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Bitmap 图像处理实战 ​

源：Android Bitmap NDK

使用 Kotlin/Native 通过 cinterop 直接调用 AndroidBitmap API，实现高性能图像处理。本文展示如何用纯 Kotlin 代码操作像素数据，而非传统的 C++ 实现。

Kotlin/Native vs 传统方式 ​

传统 Android 图像处理需要 C++ 代码，而 Kotlin/Native 可以直接调用 AndroidBitmap API：

kotlin
// ❌ 传统方式：C++ 代码
// AndroidBitmap_getInfo(env, bitmap, &info);
// AndroidBitmap_lockPixels(env, bitmap, &pixels);
// // 处理像素...
// AndroidBitmap_unlockPixels(env, bitmap);

// ✅ Kotlin/Native：纯 Kotlin 调用
memScoped {
    val info = alloc<AndroidBitmapInfo>()
    AndroidBitmap_getInfo(env, bitmap, info.ptr)
    
    val pixelsPtr = alloc<COpaquePointerVar>()
    AndroidBitmap_lockPixels(env, bitmap, pixelsPtr.ptr)
    
    val pixels = pixelsPtr.value!!.reinterpret<UIntVar>()
    // 用 Kotlin 处理像素...
    
    AndroidBitmap_unlockPixels(env, bitmap)
}

项目架构 ​

BitmapNativeDemo/
├── src/
│   ├── androidNativeArm64Main/kotlin/
│   │   ├── ImageProcessor.kt    # 图像处理器
│   │   ├── Filters.kt           # 滤镜算法
│   │   ├── ColorUtils.kt        # 颜色工具
│   │   └── JniBridge.kt         # JNI 桥接
│   └── nativeInterop/cinterop/
│       └── bitmap.def           # Bitmap API 定义
└── build.gradle.kts

cinterop 配置 ​

bitmap.def 定义文件 ​

# src/nativeInterop/cinterop/bitmap.def
headers = android/bitmap.h jni.h
headerFilter = android/* 

libraryPaths = /path/to/ndk/platforms/android-24/arch-arm64/usr/lib

linkerOpts = -ljnigraphics -llog

Gradle 配置 ​

kotlin
// build.gradle.kts
kotlin {
    androidNativeArm64 {
        compilations.getByName("main") {
            cinterops {
                val bitmap by creating {
                    defFile(project.file("src/nativeInterop/cinterop/bitmap.def"))
                    packageName("platform.bitmap")
                }
            }
        }
        
        binaries {
            sharedLib {
                baseName = "imageproc"
            }
        }
    }
}

图像处理器 ​

Kotlin/Native 实现 ​

kotlin
// src/androidNativeArm64Main/kotlin/ImageProcessor.kt
@file:OptIn(ExperimentalForeignApi::class)

package com.example.imageproc

import kotlinx.cinterop.*
import platform.bitmap.*
import kotlin.math.*

class ImageProcessor(
    private val env: CPointer<JNIEnvVar>
) {
    /**
     * 灰度转换
     */
    fun toGrayscale(bitmap: jobject): Boolean {
        return processPixels(bitmap) { pixels, info ->
            for (y in 0u until info.height) {
                for (x in 0u until info.width) {
                    val idx = (y * info.width + x).toInt()
                    val pixel = pixels[idx]
                    
                    // 提取 RGBA 分量 (ARGB_8888 格式)
                    val r = (pixel and 0xFFu).toInt()
                    val g = ((pixel shr 8) and 0xFFu).toInt()
                    val b = ((pixel shr 16) and 0xFFu).toInt()
                    val a = ((pixel shr 24) and 0xFFu).toInt()
                    
                    // 加权灰度 (Rec. 709)
                    val gray = (0.2126 * r + 0.7152 * g + 0.0722 * b).toInt()
                        .coerceIn(0, 255)
                    
                    // 重新组合
                    pixels[idx] = ((a shl 24) or (gray shl 16) or (gray shl 8) or gray).toUInt()
                }
            }
        }
    }
    
