Whites clipping at level 248

[Guy Burns]

Hi everyone. You've got an amateur here.

I have the rare chance of showing one of my audio-visuals (glorified wilderness slide-show really) at a local cinema on one of their slower afternoons. It's probably the only chance I'll ever have. I'm paying for this once off, so I want to make sure the image is as good as possible.

I was allowed 20 minutes with the tech one morning to run through my calibration disk, just to confirm that what I see at home is what I will see in the cinema. The video came from my Oppo Blu-ray player via a HDMI cable. It looked and sounded pretty good – black levels distinguishable down to level 1, colour bars spot one, audio in perfect sync, B&W images true gray (no colour cast), and correct aspect ratio (circles appeared circular). The problem was with the whites.

The calibration disk is based on the "Disney WOW" disk and it works really well at home. My 27" iMac, my BenQ 7000 (for our home theatre), and my BenQ PU9730 (for large venue shows) all can be calibrated so that I can distinguish white levels all the way to 254. But at the cinema, the whites were clipping at level 248. i.e. 248-254 were pure white.

When I was editing the scanned slides in Photoshop, I took particular care with clouds, making sure the beautiful gradations at the top end were not lost. Maybe losing levels 248-254 won't be noticeable by me or anyone else in the cinema, but I'd like them to be there.

I mentioned the blown whites to the Tech. He could see them on the screen, and he said it was because the Blu-ray format must be different from the DCP format. He was unable to alter any settings on his machine. I think they're Christie.

I think the best way to find out why these whites are being lost is for some kindly cinema tech who is interested in this sort of thing, to actually try my white calibration file in their cinema. I've uploaded the file direct from my Blu-ray calibration disk. It's a 44MB m2ts file called Calibrate Projector White and it can be downloaded here. If a DCP file would be useful, I can upload a DCP version.

The file's providence is this: ripped from the Disney WOW disk, edited in Premiere, exported to Adobe Encore from where a Blu-ray was burnt. Then I uploaded the m2ts file. The whites appear fine at home, but not in the cinema.

The lost whites: is that because of the blu-ray file, or is this particular cinema machine out of whack?

[Leo Enticknap]

Only thing I can think of off the top of my head is color space. A regular Blu-Ray Disc (BD) is restricted to Rec. 709, whereas a DCP uses DCI XYZ/P3, which is a significantly wider color space.

If I'm on the right track, doing this via BD could be the root of the problem. Maybe output wider color space still images from Photoshop, then create a DCP from those directly, using, say, Da Vinci Resolve or DCP-o-Matic?

[Carsten Kurz]

I wouldn't know why it's only happening with the white levels, but have you thought about full vs. limited range HDMI levels? Player and projector need to be configured properly to render the extreme ends of the scale. Also, your slide editing certainly happened on a full swing/sRGB display, while the standard for video/bluray is rec.709 with limited range. Also, there is a known gamma shift issue with Quicktime that can hit you with certain players.

That said - 248 vs 254 is a bit far fetched for an unknown projection system that you haven't tuned to your own liking.

HDMI/DVI inputs in digital cinema are usually the step-child of the projection system. They are often not calibrated/maintained equally well as the DCP and audio chain. Most projectors do not offer the dials and sliders for adjustment that you know from your consumer systems. They are considered calibrated and use fixed color conversion parameters. The calibration tools are usually not exposed to the local cinema tech.

How is that slide show rendered - individual images, a video file? If you want to get the best quality, you should probably create a DCP from your original files. I don't know if it's easy to do, depends on how complex your composition is (text slides, transitions, music, etc.). A DCP will be of higher tonal range than a rec.709/HDMI feed. The trouble of course is, very often you don't get enough time with the cinema system to become familiar with it's properties.

[Mark Gulbrandsen]

Beware of the Tasmanian Devil!

[Marco Giustini]

Limited vs Full range RGB would start clipping the whites at 236 as far as I know.

