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How to do particle binning in OpenGL?

Currently I'm creating a particle system and I would like to transfer most of the work to the GPU using OpenGL, for gaining experience and performance reasons. At the moment, there are multiple particles scattered through the space (these are currently still created on the CPU). I would more or less like to create a histogram of them. If I understand correctly, for this I would first translate all the particles from world coordinates to screen coordinates in a vertex shader. However, now I want to do the following:

enter image description here

So, for each pixel a hit count of how many particles are inside. Each particle will also have several properties (e.g. a colour) and I would like to sum them for every pixel (as shown in the lower-right corner). Would this be possible using OpenGL? If so, how?

Solution

  • The best tool I recomend for having the whole data (if it fits on GPU memory) is the use of SSBO.

    Nevertheless, you need data after transforming them (e.g. by a projection). Still SSBO is your best option:

    In the fragment shader you read the properties of already handled particles (let's say, the rendered pixel) and write modified properties (number of particles at this pixel, color, etc) to the same index in the buffer.

    Due to parallel nature of GPU, several instances coming from different particles may be doing concurrently the work for the same index. Thus you need to handle this on your own. Read Memory model and Atomic operations

    Another approach, but limited, is using Blending

    The idea is that each fragment increments the actual color value of the frame buffer. This can be done using GL_FUNC_ADD for glBlendEquationSeparate and using as fragment-output-color a value of 1/255 (normalized integer) for each RGB/a component.

    Limitations come from the [0-255] range: Only up to 255 particles in the same pixel, the rest amount is clamped to this range and so "lost".

    You have four components RGBA, thus four properties can be handled. But can have several renderbuffers in a FBO.

    You can read the FBO by glReadPixels. Use glReadBuffer first with a GL_COLOR_ATTACHMENTi if you use a FBO instead of the default frame buffer.