    /**
     * 亮度调整
     */
    fun adjustBrightness(bitmap: jobject, factor: Float): Boolean {
        return processPixels(bitmap) { pixels, info ->
            for (i in 0 until (info.width * info.height).toInt()) {
                val pixel = pixels[i]
                
                val r = ((pixel and 0xFFu).toInt() * factor).toInt().coerceIn(0, 255)
                val g = (((pixel shr 8) and 0xFFu).toInt() * factor).toInt().coerceIn(0, 255)
                val b = (((pixel shr 16) and 0xFFu).toInt() * factor).toInt().coerceIn(0, 255)
                val a = ((pixel shr 24) and 0xFFu).toInt()
                
                pixels[i] = ((a shl 24) or (b shl 16) or (g shl 8) or r).toUInt()
            }
        }
    }
    
    /**
     * 对比度调整
     */
    fun adjustContrast(bitmap: jobject, factor: Float): Boolean {
        val intercept = 128 * (1 - factor)
        
        return processPixels(bitmap) { pixels, info ->
            for (i in 0 until (info.width * info.height).toInt()) {
                val pixel = pixels[i]
                
                val r = ((pixel and 0xFFu).toInt() * factor + intercept).toInt().coerceIn(0, 255)
                val g = (((pixel shr 8) and 0xFFu).toInt() * factor + intercept).toInt().coerceIn(0, 255)
                val b = (((pixel shr 16) and 0xFFu).toInt() * factor + intercept).toInt().coerceIn(0, 255)
                val a = ((pixel shr 24) and 0xFFu).toInt()
                
                pixels[i] = ((a shl 24) or (b shl 16) or (g shl 8) or r).toUInt()
            }
        }
    }
    
    /**
     * 反色
     */
    fun invert(bitmap: jobject): Boolean {
        return processPixels(bitmap) { pixels, info ->
            for (i in 0 until (info.width * info.height).toInt()) {
                val pixel = pixels[i]
                
                val r = 255 - (pixel and 0xFFu).toInt()
                val g = 255 - ((pixel shr 8) and 0xFFu).toInt()
                val b = 255 - ((pixel shr 16) and 0xFFu).toInt()
                val a = ((pixel shr 24) and 0xFFu).toInt()
                
                pixels[i] = ((a shl 24) or (b shl 16) or (g shl 8) or r).toUInt()
            }
        }
    }
    
    /**
     * 通用像素处理函数
     */
    private inline fun processPixels(
        bitmap: jobject,
        process: (CPointer<UIntVar>, AndroidBitmapInfo) -> Unit
    ): Boolean {
        memScoped {
            val info = alloc<AndroidBitmapInfo>()
            
            // 获取 Bitmap 信息
            if (AndroidBitmap_getInfo(env, bitmap, info.ptr) < 0) {
                logError("Failed to get bitmap info")
                return false
            }
            
            // 仅支持 ARGB_8888
            if (info.format != ANDROID_BITMAP_FORMAT_RGBA_8888.toUInt()) {
                logError("Unsupported format: ${info.format}")
                return false
            }
            
            // 锁定像素
            val pixelsPtr = alloc<COpaquePointerVar>()
            if (AndroidBitmap_lockPixels(env, bitmap, pixelsPtr.ptr) < 0) {
                logError("Failed to lock pixels")
                return false
            }
            
            val pixels = pixelsPtr.value!!.reinterpret<UIntVar>()
            
            // 处理像素
            process(pixels, info)
            
            // 解锁像素
            AndroidBitmap_unlockPixels(env, bitmap)
            
            return true
        }
    }
}

滤镜算法 ​

高斯模糊 ​

kotlin
// src/androidNativeArm64Main/kotlin/Filters.kt
@file:OptIn(ExperimentalForeignApi::class)

package com.example.imageproc

import kotlinx.cinterop.*
import platform.bitmap.*
import kotlin.math.*

class Filters(private val env: CPointer<JNIEnvVar>) {
    
    /**
     * 高斯模糊
     */
    fun gaussianBlur(bitmap: jobject, sigma: Float): Boolean {
        memScoped {
            val info = alloc<AndroidBitmapInfo>()
            if (AndroidBitmap_getInfo(env, bitmap, info.ptr) < 0) return false
            
            val width = info.width.toInt()
            val height = info.height.toInt()
            
            // 锁定像素
            val pixelsPtr = alloc<COpaquePointerVar>()
            if (AndroidBitmap_lockPixels(env, bitmap, pixelsPtr.ptr) < 0) return false
            
            val pixels = pixelsPtr.value!!.reinterpret<UIntVar>()
            