To be honest, chances are there is a software bug somewhere. When it comes to HDMI, it's a bit of a mess and I would not be surprised if either the projector or the player do something incorrectly.
One thing you can try is to tinker with all the Blu Ray options to see if changing any of them makes any difference. I believe the projector is expecting a "limited" RGB range from the player but you can try setting it to "full range" and see if that changes anything. Then try disabling deep colour, HDMI control etc and see if it makes any difference.
You mentioned level 1, so I suppose your OPPO is set for full range. Try limited range and see if the projector behaves better.

In theory, as long as BOTH player and display are on the same page (Both on limited or both on full) you should get the same results on screen (besides that in Full you have 1-255 and in limited you have 16-235).

You did not say what projector you tested - some have many settings you can try, others will be pretty much "locked".

I've once encountered an HDMI device which was automatically changing the gamma of the picture based on the content (sort of AGC) and it could not be disabled. The manufactured acknowledged it was a bug and said they were not going to fix it as I was the only one reporting it!

[Guy Burns]

Thanks for all the feedback and suggestions. If I've understood them correctly, the projector probably won't allow the local tech to access calibration tools, so it will be up to me to alter the Oppo settings (see below). According to the manual, Contrast will adjust white levels, so I might try winding that down in the theatre.
Oppo BDP-93 Manual-62.jpg

[Marcel Birgelen]

If your BluRay player outputs everything correctly on multiple of your own devices, then it's probably the calibration of the projector at the cinema which is crushing the whites. The same calibration could also apply to DCP playback, so even creating a DCP in DCI-P3 space instead of REC.709 wouldn't be a guarantee to solve your problem.

The only sure thing that would solve your problem is to remap your color space, so that your most extreme white would be 248,248,248 and create a custom BluRay for this particular show. When reducing color resolution, you should check for banding artifacts, which can subsequently be reduced by introducing some dithering. Please notice that too much dithering will result in noticeable digital noise though...

[Guy Burns]

Thanks for the suggestions Marcel. Like you, I think it's the projector. Why would the designers of a $100k+ (?) machine skimp on the HDMI input or the conversion from REC 709? Maybe, but I doubt it.

I had thought of generating a new Blu-ray along the lines you suggested, but the 24 hour encode time out of Premiere, plus testing, says that's a no-go. I'm hoping I can simulate your suggestion of max whites at 248,248,248 by using the Oppo's controls. So, what I'm going to do is this: de-calibrate my W7000 by it's own controls so that it clips at 248, and then see if I can get full range via the Oppo's Brightness and Contrast controls. If I can do that at home, maybe it may work at the theatre.

Fortunately, the Oppo has the ability to store settings in what they call Modes. So I envisage simulating the theatre projector at home, adjusting, saving as a Mode for the theatre projector, and then calling it up when I get there. Mode 1 at home, Mode 2 at the theatre. Wish me luck!

Never thought I'd be using a $2000 machine to simulate a $100K machine.

[Marco Giustini]

If you Oppo has contrast adjustment then go for that (brightness adjustment will adjust the black level). Have you tested another BD player? In my experience the "HDMI" world is quite messy, standards tend to be interpreted slightly differently by each manufacturer. Or, it could be a small bug on the projector - I can't see many people doing what you are doing in a cinema. The vast majority of the content output by that projector is going to be DCI cinema content, hence checking 6 crushed levels on the HDMI input may have been missed when testing the HW/SW.

You still haven't told us what projector that is!

[Guy Burns]

The projector is a Christie CP2220 with a Doremi "Integrated Media Block" for the HDMI inputs. I have no idea what the latter means, but that's how it was described by the Supervisor/Projectionist when I emailed him today.

