Mr. Steve,
Take a look at:
ARRI Group: LOG C AND REC 709 VIDEO
| |
| | | | ARRI Group: LOG C AND REC 709 VIDEO The ARRI Group is the world's largest manufacturer and distributor of motion picture cameras, digital intermediate systems and lighting equipment. With ... | |
| | Preview by Yahoo |
| |
When shooting in LogC the data never goes to 0 or up to 1024. That's why it looks milky. I would expect the Rec 709 video tap on the camera to go basically from 0 to 1024 when properly exposed but that's not the actual LogC values that end up coming to me. When something is under or over exposed which I assume mean Scene 18% gray is below or above the 400 data point does that asymmetrically whack out the exposure of the image.
Traditionally with Rec 709 images if it's under exposed I increase the gain and vice versa for over exposed. Now with LogC images I have to pull down the setup and raise the gain and generally bring up the the setup gain and bring up the gamma lower end. Arri's standard LUT in Resolve seems to work well but other cameras don't seem to be so consistent with their LUT performance. I've been told that the varying manufacturers Log color math isn't implemented as consistently as Arri's or Sony's which may explain why I can't find that dream one size fits all LUT for a given camera like Canon's C300.
My math skills are very rusty when it comes to exponential/ log math but it would seem to me in the case of Arri that the unpacking/color transform of Arri footage with their LUT would get messed up if the Scene 18% grey hasn't been exposed at 400 data value. Is there an L2 mathematician that can whack me over the head with a ruler, preferably graduated in F stops or is it EI these days, and knock some Log sense into me?
---In Avid-L2@yahoogroups.com, <Steve@...> wrote :
Well, 18% gray is a standard on color charts. So I'd assume that they're saying that the LUT is developed so that a properly exposed 18% gray exposure (on a chip or color chart) comes out to 400 in 10bit. Properly exposed would mean white was 1024 and black was as close to 0 as you could get it. So basically they're defining part of the gamma curve for you - the part that's at 18%.
Steve
Avid 8.3.1 and perhaps earlier version have an option to auto apply a color transform based on ama linking but as was pointed out earlier this week on the L2 it makes a lot of mistakes. This combined with my recent LUT related trials and tribulations that made it seem like it's virtually impossible to get decent results from a once size fits all LUT for a given manufacturers Log file. Arri seems good as are some of the Sony's but I couldn't seem to get a basic LUT for my Canon C300 footage and have adopted what others here have suggested and just color corrected out the LUT.
I really don't see why the Log transforms can't just be untransformed out to 709 space but perhaps there is some aspect of the logarithmic math or the transform formula that I don't understand. Arri site says that their LogC format anchors Scene 18% Grey to a data value of 400 in 10bit math. As you adjust the exposure you get more or less stops above or below. I don't know what determines Scene 18% grey. Is that a constant value like a spectrometer probe on a monitor would measure and exact value of nits or is it something that takes the darkest to the lightest elements and from that establishes Scene 18% grey. I'd really like to understand this better.
My experience seems to suggest that with all the varying log transforms from the different manufactures that each one behaves differently and that over and under exposures gets whacky pretty quick. This is just my observation dealing with material I get in post. I hope to get with a knowledgeable DIT /Cinematographer to get a better handle on how to deal properly with the material.
---In
Avid-L2@yahoogroups.com, <mikeparsons.tv@...> wrote :
What makes more sense is for the world to get onto the Aces band wagon and adopt transforms for output.
We do everything full range now and output appropriately, seems we could add transforms to and from outboard processing.
What is distressing is that it's still down to manual labour choosing settings when every camera records all the needed info in the file header.
Id like to see auto input transforms in every system and a selection at export time - rec709, rgb or dci.
Mike
"Color correcting means correcting things like these."
Yes but if I have a perfectly legal color corrected shot and I apply the tiffen day for night I would like for it to come back with the effect in 709 space not RGB so that I then have to add a color correction effect to bring it back into legal. Now if the BCC legal check box is just clipping that's not great but usually I see it chopping the glows etc... that are whacky anyways but it doesn't take the base shot and alter the overall color space.
As you say the proper way would be to scale the effect output and I don't think that option is asking too much of an avx plugin. I do see that when the Tiffen effects open in their own interface that there is a reason for them to display as RGB because more than likely they are being seen in a computer screen that is working in RGB space. Doesn't it make sense for those RGB levels to be scaled to 709 space if that's what I'm working on in my Avid timeline?
---In
Avid-L2@yahoogroups.com, <Job_L2@...> wrote :
That's what happens if you manipulate color levels, which is what these filters do.
I think in the past, they were clipped/crushed to legal, which is not what you would normally want. Color correcting means correcting things like these.
I notice that the effects come back into Avid at illegal RGB levels
