2015-08-17, 23:14
This is where it gets confusing...
Almost all consumer video is coded in limited range 16-235 (aka Limited) YCrCb 4:2:0.
HDMI TVs will default to Limited 16-235 input range, but many can also be configured to accept Full 0-255 range. (*)
Usually Kodi handles video in the 0-255 domain, so 16-235 content is usually scaled to 0-255 internally.
The Intel GPU driver can output 16-235 limited video or 0-255 full video.
If you want Limited video outputs then the obvious thing to do is to ensure the GPU is running in Limited output range. (This is the default behaviour for 720p and 1080i/p modes in OpenElec with an Intel GPU)
In this case the video has gone 16-235 -> 0-255 -> 16-235. This can introduce banding (**) due to 8 bit scaling happening twice.
If you want Full video output then you can use xrandr to output Full range 0-255 video, and configure your display to accept Full range inputs (this is not always possible if you route through an AVR and have other sources feeding your AVR that are conventional HDMI sources like satellite receivers and DVD/Blu-ray players that only output Limited range)
In this case the video has gone 16-235 -> 0255 and there is only one scale, though this can still introduce banding (**)
HOWEVER there is a third option - which can work in some cases - which can deliver Limited video range output without scaling to 0-255 and avoiding banding.
You configure your GPU to output Full 0-255 range video. However you tell Kodi to compensate for this (by setting the Limited 16-235 range option in video output system settings) Kodi then ensures that the video remains in 16-235 level space when output. Until very recently it wasn't possible to do this if you used VAAPI rendering (needed for VAAPI deinterlacing) - as that always scaled via 0-255 - but new code has been produced that does now allow this to happen in 16-235 level space, so if you are using experimental builds you can continue to use VAAPI Rendering AND get 16-235 all the way through (***) . This code is in the experimental EGL stuff here : http://forum.kodi.tv/showthread.php?tid=231955
So in this case the video stays 16-235 all the way through (though the output is configured for full 0-255 range it is output as 16-235 via a cheat) (***)
(*) It is possible for HDMI sources to flag their signal to tell a display whether the video is in 0-255 or 16-235 space using 'info frames'. However these are usually ignored. But not by all displays.
(**) In the new experimental VAAPI with EGL code there is now the option to add dither. This can help reduce the visibility of banding by adding a small amount of dither noise to the 16-235 <-> 0-255 scaling.
(***) This works on TVs that ignore the Full 0-255 / Limited 16-235 'info frame' flags, but if they interpret them correctly, then the 'cheat' doesn't work, as the TV doesn't get tricked into accepting 0-255 Full range flagged video as 16-235 Limited range video as we hope. My 2007 Sony Full HD set ignores them, my 2014 Sony UHD set follows them. Therefore I can use the cheat that delivers clean 16-235 all the way through on my 2007 set, but not my 2014 one...
Almost all consumer video is coded in limited range 16-235 (aka Limited) YCrCb 4:2:0.
HDMI TVs will default to Limited 16-235 input range, but many can also be configured to accept Full 0-255 range. (*)
Usually Kodi handles video in the 0-255 domain, so 16-235 content is usually scaled to 0-255 internally.
The Intel GPU driver can output 16-235 limited video or 0-255 full video.
If you want Limited video outputs then the obvious thing to do is to ensure the GPU is running in Limited output range. (This is the default behaviour for 720p and 1080i/p modes in OpenElec with an Intel GPU)
In this case the video has gone 16-235 -> 0-255 -> 16-235. This can introduce banding (**) due to 8 bit scaling happening twice.
If you want Full video output then you can use xrandr to output Full range 0-255 video, and configure your display to accept Full range inputs (this is not always possible if you route through an AVR and have other sources feeding your AVR that are conventional HDMI sources like satellite receivers and DVD/Blu-ray players that only output Limited range)
In this case the video has gone 16-235 -> 0255 and there is only one scale, though this can still introduce banding (**)
HOWEVER there is a third option - which can work in some cases - which can deliver Limited video range output without scaling to 0-255 and avoiding banding.
You configure your GPU to output Full 0-255 range video. However you tell Kodi to compensate for this (by setting the Limited 16-235 range option in video output system settings) Kodi then ensures that the video remains in 16-235 level space when output. Until very recently it wasn't possible to do this if you used VAAPI rendering (needed for VAAPI deinterlacing) - as that always scaled via 0-255 - but new code has been produced that does now allow this to happen in 16-235 level space, so if you are using experimental builds you can continue to use VAAPI Rendering AND get 16-235 all the way through (***) . This code is in the experimental EGL stuff here : http://forum.kodi.tv/showthread.php?tid=231955
So in this case the video stays 16-235 all the way through (though the output is configured for full 0-255 range it is output as 16-235 via a cheat) (***)
(*) It is possible for HDMI sources to flag their signal to tell a display whether the video is in 0-255 or 16-235 space using 'info frames'. However these are usually ignored. But not by all displays.
(**) In the new experimental VAAPI with EGL code there is now the option to add dither. This can help reduce the visibility of banding by adding a small amount of dither noise to the 16-235 <-> 0-255 scaling.
(***) This works on TVs that ignore the Full 0-255 / Limited 16-235 'info frame' flags, but if they interpret them correctly, then the 'cheat' doesn't work, as the TV doesn't get tricked into accepting 0-255 Full range flagged video as 16-235 Limited range video as we hope. My 2007 Sony Full HD set ignores them, my 2014 Sony UHD set follows them. Therefore I can use the cheat that delivers clean 16-235 all the way through on my 2007 set, but not my 2014 one...
