Hi GeoffreyKenner,

Thanks for mentioning OpenEXR; I think perhaps we should move image formats to another thread, though.

The point of this thread really is to compare the GH4's internal downscaling with the external downscaling of a post workflow. The colour shift was just a noticed byproduct of the capture and we're working on identifying where it came from. Since it was in the original source as well, the image format I used to get a frame export from my NLE isn't terribly relevant - it translates what I see in the source material accurately and that's really all that matters here.

@ joethepro quote "I dont know if its the way you saved them, or just the way the GH4 handles it, but there is ZERO advantage here with the 10 bit vs 8 bit files. I put the same extremely ridiculous grades on these images to stretch them to the limit, and they all broke apart and showed banding and compression patterns in the exact same way. What gives? I figured the 10 bit file would have more highlight and shadow data retention, but it doesnt appear to at all from your files"

You can't test the TIFF files because of the conversion process from the original source. This is a common mistake. You must test the original camera or recorder files to see the true difference between 8 bit and 10 bit. Also the difference is not huge with compressed MPEG4 or H264 files due to compression artefacts which become exaggerated with heavy grading. 10 bit files still have compression artefacts like macro-blocking which is very noticeable in shadow areas. This is why using log or 'flat' profiles on cameras which record lossy compressed files is not always an advantage. The amount of grading required to bring the image back to a normal contrast range can reveal the limitations of the recording codec. This is the rationale behind true RAW recording which has minimal compression and avoids the whole encoding issue to sRGB or rec709.

Despite my comments above take a look at the attached blowups from screen grabs in Photoshop. Look below the top yellow color swatch to see the GH4 8 bit internal processing.

@GeoffreyKenner No I am not mistaking what's a format, a color space and a compression. There are quite a few inaccuracies in your post. I don't think you fully understood my post. That may be my fault for not giving an extremely long-winded and technical explanation.

Log is not a "chroma & luma arrangement" it's a gamma curve. Heavily compressed file formats have macro-blocks no matter what kind of gamma curve they have been encoded in. The issue with these formats is that one must do more extreme grading changes to the file to bring them into a linear gamma curve. This can sometimes reveal the limitations of the compression format. I've graded enough XDCAM 50mb/s 10bit 4:2:2 files that totally fall apart in DaVinci Resolve (32bit float processing) when pushed hard (due to under exposure in most cases).

RAW does not suffer the same issues as the grading happens BEFORE the conversion to rec709. BTW the rec709 EBU gamma specification is generally accepted as 2.35, but is not an exact specification and can vary.

You have not seen macro-blocking on your GH2 with Moon T7 because it's an intra-frame format or GOP1. Most standard camera compression formats have a GOP structure of 12-15 frames. However you will see chroma channel artefacts in 4:2:0 & 4:2:2 sampling schemes when the grading is pushed hard, purely due to the lower resolution in the UV channels compared to the Y channel.

@GeoffreyKenner Yeah, we are off topic. I think we agree but are using different terminology. Whichever way you record and downscale, 10 bit capture will always be superior to 8 bit capture. I really don't want to argue with anyone on this issue because I've been converting, editing and grading all kinds of broadcast formats since around 1986, where I've had proper measurement tools and support from video engineering staff. Not blowing my own trumpet, but professional experience teaches and informs in a way that theory and home testing can't. Some people think that downscaling from 4k 8 bit to 1080 10 bit will remove the banding in gradients (like sky) but it does not. It just improves the resolution of the UV chroma channels.

Thanks for having a civil discussion.

@StudioDCCreative thank you, I'll look into it when I got time.

