How to view a video hitorgam ?

[dyanglp]

Sorry to bring up this topic again. In this thread:
https://www.videohelp.com/forum/viewtopic.php?t=18990&highlight=ccir601

ffdshow can give you on the fly historgam display as well as allow you to set clipping values, This may help you to "debug" your video stream.

I would appreciate if someone could give a bit more info on how to view the historgam on the fly, as well as the screen display presented by BJ_M in that thread(what tool is that? ). Thanks.

[adam]

That link doesn't work for me but I see it involves ccir601, so I think I probably know the gist of it. I'm assuming its explained in that thread but, there are different ranges of luminence levels. Pc's use the full range of 0-255 and tv's have it compressed to leave headroom and footroom in the signal. Depending on the region its usually either a range of 16-235 or 8-235. Anyway, the point is that your've got a full range of 0-255 and your luminence levels may max it out (ie: blackest blacks will hit 0 and whitest whites will hit 255) or they may trail off around the 16 and 235 areas. A histogram can be used to determine what your source does. Play the video and watch the histogram. Pay careful attention to very dark and very light scenes. If the waves extend to the outer edges its not compressed, and is 0-255. If it consistently cuts off before reaching the edges it is 16-235.

Basically you need to know this because if you are encoding for tv then you need to compress to 16-235 (the encoder will anyway) but you want to do it correctly so as to avoid clipping. Its just a good idea to know what you got and where you need to go.

[dyanglp]

Adam,thank you for your quick reply. I'm still reading the following threads and try to have a better understanding to this "16-235" "0-255" story.
This is the begining:
https://www.videohelp.com/forum/viewtopic.php?t=244585&highlight=ccir601
followed by this long one:
https://www.videohelp.com/forum/viewtopic.php?t=177532
and this one:
https://www.videohelp.com/forum/viewtopic.php?t=177532
I have some immediate questions while reading these threads:
- Is TMPGEnc the only tool that does this color range "shrinking" and "extention" (with Y/C filter as some people said) ? What about other software, e.g. what about Ulead, how do they handle this ?
- Is that confirmed that PAL TV has full range support ? I read elsewhere that PAL people are also bothered by this 0-255/16-235 issue ?
- Some one said if you dont EDIT the movie, then you dont have to worry about this, otherwise the movie will be converted to RGB, what does EDIT mean in this case, simple cutting is considered as edit ?
- I used After effect Info pallete to display video RGB as mentioned in the above discussion thread but not convinced - I used mainconcept to capture a video from DV camera with "Native format" option, so I assume the uncompressed avi file generated should remain YUV color, but in AE when moving cursor on the clouds, info pallete shows 255,255,255 and I could never found a 0.0.0 value through out the movie, it seems to me the color range has kind of "shifting" to the white side.
Thanks.

[adam]

Is TMPGEnc the only tool that does this color range "shrinking" and "extention" (with Y/C filter as some people said) ?

Not at all. CCIR601 is a broadcast standard and is something that is incorporated into the mpeg specification. All mpeg encoders must compress the luminence levels to provide this headroom and footroom. As for any other software that processes the video along the way, the only way to know what it is doing it to test it. This is what historgrams are good for.

- Is that confirmed that PAL TV has full range support ? I read elsewhere that PAL people are also bothered by this 0-255/16-235 issue ?

This is a big misconception. PAL indeed does support a wider luminence range then NTSC, but this has nothing to do with CCIR601.

As I'll explain in response to your next question, all mpeg video will have a DIGITAL luminence range measuring 16-235. Take an mpeg and play it on PAL equipment and that 16 (blackest black) will translate to an ANALOGUE measurement of 0 IRE. Take that same mpeg and play it on NTSC equiment and it will measure 7.5 IRE, which is noticably brighter then 0 IRE. So you see the decoded (analogue) lumience ranges will vary from PAL to NTSC by nature of just what those standards can handle, but the source is encoded the same either way. So you still have to worry about getting your source to 16-235 correctly regardless of what regional format you are encodign for. To summarize:

For NTSC a digital 16=7.5 IRE.
For PAL a digital 16=0 IRE.

- Some one said if you dont EDIT the movie, then you dont have to worry about this, otherwise the movie will be converted to RGB, what does EDIT mean in this case, simple cutting is considered as edit ?

