Multiple calls to cv::ogl::Texture2D.copyFrom() results in cv::Exception (-219) - c++

Question

I am rendering two views per frame on an HMD and it's kind of complicated right now because I use OpenCV to load images and process intermediary results and the rest is OpenGL, but I still want it to work. I am using OpenCV 3.1 and any help would be greatly appreciated, even if you have just some advice.
Application details:
Per view (left and right eye) I take four images as cv::Mat and copy them into four cv::ogl::Texture2D objects. Then I bind these textures to active OpenGL textures to configure my shader and draw to a framebuffer. I read the pixels of the frame buffer again (glReadPixels()) as a cv::Mat and do some postprocessing. This cv::Mat ("synthView") is getting copied to another cv::ogl::Texture2D which is rendered on a 2D screenspace quad for the view.
Here's some console output I logged for each call to the cv::ogl::Texture2D objects. No actual code!
// First iteration for my left eye view
colorImageTexture[LEFT].copyFrom(imageLeft, true); //view1
colorImageTexture[RIGHT].copyFrom(imageRight, true); //view1
depthImageTexture[LEFT].copyFrom(depthLeft, true); //view1
depthImageTexture[RIGHT].copyFrom(depthRight, true); //view1
colorImageTexture[i].bind(); //left
depthImageTexture[i].bind(); //left
colorImageTexture[i].bind(); //right
depthImageTexture[i].bind(); //right
synthesizedImageTexture.copyFrom(synthView, true); //frame0, left_eye done
// Second iteration for my right eye view, reusing colorImageTexture[LEFT] the first time
colorImageTexture[LEFT].copyFrom(imageLeft, true); //view2 // cv::Exception!
The code was working when I catched the exceptions and used the Oculus DK2 instead of the CV1. As you can see, I can run through one rendered view, but trying to render the second view will throw an exception in the copyFrom method at gl::Buffer::unbind(ogl::Buffer::PIXEL_UNPACK_BUFFER).
The exception occurs after all ogl::Texture2D objects have been used once and the first one gets "reused", which means that it will not call ogl::Texture2D::create(...) in the copyFrom() function!
Details of the cv::Exception:
code: -219
err: The specified operation is not allowed in the current state
func: cv::ogl::Buffer::unbind
file: C:\\SDKs\\opencv3.1\\sources\\modules\\core\\src\\opengl.cpp
Call stack details:
cv::ogl::Texture2D::copyFrom(const cv::_InputArray &arr, bool autoRelease);
gets called from my calls, which invokes
ogl::Buffer::unbind(ogl::Buffer::PIXEL_UNPACK_BUFFER);
In that, there is an OpenGL call to
gl::BindBuffer(target, 0); // target is "ogl::Buffer::PIXEL_UNPACK_BUFFER"
with a direct call to CV_CheckGlError(); afterwards, which throws the cv::exception. HAVE_OPENGL is apparently not defined in my code. The GL error is a GL_INVALID_OPERATION.
According to the specification of glBindBuffer:
void glBindBuffer(GLenum target,
GLuint buffer);
While a non-zero buffer object name is bound, GL operations on the
target to which it is bound affect the bound buffer object, and
queries of the target to which it is bound return state from the bound
buffer object. While buffer object name zero is bound, as in the
initial state, attempts to modify or query state on the target to
which it is bound generates an GL_INVALID_OPERATION error.
If I understand it correctly, gl::BindBuffer(target, 0) is causing this error because the buffer argument is 0 and I somehow alter the target. I am not sure what the target actually is, but maybe my glReadPixels() interferes with it?
Can somebody point me in the right direction to get rid of this exception? I just used the sample OpenCV code to construct my code.
Update: My shader code can trigger the exception. If I simply output the unprojected coordinates or vec4(0,0,0,1.0f), the program breaks because of the exception. Else, it continues but I cannot see my color texture on my mesh.

Answer 1

Given the information in your question, I believe the issue is with asynchronous writes to a pixel buffer object (PBO). I believe your code is trying to bind a buffer to 0 (unbinding the buffer), but that buffer is still being written to by an asynchronous call prior to it.
One way to overcome this is to use sync objects. Either define a sync object and use glFenceSync() or glWaitSync(). If you wait for buffers to finish their actions, this will have a negative impact on performance. Here is some information about a sync object.
Check this question for information on where one would use fence sync objects.
Another way could be use multiple buffers and switch between them for consecutive frames, this will make it less likely that a buffer is still in use while you unbind it.

Answer 2

Actual answer is that the code from OpenCV checks for errors with glGetError(). If you don't do this in your code, the cv::ogl::Texture2D::copyFrom() code will catch the error and throw an exception.