modifications.txt

If you want to update the jgarff rpi_ws281x code you need to make some changes:
https://github.com/jgarff/rpi_ws281x

//First add in ws2811.h:
int color_size;                              //3 = RGB, 4 = RGBW
//to ws2811_channel_t;

//change type of ws2811_led_t;
typedef struct
{
    uint32_t color;
    uint32_t brightness;
}ws2811_led_t;

//change in the reder function of ws2811.c the for loop:

	for (i = 0; i < channel->count; i++)                // Led
	{
		const int brightness = scale * (channel->leds[i].brightness & 0xff) + 1;
		uint8_t color[] =
		{
			channel->gamma[(((channel->leds[i].color >> channel->rshift) & 0xff) * brightness) >> 16], // red
			channel->gamma[(((channel->leds[i].color >> channel->gshift) & 0xff) * brightness) >> 16], // green
			channel->gamma[(((channel->leds[i].color >> channel->bshift) & 0xff) * brightness) >> 16], // blue
			channel->gamma[(((channel->leds[i].color >> channel->wshift) & 0xff) * brightness) >> 16], // white
		};
		uint8_t array_size = 3; // Assume 3 color LEDs, RGB

		// If our shift mask includes the highest nibble, then we have 4
		// LEDs, RBGW.
		if (channel->strip_type & SK6812_SHIFT_WMASK)
		{
			array_size = 4;
		}

//add initialization of the led brightness table in the ws2811_return_t ws2811_init(ws2811_t *ws2811) function:
			
//add:
for (i=0;i<channel->count;i++) channel->leds[i].brightness=255;

//before:
// Set default uncorrected gamma table
if (!channel->gamma)
{

//add extra paramter to the ws2811_render function: int32_t channel_number
//add check for channel number in the for loop of ws2811_render() function:
    for (chan = 0; chan < RPI_PWM_CHANNELS; chan++)         // Channel
    {
        if (channel_number == chan || channel_number == -1) {
		//.... original for loop code here
		}
	}
	
//move the code to wait for ws2811 dma to complete (inside ws2811_render function) BEFORE the for loop that changes the raw pixel data:

    // Wait for any previous DMA operation to complete.
    if ((ret = ws2811_wait(ws2811)) != WS2811_SUCCESS)
    {
        return ret;
    }
//before:
    for (chan = 0; chan < RPI_PWM_CHANNELS; chan++)         // Channel

in readpng.c:readpng_get_image comment:
    //if (png_get_gAMA(png_ptr, info_ptr, &gamma))
    //    png_set_gamma(png_ptr, display_exponent, gamma);