在 OpenGL-OpenCL 纹理互操作中，PBO 如何从纹理中接收数据？

Question

Most examples on PBOs on the web are about making CPU-GPU transfers asynchronous.

I'm trying to use PBOs for something else - for OpenGL-OpenCL interop, as explained in this tutorial . Quote from there:

[...] Creating an intermediate (staging) Pixel-Buffer-Object for the OpenGL texture via clCreateFromGLBuffer, updating the buffer with OpenCL, and copying the results back to the texture.

The tutorial explains I need to create the PBO as follows:

GLuint pbo;
glGenBuffers(1, &pbo);
glBindBuffer(GL_ARRAY_BUFFER, pbo);
//specifying the buffer size
glBufferData(GL_ARRAY_BUFFER, width * height * sizeof(cl_uchar4), …);

I have replaced the ellipsis with NULL, GL_DYNAMIC_COPY , as I don't want to send any data from the host.

Here's the createPbo function I ended up with:

void createPbo(ivec2 sz, int elementSize) {
    glGenBuffers(1, &pbo);
    glBindBuffer(GL_ARRAY_BUFFER, pbo);
    glBufferData(GL_ARRAY_BUFFER, sz.x * sz.y * elementSize, NULL, GL_DYNAMIC_COPY);
    glBindBuffer(GL_ARRAY_BUFFER, 0);

    glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo);
    glGetTexImage(GL_TEXTURE_2D, 0, GL_RED, GL_FLOAT, NULL);
    glBindBuffer(GL_PIXEL_PACK_BUFFER, 0);
}

In the rest of my code, I do what the tutorial does, and the code works fine. The problem is the glGetTexImage call: The tutorial doesn't mention it, which means maybe I can skip doing this slow copy? Can I?

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Note about my usecase: I'm trying to send existing OpenGL texture data into OpenCL, manipulate it there, and then send it back to OpenGL.

Answer 1

You can access an image level of an OpenGL texture directly in OpenCL without copying (at least not as an explicit API action) as an OpenCL image object via clCreateFromGLTexture .

The very tutorial you mentioned also mentions this. You don't need a buffer object or a pixel buffer transfer operation.

And, for completeness, since the question was explicitly about how to use GL buffer sharing in OpenCL and then using the GL buffer to update a texture image:

The tutorial you referenced (in Method 2) also covers exactly how to do this. In this case, we do not share a level of the GL texture but we share a GL buffer object as a CL buffer object:

(the following is more or less a direct copy/paste of that tutorial's sources)

First, create the OpenGL buffer object that we want to share with OpenCL and that we will later use to update a texture image with PBO transfer:

GLuint pbo;
glGenBuffers(1, &pbo);
glBindBuffer(GL_ARRAY_BUFFER, pbo);
//specifying the buffer size
glBufferData(GL_ARRAY_BUFFER, width * height * sizeof(cl_uchar4), …);

Next, if you want to initialize the GL buffer object with a level of your original GL texture, you first need to PBO copy that image into the GL buffer object:

glBindBuffer(GL_PIXEL_PACK_BUFFER, pbo);
glBindTexture(GL_TEXTURE_2D, tex);
glGetTexImage(GL_TEXTURE_2D, 0, texFormat, texType, 0);

This will PBO-copy the GL texture's first level into the GL buffer object.

Next, share the GL buffer in OpenCL:

mem = clCreateFromGLBuffer(g_context, CL_MEM_WRITE_ONLY, pbo,  NULL);

Then, acquire the ownership of the shared GL/CL buffer object via clEnqueueAcquireGLObjects() , execute the kernel that updates the CL buffer content, and release the ownership via clEnqueueReleaseGLObjects() .

And last, update an OpenGL texture (currently bound to GL_TEXTURE_2D ) with PBO transfer out of the GL buffer object:

glBindBuffer(GL_PIXEL_UNPACK_BUFFER, pbo);
glBindTexture(GL_TEXTURE_2D, texture);
glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

Answer 2

No, you cannot because the data is accessed with buffer Mapping ( glMapBuffer ). You cannot directly map a texture image, but you can map the data store of a buffer object. Therefore you need to copy the data from the texture image into the data store of a buffer.
glGetTexImage carries out the "copying" process (and necessary format and type conversions). It get the data from the texture bound to target GL_TEXTURE_2D and stores the data in the buffer bound to target GL_PIXEL_PACK_BUFFER .