什么是JNI的图形或如何使用它？ [英] What is JNI Graphics or how to use it?

问题描述

在Android NDK中有一个名为JNI图形库。这是什么？我可以用它来装载图像的OpenGL ES与C / C ++？

In the Android NDK there is a library named JNI Graphics. What is that? Can I use that to load Images for OpenGL ES with C/C++?

推荐答案

在 jnigraphics 库可用于从<$ C $访问C / C位图缓存++ C> android.bitmap.Graphics 类（当然，在Java中，）。它的详细随附NDK下的文档中描述的：

The jnigraphics library can be used to access bitmap buffers in C/C++ from the android.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

有可用于载入图像为例如OpenGL ES的在C / C ++，但你必须做一些工作交给一个 jobject 到库，以便它可以给你直接进入缓冲区。您可以传递一个缓冲区的OpenGL通过 glTexImage2D（）。

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

首先，您需要一个Java 位图的对象，您可以获取并传递给你的本地方法是这样的：

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

这是本机方法可以是这个样子：

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

}

请记住，你应该叫 AndroidBitmap_unlockPixels（）当你使用像素缓冲区完成的，这个例子不检查错误的。

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.

更新希德·达塔的问题：你能保证输出图像格式为你加入这个期权上述预期是什么：

Update for Sid Datta's question: You can ensure that the output image format is what you're expecting by adding this to options above:

options.inPreferredConfig = Bitmap.Config.ARGB_8888;

还有一个情况下输出图像的还是的结束与一个未知格式JNI。这似乎只发生与GIF格式。调用后 BitmapFactory.de codeResource（），你可以将图像转换为正确的格式，如果必要的：

There is one case where the output image will still end up with an unknown format in JNI. This seems to happen only with GIFs. After calling BitmapFactory.decodeResource(), you can convert the image to the proper format if necessary:

if (bitmap.getConfig() != Bitmap.Config.ARGB_8888) {
    Bitmap reformatted_bitmap = bitmap.copy(Bitmap.Config.ARGB_8888, false);
    bitmap.recycle(); /* reduce memory load in app w/o waiting for GC */
    bitmap = reformatted_bitmap;
}

这篇关于什么是JNI的图形或如何使用它？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！