I have some questions. I could ask the tech, he's very accommodating, but I don't want to bother him too much. And I certainly won't be asking him to attempt calibration. This is simply a learning experience. I want to know how calibration works on this particular projector.
…

Ques 1
Attached is the only page in the CP2220 Manual that seems to describe how to calibrate for colour, and I'm assuming for White levels as well. If my understanding is correct, you ask the projector to display each colour in turn, and measure the colours at the screen with a colour meter. Type in the readings and the projector does the rest.

Would this process calibrate for white?


Ques 2
How much is a colour meter? I might offer to buy one for the cinema if they are prepared to try calibration.
Hang on. Dumb question. All he needs is his iPhone.


Ques 3
Would a projectionist in a small town cinema, quite capable but probably not an enthusiast, normally bother to do this sort of calibration?

Troubleshooting Section, Page 8.3
I noticed the comments below, taken from Page 8.3 of the manual. The writer would have assumed that the projector has been set up and calibrated properly, and that any colour problems would be due to the source:
…

8.13 Inaccurate Display Colors
Adjust the color, tint, color space, and color temperature settings of your input source.

So, it appears to me this projector can be calibrated by the operator (if they have a colour meter), and barring that, Christie suggest adjusting the source.

[Guy Burns]

Re testing a second player – not yet. But I will when I arrange for another run through. They've got an el-cheapo which I intend as a backup.

[Marcel Birgelen]

I doubt they let you mess around with the calibration of the projector. Those calibrations are usually not even done by the projectionists, but by service personnel and they should use specialized gear to do so.

Thanks for the suggestions Marcel. Like you, I think it's the projector. Why would the designers of a $100k+ (?) machine skimp on the HDMI input or the conversion from REC 709? Maybe, but I doubt it.

I don't think it's the hardware/software itself but rather the calibration. Like Carsen indicated, the HDMI inputs are often not part of the calibration routine and as such, they might just not be correctly calibrated. This kind of projector allows for very fine-grained calibration of the color output, so unless there is an underlying hardware defect in this particular machine, the projector should be able to resolve the entire REC 709 color-space just fine. Keep in mind that DCI-P3, for which the machine should be calibrated, is larger than REC 709 and DCPs support 12-bits color per channel, which should allow you to eliminate any kind of banding artifacts.

If you Oppo has contrast adjustment then go for that (brightness adjustment will adjust the black level). Have you tested another BD player? In my experience the "HDMI" world is quite messy, standards tend to be interpreted slightly differently by each manufacturer. Or, it could be a small bug on the projector - I can't see many people doing what you are doing in a cinema. The vast majority of the content output by that projector is going to be DCI cinema content, hence checking 6 crushed levels on the HDMI input may have been missed when testing the HW/SW.

You still haven't told us what projector that is!

The problem with those adjustments is that they're also usually linear, while the phenomenon you want to compensate for isn't. What you'd ideally would want is an option to manipulate the gamma curve of the output of the BR player. Usually though, such adjustments are done on the input side and not the output side of things.

[Carsten Kurz]

The type of color meter you need for this type of projector cost several thousand dollars. I think the best you can do is trying to adjust the player.
The color calibration function of these cinema projectors are password protected. Chances are high that even this cinema tech does not know these passwords.

[Guy Burns]

Thanks for the comments. You've all been very helpful. I've learned that it's very unlikely this projector has been calibrated by the local tech because he wouldn't have access to the colour controls, or if he has, he's not likely to have a suitable colour meter.

That leaves me with: I'll try adjusting the Oppo's output, and if successful in bringing back the whites I'll let you know in a week or two.

Thanks again.

[Marcel Birgelen]

Probably not a feasible option, but if you know someone with an LUT-BOX, this could fix your issue in-stream. Some rental firms that specialize on video-gear may have one which you may be able to rent. For a one-time gig, the thing will probably be too costly.

I've not configured the thing myself, but we've used one of those for a low-budget live-stream event, in order to make the picture look more "cinematic". Usualy, if you've a decent video mixing equipment, you can do your color correction there, but in this case, the setup was rather ghetto, but the result was still pretty decent.