Android NDK功能很强大,今天就来学习下jni用bitmap实现Imageview的上下翻转和左右镜像
效果图:
在 Android 通过 JNI 去调用 Bitmap,通过 CMake 去编 so 动态链接库的话,需要添加 jnigraphics 图像库。
target_link_libraries( # Specifies the target library.
native-operation
jnigraphics
${log-lib} )
在 Android 中关于 JNI Bitmap 的操作,都定义在 bitmap.h 的头文件里面了,主要就三个函数,明白它们的含义之后就可以去实践体会了。
检索 Bitmap 对象信息
AndroidBitmap_getInfo 函数允许原生代码检索 Bitmap 对象信息,如它的大小、像素格式等,函数签名如下:
/**
* Given a java bitmap object, fill out the AndroidBitmapInfo struct for it.
* If the call fails, the info parameter will be ignored.
*/
int AndroidBitmap_getInfo(JNIEnv* env, jobject jbitmap,
AndroidBitmapInfo* info);
其中,第一个参数就是 JNI 接口指针,第二个参数就是 Bitmap 对象的引用,第三个参数是指向 AndroidBitmapInfo 结构体的指针。
AndroidBitmapInfo 结构体如下:
/** Bitmap info, see AndroidBitmap_getInfo(). */
typedef struct {
/** The bitmap width in pixels. */
uint32_t width;
/** The bitmap height in pixels. */
uint32_t height;
/** The number of byte per row. */
uint32_t stride;
/** The bitmap pixel format. See {@link AndroidBitmapFormat} */
int32_t format;
/** Unused. */
uint32_t flags; // 0 for now
} AndroidBitmapInfo;
其中,width 就是 Bitmap 的宽,height 就是高,format 就是图像的格式,而 stride 就是每一行的字节数。
图像的格式有如下支持:
/** Bitmap pixel format. */
enum AndroidBitmapFormat {
/** No format. */
ANDROID_BITMAP_FORMAT_NONE = 0,
/** Red: 8 bits, Green: 8 bits, Blue: 8 bits, Alpha: 8 bits. **/
ANDROID_BITMAP_FORMAT_RGBA_8888 = 1,
/** Red: 5 bits, Green: 6 bits, Blue: 5 bits. **/
ANDROID_BITMAP_FORMAT_RGB_565 = 4,
/** Deprecated in API level 13. Because of the poor quality of this configuration, it is advised to use ARGB_8888 instead. **/
ANDROID_BITMAP_FORMAT_RGBA_4444 = 7,
/** Alpha: 8 bits. */
ANDROID_BITMAP_FORMAT_A_8 = 8,
};
如果 AndroidBitmap_getInfo 执行成功的话,会返回 0 ,否则返回一个负数,代表执行的错误码列表如下:
/** AndroidBitmap functions result code. */
enum {
/** Operation was successful. */
ANDROID_BITMAP_RESULT_SUCCESS = 0,
/** Bad parameter. */
ANDROID_BITMAP_RESULT_BAD_PARAMETER = -1,
/** JNI exception occured. */
ANDROID_BITMAP_RESULT_JNI_EXCEPTION = -2,
/** Allocation failed. */
ANDROID_BITMAP_RESULT_ALLOCATION_FAILED = -3,
};
访问原生像素缓存
AndroidBitmap_lockPixels 函数锁定了像素缓存以确保像素的内存不会被移动。
如果 Native 层想要访问像素数据并操作它,该方法返回了像素缓存的一个原生指针,
/**
* Given a java bitmap object, attempt to lock the pixel address.
* Locking will ensure that the memory for the pixels will not move
* until the unlockPixels call, and ensure that, if the pixels had been
* previously purged, they will have been restored.
*
* If this call succeeds, it must be balanced by a call to
* AndroidBitmap_unlockPixels, after which time the address of the pixels should
* no longer be used.
*
* If this succeeds, *addrPtr will be set to the pixel address. If the call
* fails, addrPtr will be ignored.
*/
int AndroidBitmap_lockPixels(JNIEnv* env, jobject jbitmap, void** addrPtr);
其中,第一个参数就是 JNI 接口指针,第二个参数就是 Bitmap 对象的引用,第三个参数是指向像素缓存地址的指针。
AndroidBitmap_lockPixels 执行成功的话返回 0 ,否则返回一个负数,错误码列表就是上面提到的。
释放原生像素缓存
对 Bitmap 调用完 AndroidBitmap_lockPixels 之后都应该对应调用一次 AndroidBitmap_unlockPixels 用来释放原生像素缓存。
当完成对原生像素缓存的读写之后,就应该释放它,一旦释放后,Bitmap Java 对象又可以在 Java 层使用了,函数签名如下:
/**
* Call this to balance a successful call to AndroidBitmap_lockPixels.
