REC.709 / REC.601 question

[madhatter300871]

Hi

So, I have been doing some reading on colourimetry, I have put three links here that I think sum up everything I have read.

http://en.wikipedia.org/wiki/Rec._709

http://avisynth.org.ru/docs/english/externalfilters/colormatrix.htm

http://forum.doom9.org/showthread.php?t=133982#post1090068

I use mediainfo and/or gspot so I do know from the outset with each source which colourmatrix my video is reported as using.

My questions are :-

1. Strictly speaking should I be converting REC.709 to REC.601 when rescaling HD to SD (to play on the ipod as it happens) provided, off course, my source uses REC.709 in the first place?

2. Do televisions/projectors have an "awareness" of colourspace ? I did read that HD TV sets expect to be fed with REC.709 colourimetry video. If thats the case, does SD material suffer from bad colour reproduction or does the TV do some converting ?

3. In ffdshow there are options for outputing REC.709 or REC.601 in the "RGB conversion" page. Does this mean that ffdshow only takes this into account if I chose to output in RGB mode ? I usually output YV12 only.

4. Does the graphics card have an "awareness" of colourimetry, does it expect a certain colourimetry or does the graphics card only output the RGB video ?

I'm a little confused here and have just started "playing" to see if I can see differences, and as always your experiences are welcome.

Thanks.

[nm]

1. Strictly speaking should I be converting REC.709 to REC.601 when rescaling HD to SD (to play on the ipod as it happens) provided, off course, my source uses REC.709 in the first place?

If all decoders and players did the right thing, they'd check the colorspace flags in the stream and act accordingly. But I guess there are cases where this doesn't happen, so you might get better results overall by using Rec.601 for SD streams (and flag them accordingly). At least that's what people generally recommend.

I don't know what iPod does.

2. Do televisions/projectors have an "awareness" of colourspace ? I did read that HD TV sets expect to be fed with REC.709 colourimetry video. If thats the case, does SD material suffer from bad colour reproduction or does the TV do some converting ?

I have no practical experience on this, but I guess both alternatives are possible.

3. In ffdshow there are options for outputing REC.709 or REC.601 in the "RGB conversion" page. Does this mean that ffdshow only takes this into account if I chose to output in RGB mode ?

Yes.

4. Does the graphics card have an "awareness" of colourimetry, does it expect a certain colourimetry or does the graphics card only output the RGB video ?

If you only use it to output RGB video, it doesn't touch the colors. But if you use an overlay or other hardware-accelerated rendering that involves a YUV->RGB conversion, it either uses some mechanism to choose which color matrix to use or defaults to one or the other.

Use software conversion or a renderer that says what it does and can be overridden if necessary.

[madhatter300871]

nm, thanks for the reply.

OK, I'll convert to REC.601 I think from now on when I downscale my videos to SD, that seems to be the general concensus.

I am presuming unless I somehow instruct the graphics card to do something different, that it always outputs RGB data ? I am a bit ignorant to the actual job of the graphics card, just assuming it deals with presenting video to the display :confused:

[nm]

I am presuming unless I somehow instruct the graphics card to do something different, that it always outputs RGB data ?

Yes.

I am a bit ignorant to the actual job of the graphics card, just assuming it deals with presenting video to the display :confused:

Usually colorspace conversions are done by the video renderer: madVR, Haali's renderer or the EVR/VMR7/VMR9 renderers, or overlay. Alternatively there may be a software filter somewhere in the playback chain. But after the renderer has done its job, GPU will simply display the resulting RGB frames as-is. (Unless you set it to do some final output conversions that concern all graphics).

If you use madVR or Haali, display driver settings don't matter. If you use overlay, they might matter if such settings exist. I don't know about Microsoft's renderers. There are many alternatives to choose from and they may do different things depending on their own settings, the display driver settings or the phase of the moon.

[madhatter300871]

HA !! The phase of the moon ..... I am sensing the mystic language of a Linux Lover here .....:)

I am actually a VTS systems engineer and all of my systems are linux based .... I would recognise that language anywhere !

I'm just pulling your leg, I am gratefull for the pointers and slowly but surely I am getting a reasonable idea of how video is processed and presented to the display ... on a windows machine at least :)

Would you humour me in one last question then, is this a typical video chain :-

Video source -> splitter -> video decoder -> video renderer -> video card -> display

I should be aware off :-
.Video source colourspace and colourimetry.
.Video decoder colourspace/colourimetry conversions (if any)
.Video renderer required colourspace input
.Video renderer colourspace output

Is that a fair assumtion and a reasonable place to start ?

[nm]

HA !! The phase of the moon ..... I am sensing the mystic language of a Linux Lover here .....:)
Ah, caught me. ;)

Would you humour me in one last question then, is this a typical video chain :-

Video source -> splitter -> video decoder -> video renderer -> video card -> display

I should be aware off :-
.Video source colourspace and colourimetry.
.Video decoder colourspace/colourimetry conversions (if any)
.Video renderer required colourspace input
.Video renderer colourspace output

Yep. That's about it, as far as I know.

As you noted, most decoders don't do conversions unless set otherwise. Only some decoders have these settings anyway. Renderers may switch between Rec.709 and Rec.601 based on the input resolution. Splitters and decoders might read flags from the source and notify other components (I don't know if this happens in reality).

[madhatter300871]

nm, great. My knowledge is ever growing ....

So I think I now need to play some more and have a play with colorimetry conversions at different stages and just see what the results are.

All good fun ....

Thanks for the replies.