qmk_firmware/quantum/rgb_matrix/animations/gradient_left_right_anim.h

#ifdef ENABLE_RGB_MATRIX_GRADIENT_LEFT_RIGHT
RGB_MATRIX_EFFECT(GRADIENT_LEFT_RIGHT)
#    ifdef RGB_MATRIX_CUSTOM_EFFECT_IMPLS

bool GRADIENT_LEFT_RIGHT(effect_params_t* params) {
    RGB_MATRIX_USE_LIMITS(led_min, led_max);

    HSV     hsv   = rgb_matrix_config.hsv;
    uint8_t scale = scale8(64, rgb_matrix_config.speed);
    for (uint8_t i = led_min; i < led_max; i++) {
        RGB_MATRIX_TEST_LED_FLAGS();
        // The x range will be 0..224, map this to 0..7
        // Relies on hue being 8-bit and wrapping
        hsv.h   = rgb_matrix_config.hsv.h + (scale * g_led_config.point[i].x >> 5);
        RGB rgb = rgb_matrix_hsv_to_rgb(hsv);
        rgb_matrix_set_color(i, rgb.r, rgb.g, rgb.b);
    }
    return rgb_matrix_check_finished_leds(led_max);
}

#    endif  // RGB_MATRIX_CUSTOM_EFFECT_IMPLS
#endif      // DISABLE_RGB_MATRIX_GRADIENT_LEFT_RIGHT