Re: [xine-devel] Converting yuv to rgb

[Simon T. <sim...@bi...>]

Bob van Loosen wrote:
> Matthias Hopf wrote:
>> On Jul 29, 07 20:51:27 +0200, Bob van Loosen wrote:
>>   
>>>> finally back to RGB for your LED's. You will also need to involve the gamma 
>>>> curves in the transformation.
>>>>   
>>>>       
>>> I'm using 80 red leds, 60 green leds and 60 blue leds. The red leds need 
>>> 2 volts and the green and blue leds need 3,8 volts.
>>> The current through the leds is 50 mA.
>>> They're hooked up to a six channel pwm controller made with a 20 mhz 
>>> PIC16F84A which is connected to the serial port.
>>>     
>> So you have linear output, while the CRT has a gamma of approx. 2.3
>> (which is resembled in the electronics of LCDs as well). So you still
>> have to apply gamma (or inverse gamma, depending on how you see it).
>>   
> I read this on Wikipedia:
> 
> In PC video, images are encoded with a gamma of about 0.45 and decoded 
> with a gamma of 2.2;
> in Mac systems, the corresponding typical values are 0.55 and 1.8.
> 
> So after the YUV to RGB conversion, I have to apply a gamma of 2.2 to 
> get linear output, is this correct?
> 
> This still leaves the problem of a proper YUV to RGB conversion,
> because I still have the problem of the dark gray showing up as green.
> I would really like to use xine's own yuv2rgb functions, however I can't 
> figure out how to use them.
> I could implement the ambilight in one of the video output modules, 
> since the conversion is already done there.

Marcel gave a good reply and I think only one important thing was
missing. broadcast SDTV uses ITU REC601 standard, HDTV should use ITU
REC709. That is the program material is transmitted with the assumption
that the equipment being used at the receiver end uses these standards
and thus will produce the correct output. These standards should provide
you with the details you need to calibrate your system to. The sites
listed elsewhere in the thread cover all the conversion you will need to
perform.

There are a number of cheap kits that can assist in colour calibration.
They tend to use a cheap sensor that responds to white light and then
you place colour filters in front of it to measure RGB, this should do
well enough for black level and gamma. I personally would not trust them
for anything else. Professional camera shops often have equipment for
hire. A good colourimeter should be quite cheap to hire for a few days.
just be sure you have calibration software first and you know how to use
it. be aware also that some meters need switching to a different mode or
just plain don't work with projection systems. I would expect that
colour LEDs have a small number of narrow wavelengths that make up the
colours rather than the broader spread found in natural light.

I also recommend Bruce Lindblooms web site as others have mentioned. It
is also worth checking out the Colour FAQ and Gamma FAQ on Charles
Poynton's web page.

http://www.poynton.com/

Simon