I don't believe LED-pixel displays use PWM. I would expect them to use bit planes: for each pixel transform the gamma-compressed intensity to the linear photon-proportional domain. Represent the linear intensity as a binary number. Start with the most significant bit, and all pixels with that bit get a current pulse, then for the next bitplane all the pixels having the 2nd bit set are turned on with half that current for the same duration, each progressive bitplane sending half as much current per pixel. After the least significant bitplane has been lit each pixel location has emitted a total number of photons proportional to what was requested in the linear domain.