Color Calibration in FLS-M Config-Builder NOT Reflected in Matter Device

Dresden-Elektronik Hardware
1

To match the LEDs in my strip, I used Config-Builder to create two CT lights on the FLS-M (firmware version 1.1). Both with Advanced Light Options > Color Calibration > Warm White X, Y Position as 0.48614, 0.41467 (2,400K/417 mired) and Cool White X, Y Position as 0.31352. 0.32363 (6,500K/154 mired). However the color temperature range that appears in Apple Home via matter is 2,695K/371 mired to 6,535K/153 mired.

How do I get the FLS-M to advertise the color temperature range that matches my LED strips? I know it’s possible to do this with the ESP32-C6, because I’ve got an ESP-IDF Matter SDK project where I did this very thing. Thanks!

2

The FLS-M internally only uses CIE1931 xy chromaticity for all LED types. The possible CT range is based on the displayable color gamut.

The formula used for that is as follows:

void xy2mired(fixedPoint_t x, fixedPoint_t y, unsigned int *mired)
{
    // McCamy's approximation
    fixedPoint_t n = FIXEDPOINT_DIVIDE(
        FIXEDPOINT_SUBTRACT(x, FLOAT_TO_FIXEDPOINT(0.3320)),
        FIXEDPOINT_SUBTRACT(FLOAT_TO_FIXEDPOINT(0.1858), y)
    );
    fixedPoint_t nSquared = FIXEDPOINT_MULTIPLY(n, n);
    fixedPoint_t nCubed = FIXEDPOINT_MULTIPLY(nSquared, n);

    fixedPoint_t ct = // Scaled to 0K - 10000K -> 0.0 - 1.0
        FIXEDPOINT_ADD(
            FIXEDPOINT_ADD(
                FIXEDPOINT_ADD(
                    FIXEDPOINT_MULTIPLY(CONVERT_TO_FIXEDPOINT(437,0,10000), nCubed), 
                    FIXEDPOINT_MULTIPLY(CONVERT_TO_FIXEDPOINT(3601,0,10000), nSquared)
                ),
                FIXEDPOINT_MULTIPLY(CONVERT_TO_FIXEDPOINT(6861,0,10000), n)
            ), 
            CONVERT_TO_FIXEDPOINT(5517,0,10000)
        );

    // TODO: rework
    *mired = // Scaled to 0 - 1000 -> 0.0 - 1.0
        CONVERT_FROM_FIXEDPOINT(
            FIXEDPOINT_DIVIDE(
                FIXEDPOINT_ONE_VALUE, 
                FIXEDPOINT_MULTIPLY(ct, CONVERT_TO_FIXEDPOINT(10,0,1))
            ),
            0,
            1000
        );
}

Because the ESP32-C6 lacks a floating-point unit, we have to approximate with fixed-point math (in this case, 13-bit fractional depth). This leads to rounding errors, which are further amplified by the way mired is transformed into color temperature.

I’ll take a closer look at this for the next firmware release—it might make sense to add a dedicated color temperature field to the config builder.

Sorry for the inconvenience.

3

Explicit support for WW/CW strips would be greatly appreciated.

Using the ESP-IDF Matter SDK, if I create a Matter color temperature light with following snippet:

color_temperature_light::config_t light_config;
light_config.color_control_color_temperature.color_temp_physical_min_mireds = 154;
light_config.color_control_color_temperature.color_temp_physical_max_mireds = 417;

The range appears in Apple Home via Matter as 2,398K/415 mired to 6,493K/154 mired, which is a near perfect match for the LEDs in my strip.

4

@Haerteleric, with the resolution of 6449, being able to specify the minimum and maximum mired range on Matter Color Temperature Light devices is my only outstanding wish list item for the FLS-M. Any idea when this capability might be added to the config builder?