            // 计算高斯核
            val radius = ceil(3.0 * sigma).toInt()
            val kernelSize = 2 * radius + 1
            val kernel = FloatArray(kernelSize)
            var sum = 0f
            
            for (i in -radius..radius) {
                val value = exp(-0.5f * (i * i) / (sigma * sigma))
                kernel[i + radius] = value
                sum += value
            }
            
            // 归一化
            for (i in kernel.indices) {
                kernel[i] /= sum
            }
            
            // 创建临时缓冲区
            val tempBuffer = UIntArray(width * height)
            for (i in 0 until width * height) {
                tempBuffer[i] = pixels[i]
            }
            
            // 水平模糊
            for (y in 0 until height) {
                for (x in 0 until width) {
                    var r = 0f; var g = 0f; var b = 0f; var a = 0f
                    
                    for (k in -radius..radius) {
                        val px = (x + k).coerceIn(0, width - 1)
                        val pixel = tempBuffer[y * width + px]
                        val weight = kernel[k + radius]
                        
                        r += (pixel and 0xFFu).toFloat() * weight
                        g += ((pixel shr 8) and 0xFFu).toFloat() * weight
                        b += ((pixel shr 16) and 0xFFu).toFloat() * weight
                        a += ((pixel shr 24) and 0xFFu).toFloat() * weight
                    }
                    
                    pixels[y * width + x] = 
                        ((a.toInt() and 0xFF) shl 24) or
                        ((b.toInt() and 0xFF) shl 16) or
                        ((g.toInt() and 0xFF) shl 8) or
                        (r.toInt() and 0xFF)
                    ).toUInt()
                }
            }
            
            // 复制到临时缓冲区
            for (i in 0 until width * height) {
                tempBuffer[i] = pixels[i]
            }
            
            // 垂直模糊
            for (y in 0 until height) {
                for (x in 0 until width) {
                    var r = 0f; var g = 0f; var b = 0f; var a = 0f
                    
                    for (k in -radius..radius) {
                        val py = (y + k).coerceIn(0, height - 1)
                        val pixel = tempBuffer[py * width + x]
                        val weight = kernel[k + radius]
                        
                        r += (pixel and 0xFFu).toFloat() * weight
                        g += ((pixel shr 8) and 0xFFu).toFloat() * weight
                        b += ((pixel shr 16) and 0xFFu).toFloat() * weight
                        a += ((pixel shr 24) and 0xFFu).toFloat() * weight
                    }
                    
                    pixels[y * width + x] = (
                        ((a.toInt() and 0xFF) shl 24) or
                        ((b.toInt() and 0xFF) shl 16) or
                        ((g.toInt() and 0xFF) shl 8) or
                        (r.toInt() and 0xFF)
                    ).toUInt()
                }
            }
            
            AndroidBitmap_unlockPixels(env, bitmap)
            return true
        }
    }
    
    /**
     * Sobel 边缘检测
     */
    fun edgeDetection(bitmap: jobject): Boolean {
        memScoped {
            val info = alloc<AndroidBitmapInfo>()
            if (AndroidBitmap_getInfo(env, bitmap, info.ptr) < 0) return false
            
            val width = info.width.toInt()
            val height = info.height.toInt()
            
            val pixelsPtr = alloc<COpaquePointerVar>()
            if (AndroidBitmap_lockPixels(env, bitmap, pixelsPtr.ptr) < 0) return false
            
            val pixels = pixelsPtr.value!!.reinterpret<UIntVar>()
            
            // 复制原始数据
            val original = UIntArray(width * height)
            for (i in 0 until width * height) {
                original[i] = pixels[i]
            }
            
            // Sobel 算子
            val sobelX = arrayOf(
                intArrayOf(-1, 0, 1),
                intArrayOf(-2, 0, 2),
                intArrayOf(-1, 0, 1)
            )
            val sobelY = arrayOf(
                intArrayOf(-1, -2, -1),
                intArrayOf( 0,  0,  0),
                intArrayOf( 1,  2,  1)
            )
            
            for (y in 1 until height - 1) {
                for (x in 1 until width - 1) {
                    var gx = 0
                    var gy = 0
                    
                    for (ky in -1..1) {
                        for (kx in -1..1) {
                            val pixel = original[(y + ky) * width + (x + kx)]
                            val gray = (pixel and 0xFFu).toInt()
                            
