PixelBuffer对象和Android上的glReadPixel(ARCore)阻塞

Question

我知道默认的glReadPixels()会等到GL线程上执行所有绘图命令.但是当你绑定一个PixelBuffer对象然后调用glReadPixels()时,它应该是异步的,不会等待任何事情.但是当我绑定PBO并执行glReadPixels()时,它会阻塞一段时间.

以下是我初始化PBO的方法:

mPboIds = IntBuffer.allocate(2); 

GLES30.glGenBuffers(2, mPboIds);

GLES30.glBindBuffer(GLES30.GL_PIXEL_PACK_BUFFER, mPboIds.get(0));
GLES30.glBufferData(GLES30.GL_PIXEL_PACK_BUFFER, mPboSize, null, GLES30.GL_STATIC_READ); //allocates only memory space given data size

GLES30.glBindBuffer(GLES30.GL_PIXEL_PACK_BUFFER, mPboIds.get(1));
GLES30.glBufferData(GLES30.GL_PIXEL_PACK_BUFFER, mPboSize, null, GLES30.GL_STATIC_READ);

GLES30.glBindBuffer(GLES30.GL_PIXEL_PACK_BUFFER, 0);

然后我用两个缓冲区来打乒乓:

    GLES30.glBindBuffer(GLES30.GL_PIXEL_PACK_BUFFER, mPboIds.get(mPboIndex)); //1st PBO
    JNIWrapper.glReadPixels(0, 0, mRowStride / mPixelStride, (int)height, GLES30.GL_RGBA, GLES30.GL_UNSIGNED_BYTE); //read pixel from the screen and write to 1st buffer(native C++ code)

    //don't load anything in the first frame
    if (mInitRecord) {
        GLES30.glBindBuffer(GLES30.GL_PIXEL_PACK_BUFFER, 0);

        //reverse the index
        mPboIndex = (mPboIndex + 1) % 2;
        mPboNewIndex = (mPboNewIndex + 1) % 2;
        mInitRecord = false;
        return;
    }

    GLES30.glBindBuffer(GLES30.GL_PIXEL_PACK_BUFFER, mPboIds.get(mPboNewIndex)); //2nd PBO
    //glMapBufferRange returns pointer to the buffer object
    //this is the same thing as calling glReadPixel() without a bound PBO
    //The key point is that we can pipeline this call

    ByteBuffer byteBuffer = (ByteBuffer) GLES30.glMapBufferRange(GLES30.GL_PIXEL_PACK_BUFFER, 0, mPboSize, GLES30.GL_MAP_READ_BIT); //downdload from the GPU to CPU

    Bitmap bitmap = Bitmap.createBitmap((int)mScreenWidth,(int)mScreenHeight, Bitmap.Config.ARGB_8888);
    bitmap.copyPixelsFromBuffer(byteBuffer);

    GLES30.glUnmapBuffer(GLES30.GL_PIXEL_PACK_BUFFER);

    GLES30.glBindBuffer(GLES30.GL_PIXEL_PACK_BUFFER, 0);

    //reverse the index
    mPboIndex = (mPboIndex + 1) % 2;
    mPboNewIndex = (mPboNewIndex + 1) % 2;

每帧都会在我的绘图方法中调用它.根据我的理解,glReadPixels不应该花费任何时间,但它需要大约25ms(在Google Pixel 2上)并且创建位图需要另外40ms.这只能达到13 FPS,比没有PBO的glReadPixels差.

我的代码中是否有任何遗漏或错误的内容？

Answer 1

编辑，因为你指出我原来的假设是不正确的（初始 PboIndex == PboNextIndex）。希望对您有所帮助，这是我刚刚使用 GLES 3 在 Android 中通过 JNI 调用的本机端编写的 C++ 代码。它似乎可以工作并且不会阻塞 glReadPixels(...)。请注意，只有一个 glPboIndex 变量：

  glBindBuffer(GL_PIXEL_PACK_BUFFER, glPboIds[glPboIndex]);
  glReadPixels(0, 0, frameWidth_, frameHeight_, GL_RGBA, GL_UNSIGNED_BYTE, 0);
  glPboReady[glPboIndex] = true;
  glPboIndex = (glPboIndex + 1) % 2;
  if (glPboReady[glPboIndex]) {
    glBindBuffer(GL_PIXEL_PACK_BUFFER, glPboIds[glPboIndex]);
    GLubyte* rgbaBytes = (GLubyte*)glMapBufferRange(
        GL_PIXEL_PACK_BUFFER, 0, frameByteCount_, GL_MAP_READ_BIT);
    if (rgbaBytes) {
      size_t minYuvByteCount = frameWidth_ * frameHeight_ * 3 / 2; // 12 bits/pixel
      if (videoFrameBufferSize_ < minYuvByteCount) {
        return; // !!! not logging error inside render loop
      }
      convertToVideoYuv420NV21FromRgbaInverted(
          videoFrameBufferAddress_, rgbaBytes,
          frameWidth_, frameHeight_);
    }
    glUnmapBuffer(GL_PIXEL_PACK_BUFFER);
    glPboReady[glPboIndex] = false;
  }
  glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);

...

先前毫无根据的假设：

您的问题没有显示设置 mPboIndex 和 mPboNewIndex 初始值的代码，但是如果它们被设置为相同的初始值，例如 0，那么它们将在每个循环中具有匹配值，这将导致映射相同的 PBO刚刚读过。在那个假设/真实场景中，即使使用了 2 个 PBO，它们也不会在 glReadPixels 和 glMapBufferRange 之间交替，然后会阻塞，直到 GPU 完成数据传输。我建议进行此更改以确保 PBO 交替：

mPboNewIndex = mPboIndex;
mPboIndex = (mPboNewIndex + 1) % 2;