ESPxRGB is an Arduino library of RGB, RGBW, HSV conversion functions and Gamma Correction written in Xtensa assembler for ESP SoCs that use Xtensa cores (ESP32).

• Why an assembler RGB manipulation library
• Features
• Performance
• Options and Use
• Versions
• History and References
• License

Looking to use small, low-powered SoCs to drive long strings of LEDs with the optimum speed and power efficiency places a premium on efficient algorythms and code, and Assembler is as efficient as you can get. The ESP SoCs are great given their feature set, conectivity, power and price. Plus, they are easy to program with a nice instruction set.

The assembler itself is contained in GNU-format assembler .S files, and is not inline assembler. The .S files are compiled along with the ESPxRGB.h header when pulled into a project.

ESPxRGB currently works in the 8-bit RGB/W space only. It covers functions supporting:

• RGB to RGBW
• HSV to RGB and RGBW, with a choice of four HSV algorythms
• RGB gamma correction (for normalizing the perception of pulsing luminance)
• RGB chroma correction (for normalizing the perception of luminance across the spectrum)

plus intersections of some of the above.

External C headers provide regular Arduino and ESP-IDF code access to the functions.

From the HSV Benchmark example

Benching HSV Conversions for 6291456 iterations

Impl			Time ms		Calls per Second
FastHSV        		 2232.91	  894121
KKs Spectrum   		 2123.05	  940389
ESPxRGB Spectrum	 1972.31	 1012261
ESPxRGB Efficient	 1987.78	 1004383
ESPxRGB Wave   		 2188.28	  912356
ESPxRGB Tweak  		 2040.31	  978521

Taking KasperKampemans C impementation of HSV as the baseline:

Drop the library into your ~Arduino/libraries folder and include the ESPxRGB.h header in your project. The library is compiled based on flags in the src/options.h file: The functions are grouped and can be included/excluded in the compiled code as dictated by pre-processor definitions.

The examples folder contains sketches than can test the functions and generate look-up tables used by the code.

• xgamma8 - Normalizes the value of a single 8-bit color value
• xgammaarray8 - Normalizes a range of 8-bit color values
• xrgbgamma8 - Normalizes a set of 8-bit RGB values
• xrgbwgamma8 - Normalizes a set of 8-bit RGBW values

• xrgb2rgbw8 - RGB to RGBW
• xhsv2rgb8s - HSV to RGB using the Spectrum algorythm
• xhsv2rgb8e - HSV to RGB using a power efficient function
• xhsv2rgb8t - HSV to RGB using a tweaked function aproximating FastLED Rainbow HSV
• xhsv2rgb8w - HSV to RGB using a (sine) wave function
• xhsv2rgb8 - HSV to RGB default function that points to an above function of your choice.

• xhsv2rgbgamma8 - HSV to RGB with gamma correction
• xhsv2rgbwgamma8 - HSV to RGB with gamma correction
• xrgb2rgbwgamma8 - RGB to RGBW with gamma correction

• 1.0.0 Initial release
• 0.1.0 Initial commit and pre-release

I came to write this library after looking into options to do HSV-to-RGB conversion. Initially I looked at FastLED but determined it too broad a library for my use, and it dictated that FastLED be at the center of whatever you were doing which was not what I wanted.

Next I came to use fast_hsv2rgb from Vagrearg: Lots of interesting math and theory on spectrum HSV, with C implementations, and also AVR, which got me thinking of why not an Xtensa version. I tried copying the logic of the AVR in fast_hsv2rgb, and then the C logic, both of which use opaque iterative pointer-swapping, but I decided to use a jump table, and then to move to simplify the whole thing.

And the simplest HSV to RGB code out there, I found, is Kasper Kamperman's, with a simple flow through and case at the end. Looking at it, I found I could simplify it a little more by pulling common calculation up out of the case; I then implemented that in Xtensa. Kasper also addressed dimming for luminance normalization.

Reading up on HSV again on Instructables, Ontaelio also spoke to power efficiency through using wave forms and in to how they effected percieved color. I added both power-efficient (saw-tooth) and smoother looking (sine) wave implementation leveraging the spectrum HSV code.

Yet the discussion of more natural looking HSV color-wheels led me to look back at FastLED HSV to RGB and at their Rainbow HSV. Not being able to quickly grok the code, looking at the rainbow waveform image led me to implement an approximation of that as simply as I could.

ESPxRGB - Copyright 2019 technosf [http://github.com/technosf]

Licensed under the GNU LESSER GENERAL PUBLIC LICENSE, Version 3.0 or greater (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

         http://www.gnu.org/licenses/lgpl-3.0.en.html         

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.