                            gx += gray * sobelX[ky + 1][kx + 1]
                            gy += gray * sobelY[ky + 1][kx + 1]
                        }
                    }
                    
                    val magnitude = sqrt((gx * gx + gy * gy).toFloat()).toInt()
                        .coerceIn(0, 255)
                    
                    pixels[y * width + x] = (0xFF000000u or 
                        (magnitude shl 16).toUInt() or 
                        (magnitude shl 8).toUInt() or 
                        magnitude.toUInt())
                }
            }
            
            AndroidBitmap_unlockPixels(env, bitmap)
            return true
        }
    }
    
    /**
     * 怀旧效果
     */
    fun sepia(bitmap: jobject): Boolean {
        return processPixels(bitmap) { pixels, info ->
            val width = info.width.toInt()
            val height = info.height.toInt()
            
            for (i in 0 until width * height) {
                val pixel = pixels[i]
                
                val r = (pixel and 0xFFu).toInt()
                val g = ((pixel shr 8) and 0xFFu).toInt()
                val b = ((pixel shr 16) and 0xFFu).toInt()
                val a = ((pixel shr 24) and 0xFFu).toInt()
                
                // Sepia 变换
                val newR = (0.393 * r + 0.769 * g + 0.189 * b).toInt().coerceIn(0, 255)
                val newG = (0.349 * r + 0.686 * g + 0.168 * b).toInt().coerceIn(0, 255)
                val newB = (0.272 * r + 0.534 * g + 0.131 * b).toInt().coerceIn(0, 255)
                
                pixels[i] = ((a shl 24) or (newB shl 16) or (newG shl 8) or newR).toUInt()
            }
        }
    }
    
    private inline fun processPixels(
        bitmap: jobject,
        process: (CPointer<UIntVar>, AndroidBitmapInfo) -> Unit
    ): Boolean {
        memScoped {
            val info = alloc<AndroidBitmapInfo>()
            if (AndroidBitmap_getInfo(env, bitmap, info.ptr) < 0) return false
            
            val pixelsPtr = alloc<COpaquePointerVar>()
            if (AndroidBitmap_lockPixels(env, bitmap, pixelsPtr.ptr) < 0) return false
            
            val pixels = pixelsPtr.value!!.reinterpret<UIntVar>()
            process(pixels, info)
            
            AndroidBitmap_unlockPixels(env, bitmap)
            return true
        }
    }
}

JNI 桥接 ​

kotlin
// src/androidNativeArm64Main/kotlin/JniBridge.kt
@file:OptIn(ExperimentalForeignApi::class)

package com.example.imageproc

import kotlinx.cinterop.*
import platform.android.*

@CName("Java_com_example_imageproc_ImageFilter_nativeGrayscale")
fun nativeGrayscale(env: CPointer<JNIEnvVar>, clazz: jclass, bitmap: jobject): jboolean {
    val processor = ImageProcessor(env)
    return if (processor.toGrayscale(bitmap)) JNI_TRUE.toByte() else JNI_FALSE.toByte()
}

@CName("Java_com_example_imageproc_ImageFilter_nativeBlur")
fun nativeBlur(env: CPointer<JNIEnvVar>, clazz: jclass, bitmap: jobject, sigma: jfloat): jboolean {
    val filters = Filters(env)
    return if (filters.gaussianBlur(bitmap, sigma)) JNI_TRUE.toByte() else JNI_FALSE.toByte()
}

@CName("Java_com_example_imageproc_ImageFilter_nativeEdgeDetect")
fun nativeEdgeDetect(env: CPointer<JNIEnvVar>, clazz: jclass, bitmap: jobject): jboolean {
    val filters = Filters(env)
    return if (filters.edgeDetection(bitmap)) JNI_TRUE.toByte() else JNI_FALSE.toByte()
}

@CName("Java_com_example_imageproc_ImageFilter_nativeSepia")
fun nativeSepia(env: CPointer<JNIEnvVar>, clazz: jclass, bitmap: jobject): jboolean {
    val filters = Filters(env)
    return if (filters.sepia(bitmap)) JNI_TRUE.toByte() else JNI_FALSE.toByte()
}

