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

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

Qt Code: 
  1. Material {
  2. 	property PointBuffer dataBuffer;
  3.  
  4. 	ShaderProgram {
  5. 		id: computeShader
  6. 		computeShaderCode:  loadSource("qrc:/shaders/pointcloud.comp")
  7. 	}
  8.  
  9. 	effect: Effect {
  10. 		techniques: [
  11. 			Technique {
  12. 				renderPasses: [
  13. 					RenderPass {
  14. 						shaderProgram: computeShader
  15. 						parameters: [
  16. 							// Point buffer
  17. 							Parameter { name: "Particles"; value: dataBuffer }
  18. 						]
  19. 					}
  20. 				] // renderpasses
  21. 				filterKeys: [
  22. 					FilterKey { name: "type"; value: "compute" }
  23. 				]
  24. 				graphicsApiFilter {
  25. 					api: GraphicsApiFilter.OpenGL
  26. 					profile: GraphicsApiFilter.CoreProfile
  27. 					majorVersion: 4
  28. 					minorVersion: 3
  29. 				}
  30. 			} // technique
  31. 		] // techniques
  32. 	}
  33. }
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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 (http://doc.qt.io/qt-4.8/http://doc.qt.io/qt-5/qt3drender-qbuffer.html#UsageType-enum) to various things, but no dice.

In my OpenGL book, I see that one can [I]glMapBuffer[I] 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 on forum.qt.io and SO)