))

extern "C" JNIEXPORT void JNICALL
Java_com_dorachat_myapplication_MainActivity_nativeProcessBitmap(JNIEnv *env,
                                                                 jobject instance,
                                                                 jobject bitmap) {

    if (bitmap == NULL) {
        return;
    }

    AndroidBitmapInfo bitmapInfo;
    memset(&bitmapInfo, 0, sizeof(bitmapInfo));
    // Need add "jnigraphics" into target_link_libraries in CMakeLists.txt
    AndroidBitmap_getInfo(env, bitmap, &bitmapInfo);
    // Lock the bitmap to get the buffer
    void *pixels = NULL;
    int res = AndroidBitmap_lockPixels(env, bitmap, &pixels);
    // From top to bottom
    int x = 0, y = 0;
    for (y = 0; y < bitmapInfo.height; ++y) {
        // From left to right
        for (x = 0; x < bitmapInfo.width; ++x) {
            int a = 0, r = 0, g = 0, b = 0;
            void *pixel = NULL;
            // Get each pixel by format
            if (bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGBA_8888) {
                pixel = ((uint32_t *) pixels) + y * bitmapInfo.width + x;
                int r, g, b;
                uint32_t v = *((uint32_t *) pixel);
                r = RGB8888_R(v);
                g = RGB8888_G(v);
                b = RGB8888_B(v);
                int sum = r + g + b;
                *((uint32_t *) pixel) = MAKE_ARGB(0xff, sum / 3, sum / 3, sum / 3);
            } else if (bitmapInfo.format == ANDROID_BITMAP_FORMAT_RGB_565) {
                pixel = ((uint16_t *) pixels) + y * bitmapInfo.width + x;
                int r, g, b;
                uint16_t v = *((uint16_t *) pixel);
                r = RGB565_R(v);
                g = RGB565_G(v);
                b = RGB565_B(v);
                int sum = r + g + b;
                *((uint16_t *) pixel) = MAKE_RGB565(sum / 3, sum / 3, sum / 3);
            }
        }
    }
    AndroidBitmap_unlockPixels(env, bitmap);
}

效果演示

知道你们喜欢清纯的，来了。最后不忘留个赞吧。