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在AS中，有时会要传递bitmap给c++函数，使用opencv等工具进行处理。这时会将bitmap传递给jni函数。bitmap在jni的函数参数中，是jobject类型。下面介绍如何将传入的bitmap转化成opencv常用的mat类型，从而进行处理

首先要包含bitmap的头文件<android/bitmap.h>。这个头文件中定义了一些重要信息。了解这些信息非常必要

AndroidBitmapInfo 一个包含bitmap常用信息的结构体

/** 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;

AndroidBitmapFormat 图片格式的枚举类型。格式为图片的像素位数。最常见的是ANDROID_BITMAP_FORMAT_RGBA_8888

/** 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,
};

AndroidBitmap_getInfo 获取bitmap信息的函数。返回值为后面的枚举类型。成功读取的返回值为0

/**
 * 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);

/** 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,
};

AndroidBitmap_lockPixels 非常关键的一个函数。将bitmap的像素在内存中的地址进行锁定，确保这一块内存不会被移动。传入的addrPtr是一个二级指针，*addPtr是指向bitmap像素地址的指针。该函数正确调用后，一定要在该像素地址指针不再使用后，使用AndroidBitmap_unlockPixels进行释放。

/**
 * 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);

AndroidBitmap_unlockPixels

/**
 * Call this to balance a successful call to AndroidBitmap_lockPixels.
 */
int AndroidBitmap_unlockPixels(JNIEnv* env, jobject jbitmap);

了解以上信息之后，就可以探索如何将传入的bitmap转为需要的mat了。通用的做法是先获取bitmap的信息

AndroidBitmapInfo img_info;
CV_Assert(AndroidBitmap_getInfo(env, bitmap, &img_info) >= 0);
CV_Assert(img_info.format == ANDROID_BITMAP_FORMAT_RGBA_8888);

使用CV_Assert确保获取信息成功

然后初始化一个指针，并将该指针的地址传递给AndroidBitmap_lockPixels，若成功则该指针即指向bitmap的像素在内存中的地址

void *ori_img_pxl = NULL;
CV_Assert(AndroidBitmap_lockPixels(env, bitmap, &ori_img_pxl) >= 0);
CV_Assert(ori_img_pxl);

然后就可以将该指针传递给mat的构造函数，创建出我们需要的mat了。

Mat ori_img_mat(img_info.height, img_info.width, CV_8UC4, ori_img_pxl);

需要注意得到的mat的数据指针是指向bitmap的像素内存的，因此mat中对像素的操作会直接反映在bitmap中

AndroidBitmap_unlockPixels(env, bitmap);

最后要记得释放锁定！