fix gamma curve, it was all broken! #5
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Cycle counting the PIO program, each data word should take 2 PIO clocks of clock_get_hz while each repetition of the delay loop should take 1 PIO clock. However, with a non-gamma-corrected ramp, discontinuities (decreases) in brightness were seen for the bitplanes that needed the additional delay with oe enabled. Empirically, the value of 128 gives a plausible linear ramp and also fixes the big buck bunny rendering artifacts with 10 planes. It also makes the granularity of the on-time twice as fine, by being able to turn off during either the "clock on" or "clock off" phase of the shift register loading process, when the output enable time is short. Thus, with 10 bitplanes and a 64x32 panel, only the most significant bitplane needs any extra delay.
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there's still a problem where, with a small number of bitplanes, the overall brightness decreases. I think this is because the longest on time is less than the total data transmission time for a row of pixels. |
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