@caveport: sorry I uploaded the clips from the 1080p internal/external comparison, not the original clips from the 4k downscale. My mistake - I was sending these to Atomos too. On the side note this topic has gone to, the difference is, literally, that while the HDMI is being output as 0-255 footage, the Ninja Star is writing that (with the same exact digital values) tagged as 16-235, meaning that what the GH4's internal footage is marking as 0 IRE digital black is coming out of the Ninja as -7.5IRE superblack, and vice-versa for the whites (although that is 9 IRE over, not 7.5). So, a simple levels/gain adjustment and you're back to identical for each of them. Although I haven't tested it yet, I'd assume that if you set the GH4 internally to 16-235 you're at the same exact point without the adjustment necessary, although you'll then have to correct BOTH pieces of footage for superblack and superwhite as the GH4 appears to happily include them in the signal all the same. Pick your poison.

Anyways, if you want the 4k downscaling samples you'll have to wait a bit longer as I just finished a very important shoot and am heading off promptly to handle / process that footage, before catching an intercontinental flight. However, I am perfectly happy to post them as soon as I get one of those round tuits, if you're genuinely interested, as it seems you are!

Any news / more opinions on this?

Thinking of buying Ninja Star for my GH4 but not so sure any more..

Even Ninja Blade (second hand) is almost the same price (and even cheaper if we consider cost of additional CFast media), but for me the most attractive aspect of Ninja Star is how tiny and lightweight it is, as well as long battery life.

If it doesn't significantly improve my footage, then there's not much point though.. But it's kinda useless to compare still images, I'd like to see some real motion and some heavy grading here.

Thanks for the update!

Probably not worth it indeed then. Video Devices PIXEs look great, however I wish there was something as small of a form factor as Ninja Star for the 4K recording.. Don't need the screen so for me that only adds to the price, the bulk and shortens the battery life.

I have done my own tests recently of 4k to HD via HDMI on the GH4 and internal 4k to HD in post. The sharpness difference people are seeing is due to the scaling method the GH4 is using. Basically since the GH4 processor is already strained the camera is doing a nearest neighbor type scaling on the 4k sensor down to HD for the HDMI port. This is the sharpest form of down scaling but it can result in increased aliasing of fine details.

I did a comparison side by side in Blackmagic Design Fusion so I could test out different scaling methods to determine what was going on with the GH4 HDMI down conversion. Sure enough the 4k to HD down scaled in Fusion using nearest neighbor resulted in the same type of ultra sharp details as the HDMI down conversion from the GH4.

At first I considered this a bad thing since nearest neighbor is considered the lowest form of image scaling in the image processing world. So why does it work so well on the GH4 then? Because it is reducing the width and height by exactly 1/2 for each (1/4 resolution total). Nearest neighbor can do a decent job when down converting at that exact ratio. Is it perfect? Nope and you will get increased aliasing.

Nearest Neighbor on the GH4 HDMI scaling basically looks at every group of 2x2 pixels and tosses out three of them and keeps one. No averaging of those 4 pixels or other surrounding takes place. So it basically just tosses out pixels to make the image 1/4 the size. Extreme fine detail can be lost this way in much the same way that interlacing can lose very fine detail because a 1 pixel wide item like a hair could be there one frame and gone the next frame if it moves over by 1 pixel. With that said it is pretty rare to have details that only fall on 1 pixel but it can happen. In practice the HDMI scaling is actually pretty good. Just be aware that it can have added artifacts. I have found shooting with the sharpness at -5 and using adapted lenses can help because there really is no longer such a thing as a 1 pixel wide item in the 4k. Plus the de-bayering means it is virtually impossible to have a pixel value that isn't made up of averaging surrounding pixels to a certain degree.

FCPX and other applications tend to a form of bi-cubic or bi-linear scaling which is why those down conversions are slightly softer looking. Each pixel is an average of surrounding pixels which tends to slightly soften the image but it results in a very clean alias free down conversion in the process.

Down conversion tends to have a triangle of artifacts with one side of that triangle must always be the result. You have to choose one and not a single method eliminates all three. These artifacts are Aliasing, blurring or ringing. Pick your poison.

FCPX and other applications tend to a form of bi-cubic or bi-linear scaling which is why those down conversions are slightly softer looking. Each pixel is an average of surrounding pixels which tends to slightly soften the image but it results in a very clean alias free down conversion in the process

Normal practice is to always sharpen slightly downsized photos and footage.