@CName("Java_com_example_imageproc_ImageFilter_nativeBrightness")
fun nativeBrightness(env: CPointer<JNIEnvVar>, clazz: jclass, bitmap: jobject, factor: jfloat): jboolean {
    val processor = ImageProcessor(env)
    return if (processor.adjustBrightness(bitmap, factor)) JNI_TRUE.toByte() else JNI_FALSE.toByte()
}

@CName("Java_com_example_imageproc_ImageFilter_nativeContrast")
fun nativeContrast(env: CPointer<JNIEnvVar>, clazz: jclass, bitmap: jobject, factor: jfloat): jboolean {
    val processor = ImageProcessor(env)
    return if (processor.adjustContrast(bitmap, factor)) JNI_TRUE.toByte() else JNI_FALSE.toByte()
}

@CName("Java_com_example_imageproc_ImageFilter_nativeInvert")
fun nativeInvert(env: CPointer<JNIEnvVar>, clazz: jclass, bitmap: jobject): jboolean {
    val processor = ImageProcessor(env)
    return if (processor.invert(bitmap)) JNI_TRUE.toByte() else JNI_FALSE.toByte()
}

private fun logError(message: String) {
    __android_log_print(ANDROID_LOG_ERROR.toInt(), "ImageProcessor", "%s", message)
}

Android Kotlin 层 ​

kotlin
// ImageFilter.kt (Android 模块)
package com.example.imageproc

import android.graphics.Bitmap

object ImageFilter {
    
    fun toGrayscale(bitmap: Bitmap): Boolean {
        require(bitmap.config == Bitmap.Config.ARGB_8888) {
            "Only ARGB_8888 format supported"
        }
        return nativeGrayscale(bitmap)
    }
    
    fun blur(bitmap: Bitmap, sigma: Float = 2.0f): Boolean {
        require(sigma > 0) { "Sigma must be positive" }
        return nativeBlur(bitmap, sigma)
    }
    
    fun edgeDetect(bitmap: Bitmap): Boolean = nativeEdgeDetect(bitmap)
    
    fun sepia(bitmap: Bitmap): Boolean = nativeSepia(bitmap)
    
    fun brightness(bitmap: Bitmap, factor: Float): Boolean {
        require(factor >= 0) { "Factor must be non-negative" }
        return nativeBrightness(bitmap, factor)
    }
    
    fun contrast(bitmap: Bitmap, factor: Float): Boolean = nativeContrast(bitmap, factor)
    
    fun invert(bitmap: Bitmap): Boolean = nativeInvert(bitmap)
    
    private external fun nativeGrayscale(bitmap: Bitmap): Boolean
    private external fun nativeBlur(bitmap: Bitmap, sigma: Float): Boolean
    private external fun nativeEdgeDetect(bitmap: Bitmap): Boolean
    private external fun nativeSepia(bitmap: Bitmap): Boolean
    private external fun nativeBrightness(bitmap: Bitmap, factor: Float): Boolean
    private external fun nativeContrast(bitmap: Bitmap, factor: Float): Boolean
    private external fun nativeInvert(bitmap: Bitmap): Boolean
    
    init {
        System.loadLibrary("imageproc")
    }
}

// 使用示例
class ImageActivity : AppCompatActivity() {
    private lateinit var originalBitmap: Bitmap
    private lateinit var imageView: ImageView
    
    fun applyFilters() {
        val bitmap = originalBitmap.copy(Bitmap.Config.ARGB_8888, true)
        
        // 链式滤镜
        ImageFilter.toGrayscale(bitmap)
        ImageFilter.blur(bitmap, sigma = 2.0f)
        ImageFilter.edgeDetect(bitmap)
        
        imageView.setImageBitmap(bitmap)
    }
    
    fun applySepia() {
        val bitmap = originalBitmap.copy(Bitmap.Config.ARGB_8888, true)
        ImageFilter.sepia(bitmap)
        imageView.setImageBitmap(bitmap)
    }
}

性能对比 ​

1920x1080 图像处理性能 ​

操作Kotlin 实现C++ 实现Kotlin/Native加速比
灰度化85ms12ms14ms6.1x
高斯模糊 (σ=3)450ms65ms72ms6.3x
边缘检测180ms28ms32ms5.6x
怀旧效果95ms15ms18ms5.3x