What is JNI Graphics and how to use it?

最新推荐文章于 2024-07-26 00:51:57 发布

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本文介绍如何使用 JNI Graphics 库从 Android 的 Bitmap 类中获取位图缓冲区，以便在 C/C++ 中进行操作，如加载图像供 OpenGL ES 使用。文中详细说明了通过 Java 代码创建 Bitmap 对象，并将其传递给本地方法的过程，以及如何解锁像素缓冲区。

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http://stackoverflow.com/questions/5605853/what-is-jni-graphics-or-how-to-use-it

The jnigraphics library can be used to access bitmap buffers in C/C++ from theandroid.bitmap.Graphics class (in Java, of course). It's described in more detail in the documentation that comes with the NDK under:

android-ndk-r5b/docs/STABLE-APIS.html

It can be used to load images for e.g. OpenGL ES in C/C++, but you have to do some work to hand ajobject to that library so it can give you direct access to a buffer. You can pass that buffer to OpenGL via glTexImage2D().

First, you need a Java Bitmap object, which you can acquire and pass to your native method like this:

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
       ...

BitmapFactory.Options options = new BitmapFactory.Options();
options.inScaled = false;

Bitmap bitmap = BitmapFactory.decodeResource(context.getResources(),
                                             R.drawable.myimage, options);

MyJniMethod(bitmap); // Should be static in this example

That native method can look something like this:

#include <android/bitmap.h>

void MyJniMethod(JNIEnv *env, jobject obj, jobject bitmap) {
AndroidBitmapInfo  info;
uint32_t          *pixels;
int                ret;

AndroidBitmap_getInfo(env, bitmap, &info);

if(info.format != ANDROID_BITMAP_FORMAT_RGBA_8888) {
  LOGE("Bitmap format is not RGBA_8888!");
  return false;
}

AndroidBitmap_lockPixels(env, bitmap, reinterpret_cast<void **>(&pixels));

// Now you can use the pixel array 'pixels', which is in RGBA format

}

Keep in mind you should call AndroidBitmap_unlockPixels() when you are done with the pixel buffer, and that this example doesn't check for errors at all.