57 lines
2.3 KiB
Markdown
57 lines
2.3 KiB
Markdown
I was dissatisfied with a gamma corrected ramp using the APA102HD mode (it seemed to visibly step at one or more points), and some analysis of the 8 bit rgb + 5 bit brightness function showed why:
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https://www.argyllcms.com/APA102_loglog.svg.
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So here is a suggested replacement for five_bit_bitshift() in fl/five_bit_hd_gamma.cpp, that fixed the problem for me:
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// Improved FastLED five_bit_bitshift() function
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// This appears to exactly match the floating point reference,
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// with a worst case resulting error of 0.2% over the full 16 bit input range,
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// and an average error of 0.05% over that range. Errors scale with maximum
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// magnitude.
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// ix/31 * 255/65536 * 256 scaling factors, valid for indexes 1..31
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static uint32_t bright_scale[32] = {
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0, 2023680, 1011840, 674560, 505920, 404736, 337280, 289097,
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252960, 224853, 202368, 183971, 168640, 155668, 144549, 134912,
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126480, 119040, 112427, 106509, 101184, 96366, 91985, 87986,
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84320, 80947, 77834, 74951, 72274, 69782, 67456, 65280
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};
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// Since the return value wasn't used, it has been omitted.
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// It's not clear what scale brightness is, or how it is to be applied,
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// so we assume 8 bits applied over the given rgb values.
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void five_bit_bitshift(uint16_t r16, uint16_t g16, uint16_t b16, uint8_t brightness,
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CRGB *out, uint8_t *out_power_5bit) {
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uint8_t r8 = 0, g8 = 0, b8 = 0;
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// Apply any brightness setting (we assume brightness is 0..255)
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if (brightness != 0xff) {
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r16 = scale16by8(r16, brightness);
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g16 = scale16by8(g16, brightness);
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b16 = scale16by8(b16, brightness);
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}
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// Locate the largest value to set the brightness/scale factor
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uint16_t scale = max3(r16, g16, b16);
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if (scale == 0) {
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*out = CRGB(0, 0, 0);
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*out_power_5bit = 0;
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return;
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} else {
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uint32_t scalef;
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// Compute 5 bit quantized scale that is at or above the maximum value.
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scale = (scale + (2047 - (scale >> 5))) >> 11;
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// Adjust the 16 bit values to account for the scale, then round to 8 bits
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scalef = bright_scale[scale];
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r8 = (r16 * scalef + 0x808000) >> 24;
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g8 = (g16 * scalef + 0x808000) >> 24;
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b8 = (b16 * scalef + 0x808000) >> 24;
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*out = CRGB(r8, g8, b8);
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*out_power_5bit = scale;
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return;
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}
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} |