To put it simply, the sources you are going to see are either going to be in the RGB colorspace or in some form of YUV colorspace. The YUV colorspace only supports a luminence range between 16-235, and of course this is what mpeg uses. So if your source is YUV, and you keep it that way all the way up until it is encoded to mpeg, then there is no way the lumience scale can be altered and thus this whole concern becomes a non-issue. Unfortunately, or probably just unavoidably, most software programs that process video are going to convert a YUV source to RGB and they will probably also expand the 16-235 range out to 0-255. The reason is because if you view a 16-235 source on a pc monitor, which supports the full range of 0-255, then it will look washed out and contrasty. Since pc monitors operate in the 0-255 range, it does make sense to view your video in this range when editing. So when others talk about "EDITING" they just mean running your source through anything that operates in the RGB colorspace.

- I used After effect Info pallete to display video RGB as mentioned in the above discussion thread but not convinced - I used mainconcept to capture a video from DV camera with "Native format" option, so I assume the uncompressed avi file generated should remain YUV color, but in AE when moving cursor on the clouds, info pallete shows 255,255,255 and I could never found a 0.0.0 value through out the movie, it seems to me the color range has kind of "shifting" to the white side.
Thanks.

DV is generally said to have a luminence range of 16-235. Now the camcorders certainly don't place any arbitrary limits on the luminence ranges they actually capture, but whatever is captured is going to be mapped to 16-235 by just about any DV codec on the market. So whenever you actually do anything with the video you are going to get already compressed, 16-235, video.

But like I said above, NLE's like premiere and After Effects are going to expand this to 0-255 when its decoded. So you can't really use a histogram in these programs to test the lumience range of your source, rather you use the histogram to test the output you will get while using that program. I wish there were an easier way to tell you what you have and where you will go, but the only way is to run tests. If you are using a DV source then your source is going to be 16-235 unless maybe you use the Canapus codec which supports 0-255 output. From there, you can be assured that Premiere or after effects will expand this to 0-255. If you then send this output to TMPGenc you will need to compress it back to 16-235. TMPGenc does this by default. So you see you need to track your process and make sure that you never compress the range multiple times, or convert between YUV and RGB more than necessary. You also have to be careful of a few rogue codecs out there that hard clip rather then compress. Run a 0-255 source through these, while calling for 16-235 output, and it will simply discard all luma values not falling within this scale.

I'm sure you'll have more questions after reading this so fire away.

[edDV]

Start with understanding color bars and work up from there.
http://www.videouniversity.com/tvbars2.htm

PS: In my experience, if you are working in a zero IRE black DV (similar to CCIR-601) environment, setup should only apply to captures obtained from over the air TV, or matereial obtained from traditional NTSC recorded analog video tapes. When digitizing this material, black should be set to 7.5 IRE (nonmial, use your scopes) and white to 100 IRE. Any digital produced original material should have black at zero IRE and white at 100IRE. And yes, luminance (Y) black should be digital 16 and white should be 235 in an 8 bit DV space (CCIR 601 and SMPTE 259M can be 8 or 10 bit)

[dyanglp]

Adam, I really appreciate your detailed explaination, also thank edDV's link. After knowing this color range issue, I am hoping to find some sysmatic way to do thing right, including utilizing certain software tools to discover color problem at each stage and fix it if possible. But like you (and many other people) suggest, this is something based on experience and can only achieved through many tests. So now we are in a delimma (correct me if wrong): what we see from computer is not we'll finally have on TV and I have to fight with my eyes while processing movie in computer. I am thinking to use a TV as the external monitor but not sure how practical it will be, i.e, how would hardware and software support it (my AGP tv card only allows to play back on one monitor at a time), and if this is a common solution out there ?

[adam]

Yes what you see on one will not be what you see on the other. They make actual NTSC monitors but I suppose you could signal out to a regular tv but I think that would drive me nuts personally.

My personal method is to go ahead and expand to 0-255 as soon as possible in my process, and then ensure that it never gets compressed back down to CCIR601 until my actual encode. This gives me proper visible luminence levels at all stages of my editing process, and with most sources you are going to have to go to RGB at some stage anyway so its not like you are losing anything.

The easiest way to tackle this problem is to use as few programs as possible (ie: DV->NLE->Encoder) so there's less things to keep tabs on. Depending on your budget this isn't always practical though.