Computing power per pixel required for stereo matching

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I want to use the StereoBM and StereoSGBM methods for stereo matching. It would be very useful for me to know the average computing power per source image pixel that is needed to process input stereo images within given time. The computing power could be measured in some units that are architecture independent (GFLOPS?) so that x86, GPU, ARM etc. could be compared against each other.

Example: Say I have X Mpix source stereo images and want to use StereoBM and need to get Y frames per second (FPS). The result should say I need Z GFLOPS to do that task.