Reading a buffer operated on by a compute shader

[kheaactua]

I have a compute shader operating on my buffer in my QML (5.8) app, the problem is that I cannot seem to read this buffer, only write to it.

My buffer is a Qt3DRender::QBuffer, I set its contents with

Qt3DRender::QBuffer::setData(QByteArray::fromRawData(reinterpret_cast<const char*>(points_.linearize()), static_cast<int>(sizeof(PointType) * pc)));

where pc is the number of points.

To use it, I created a QGeometry which sets up QAttributes, and apply the computer shader as a material component to the Entity

My material:

Material {
	property PointBuffer dataBuffer;

	ShaderProgram {
		id: computeShader
		computeShaderCode:  loadSource("qrc:/shaders/pointcloud.comp")
	}

	effect: Effect {
		techniques: [
			Technique {
				renderPasses: [
					RenderPass {
						shaderProgram: computeShader
						parameters: [
							// Point buffer
							Parameter { name: "Particles"; value: dataBuffer }
						]
					}
				] // renderpasses
				filterKeys: [
					FilterKey { name: "type"; value: "compute" }
				]
				graphicsApiFilter {
					api: GraphicsApiFilter.OpenGL
					profile: GraphicsApiFilter.CoreProfile
					majorVersion: 4
					minorVersion: 3
				}
			} // technique
		] // techniques
	}
}

In my buffer code though, whenever I read from the buffer (Qt3DRender::buffer::data()), I only get the data I wrote to it, not the result of the compute shader.

I've tried setting the usage to various things, but no dice.

In my OpenGL book, I see that one can glMapBuffer to have read/write access, and there's even a QOpenGLBuffer that has an ENUM to set the access in a similar way, but I can't seem to make use of either of these; i.e. I can't find a way to map my buffer, and the QOpenGLBuffer doesn't seem compatible with any of the renderers.

(cross posted this on SO and Qt Center)