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View Full Version : AVC rears it's ugly head.



cheezweezl
06-21-2011, 12:14 AM
Let me start by saying that I have been thoroughly impressed by how well the avc codec holds up during basic grading. i have lifted mids several stops with useable results. This isn't meant to be a slam to the codec or to say that it's no good, because in general, I like it. In fact, I was almost convinced that I didn't need to buy an external recorder, until tonight.

I finally got out from under a mountain of work and had a chance to throw some fs100 footage on the davinci. The footage was transcoded to prores hq prior to flying in. Where the codec seems to fail is when doing more complicated grading. More specifically, when pulling a key. I did a simple luma key and then further processed by isolating just the areas with less saturation. This is where I saw the big ugly blocks show up. Now, I could smooth this out a bit by softening the key and adding blur to it, but getting rid of those big giant blocks may be impossible. This would really limit how far I could take the correction before you would start to see those blocks.

So, external recorder it is.

Below is a screenshot of the key. Mind you that this key could be improved slightly with some tweaking. I just left it ugly to demonstrate the starting point and the blocks.


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full size pic (https://files.me.com/fisherfilmworks/n34t4f)

PabloOzzy
06-21-2011, 06:00 AM
I cant really tell anything from the screen grab but I'd expect there to be limits in such a high compressed codec. So are you going to buy the Atmos Ninja? $995 on B&H looks great - few bad reviews on B&H but those are questionable - one clown gave it one star because it didnt work with his 7D...whats that saying about a fool and his money? ;-)

The only issue I can see with the Ninja though is at this stage it doesn't accept a 1080/60p signal - only records 60i - kind of a deal breaker for me unless they can change that in a firmware upgrade.

EDIT

Forgot to ask - when it comes to pushing the codec in a harsh grade - would you say its better or worse than that of the 5D / 7D ?

Duke M.
06-21-2011, 06:22 AM
I cant really tell anything from the screen grab

Look at the macro blocking on the ground and in the trees. Its a little more understandable in the busy leaf areas, but the ground shouldn't look like that.

Richard Allen Crook
06-21-2011, 07:18 AM
Isn't this more a casualty of the 8bit codec than AVCHD? The 256 steps that 8bit provides is a severe limitation...just as it is with any other 8bit codec. Does this issue pop up from Canon DSLR h264 footage too? Just curious. Does an 8bit 4:4:4 or 4:2:2 key better than 8bit 4:2:0? How does it compare to P2 footage from an hvx200 which is 8bit as well but considered a great codec?

I'm not asking as a challenge to your findings, just truly interested in your thought of the AVCHD codec compared to others. I've been grading dslr footage for so long, when I did some grading tests with AVCHD from the fs100 it seemed that I could push it further than the crappy h264 codec.

cheezweezl
06-21-2011, 11:48 AM
Isn't this more a casualty of the 8bit codec than AVCHD? The 256 steps that 8bit provides is a severe limitation...just as it is with any other 8bit codec. Does this issue pop up from Canon DSLR h264 footage too? Just curious. Does an 8bit 4:4:4 or 4:2:2 key better than 8bit 4:2:0? How does it compare to P2 footage from an hvx200 which is 8bit as well but considered a great codec?

I'm not asking as a challenge to your findings, just truly interested in your thought of the AVCHD codec compared to others. I've been grading dslr footage for so long, when I did some grading tests with AVCHD from the fs100 it seemed that I could push it further than the crappy h264 codec.

I don't think so. Here's why. My ex1 is 8bit 4:2:0 as well. When it's footage is converted to prores, I can do grades like this without seeing the giant blocks. However, I have seen this before with h.264 footage that was converted. Avc and h.264 are essentially the same from what I understand. 8 bit footage may mean visible banding at times but the macro blocks are from the compression for sure.

As far as this compared to canon dslr footage, unfortunately I have to say that the canon does better in this type of situation.

Richard Allen Crook
06-21-2011, 12:04 PM
I thought they were essentially the same as well, but I guess according to this thread AVCHD is far more superior: http://www.dvxuser.com/V6/showthread.php?208882-HMC150-AVCHD-VS-Canon-SLR-MP4

Suprising to hear that Canon's H264 holds up to grading better than AVCHD in your tests and not mine. But I use Premiere which is native for both codecs so I don't convert before grading...maybe that's the reason? Converting to ProRes is cleaner from Canon H264 vs AVCHD? Hmmm...

Anyway thanks for the findings...damn I hope they give us the paid option to upgrade the FS100 to a 10bit codec. I doubt they'll undercut their F3...but we can always hope right?

cheezweezl
06-21-2011, 02:01 PM
i should have been clearer. the fs100 footy holds up better for general cc, such as lifting dark areas. it holds up well. i suspect the cleaner sensor and lower overall noise are to thank for this.

it's only when i am pulling a key like this that the canon footage is better. it has blocking but they seem to be about half the size and not as apparent. i suspect that recording hdmi direct to prores will be just the ticket.

i TOTALLY agree with you on 10bit. i would pay for it in a second...

also, i don't think sony is conspiring to cripple the fs100 to protect the f3. they already gave us 4:4:4: out of the box, 1080/60p internally, and overcranking to 60 frames at 1080. not to mention 30db gain. the f3 can't do any of these things out of the box. on the contrary, while your paid upgrade to 10 bit sounds awesome, i doubt it will ever happen...

Everts
06-21-2011, 02:04 PM
Look at the macro blocking on the ground and in the trees. Its a little more understandable in the busy leaf areas, but the ground shouldn't look like that.


I dont think its a good idea to key a forest :)
No matter what codec....it will always be ugly.

Rick Burnett
06-21-2011, 02:32 PM
I've not done any green screen work with my FS100 yet, but I have been doing working with 5D footage for a week. I find the macroblocking on the 5D footage to be horrible and VERY hard to keep from getting edges that chatter. It's funny because certain parts of the scene key just fine, but other parts are horrible.

I think it's more H.264 related from all that I have seen. While 4:2:0 does have a lot more reduction in color resolution, as long as it is consistent, it's not as hard to pull. With the macroblocks, the differences between the blocks with a keyer that does despilling, this is where the problems come in.

I think keying from footage off an external recorder in the future is going to be a must for me as well. (And this includes the AF100 too as I have keyed that and saw the same thing)

cheezweezl
06-21-2011, 03:06 PM
yeah rick. i had the same issue with greenscreen and the 7d. it worked marginally for some stuff, like if it was a simple talking head with plain clothing. but the minute a striped shirt or a lot of movement was added, the key took the hit. i'm not even going to attempt any paying greenscreen gigs with this camera without an external recording solution.