*/
int AndroidBitmap_unlockPixels(JNIEnv* env, jobject jbitmap);
其中,第一个参数就是 JNI 接口指针,第二个参数就是 Bitmap 对象的引用,如果执行成功返回 0,否则返回 1。
对 Bitmap 的操作,最重要的就是 AndroidBitmap_lockPixels 函数拿到所有像素的缓存地址,然后对每个像素值进行操作,从而更改 Bitmap 。
具体代码如下:
Activity 部分:
private void initview(){
source_img = (ImageView)findViewById(R.id.source_img);
operation_img = (ImageView)findViewById(R.id.operation_img);
Bitmap bitmap = BitmapFactory.decodeResource(getResources(),R.drawable.newpic);
operation_img.setImageBitmap(rotateBitmap(bitmap));
}
/*
* 上下翻转和左右镜像
*/
public native Bitmap rotateBitmap(Bitmap bitmap);
2.jni部分:
#include
#include
#include
#include
#include
#include "AndroidLog.h"
#include
jobject generateBitmap(JNIEnv *env, uint32_t width, uint32_t height) {
jclass bitmapCls = env->FindClass("android/graphics/Bitmap");
jmethodID createBitmapFunction = env->GetStaticMethodID(bitmapCls,
"createBitmap",
"(IILandroid/graphics/Bitmap$Config;)Landroid/graphics/Bitmap;");
jstring configName = env->NewStringUTF("ARGB_8888");
jclass bitmapConfigClass = env->FindClass("android/graphics/Bitmap$Config");
jmethodID valueOfBitmapConfigFunction = env->GetStaticMethodID(
bitmapConfigClass, "valueOf",
"(Ljava/lang/String;)Landroid/graphics/Bitmap$Config;");
jobject bitmapConfig = env->CallStaticObjectMethod(bitmapConfigClass,
valueOfBitmapConfigFunction, configName);
jobject newBitmap = env->CallStaticObjectMethod(bitmapCls,
createBitmapFunction,
width,
height, bitmapConfig);
return newBitmap;
}
extern "C" JNIEXPORT jobject JNICALL
Java_com_xinrui_ndkapp_MainActivity_rotateBitmap(
JNIEnv *env,
jobject /* this */,jobject bitmap) {
AndroidBitmapInfo bitmapInfo;
int ret;
if ((ret = AndroidBitmap_getInfo(env, bitmap, &bitmapInfo)) < 0) {
LOGE("AndroidBitmap_getInfo() failed ! error=%d", ret);
return NULL;
}
// 读取 bitmap 的像素内容到 native 内存
void *bitmapPixels;
if ((ret = AndroidBitmap_lockPixels(env, bitmap, &bitmapPixels)) < 0) {
LOGE("AndroidBitmap_lockPixels() failed ! error=%d", ret);
return NULL;
}
// 旋转操作,新 Bitmap 的宽等于原来的高,新 Bitmap 的高等于原来的宽
uint32_t newWidth = bitmapInfo.width;
uint32_t newHeight = bitmapInfo.height;
// 创建一个新的数组指针,把这个新的数组指针填充像素值
uint32_t *newBitmapPixels = new uint32_t[newWidth * newHeight];
int whereToGet = 0;
//上下翻转
for (int y = 0; y < newHeight; ++y) {
for (int x = 0; x < newWidth; x++) {
uint32_t pixel = ((uint32_t *) bitmapPixels)[whereToGet++];
newBitmapPixels[newWidth * (newHeight - 1 - y) + x] = pixel;
}
}
//左右镜像
// for (int y = 0; y < newHeight; ++y) {
// for (int x = newWidth - 1; x >= 0; x--) {
// uint32_t pixel = ((uint32_t *) bitmapPixels)[whereToGet++];
// newBitmapPixels[newWidth * y + x] = pixel;
// }
// }
AndroidBitmap_unlockPixels(env, bitmap);
jobject newBitmap = generateBitmap(env,newWidth,newHeight);
void *resultBitmapPixels;
if ((ret = AndroidBitmap_lockPixels(env, newBitmap, &resultBitmapPixels)) < 0) {
LOGE("AndroidBitmap_lockPixels() failed ! error=%d", ret);
return NULL;
}
int pixelsCount = newWidth * newHeight;
memcpy((uint32_t *) resultBitmapPixels, newBitmapPixels, sizeof(uint32_t) * pixelsCount);
AndroidBitmap_unlockPixels(env, newBitmap);
delete []newBitmapPixels;
return newBitmap;
}