[Android]通过JNI访问并操作Bitmap的元素，支持RGB565和ARGB8888

一段简单的JNI例子，输入是Bitmap（需要是Mutable），结果是把Bitmap变成灰度图。

为了看起来有点价值，所以同时支持了RGB565和ARGB8888（囧rz）

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <jni.h>
#include <android/bitmap.h>
#include <android/log.h>

#ifndef eprintf
#define eprintf(...) __android_log_print(ANDROID_LOG_ERROR,"@",__VA_ARGS__)
#endif

#define RGB565_R(p) ((((p) & 0xF800) >> 11) << 3)
#define RGB565_G(p) ((((p) & 0x7E0 ) >> 5)  << 2)
#define RGB565_B(p) ( ((p) & 0x1F  )        << 3)
#define MAKE_RGB565(r,g,b) ((((r) >> 3) << 11) | (((g) >> 2) << 5) | ((b) >> 3))

#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))

JNIEXPORT void JNICALL Java_com_yxcorp_hello_Effect_update
  (JNIEnv *env, jclass clazz, jobject zBitmap) {
	JNIEnv J = *env;

	if (zBitmap == NULL) {
		eprintf("bitmap is null\n");
		return;
	}

	// Get bitmap info
	AndroidBitmapInfo info;
	memset(&info, 0, sizeof(info));
	AndroidBitmap_getInfo(env, zBitmap, &info);
	// Check format, only RGB565 & RGBA are supported
	if (info.width <= 0 || info.height <= 0 ||
		(info.format != ANDROID_BITMAP_FORMAT_RGB_565 && info.format != ANDROID_BITMAP_FORMAT_RGBA_8888)) {
		eprintf("invalid bitmap\n");
		J->ThrowNew(env, J->FindClass(env, "java/io/IOException"), "invalid bitmap");
		return;
	}

	// Lock the bitmap to get the buffer
	void * pixels = NULL;
	int res = AndroidBitmap_lockPixels(env, zBitmap, &pixels);
	if (pixels == NULL) {
		eprintf("fail to lock bitmap: %d\n", res);
		J->ThrowNew(env, J->FindClass(env, "java/io/IOException"), "fail to open bitmap");
		return;
	}

	eprintf("Effect: %dx%d, %d\n", info.width, info.height, info.format);

	int x = 0, y = 0;
	// From top to bottom
	for (y = 0; y < info.height; ++y) {
		// From left to right
		for (x = 0; x < info.width; ++x) {
			int a = 0, r = 0, g = 0, b = 0;
			void *pixel = NULL;
			// Get each pixel by format
			if (info.format == ANDROID_BITMAP_FORMAT_RGB_565) {
				pixel = ((uint16_t *)pixels) + y * info.width + x;
				uint16_t v = *(uint16_t *)pixel;
				r = RGB565_R(v);
				g = RGB565_G(v);
				b = RGB565_B(v);
			} else {// RGBA
				pixel = ((uint32_t *)pixels) + y * info.width + x;
				uint32_t v = *(uint32_t *)pixel;
				a = RGBA_A(v);
				r = RGBA_R(v);
				g = RGBA_G(v);
				b = RGBA_B(v);
			}

			// Grayscale
			int gray = (r * 38 + g * 75 + b * 15) >> 7;

			// Write the pixel back
			if (info.format == ANDROID_BITMAP_FORMAT_RGB_565) {
				*((uint16_t *)pixel) = MAKE_RGB565(gray, gray, gray);
			} else {// RGBA
				*((uint32_t *)pixel) = MAKE_RGBA(gray, gray, gray, a);
			}
		}
	}

	AndroidBitmap_unlockPixels(env, zBitmap);
}