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4k dci vs normal 4k


Yannick Willox
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Hello,

i have a not so trivial question. I have been shooting with my bmpcc4k cams in 4k dci, because intuitively, it has the best quality. However, I always render at 1080p, delivery is full hd only.

I actually prefer the 4k dci aspect ratio, but does this method give the best quality ?

downscaling 4k to 1080p is maybe easier, as 4k dci to full hd while keeping the aspect ratio, means you actually do not have a 2:1 line ratio anymore. More complex math involved ?

Even if quality is not an issue, would this possibly cause a performance hit on my cpu ? I imagine scaling 4k to full hd is easier.

To summarize: I shoot in 4k dci, because of the aspect ratio and also because my lenses actually get a little wider. But would it be better practice to shoot in normal 4k, to downscale to 1080p ? Performance and or quality wise ?

maybe I am overthinking this.

thanks,

 

yannick

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The quality difference between 4kUHD and 4kDCI will be exactly 0, and performance difference will be close to the same. The only difference is DCI has extra pixels on the side. Sharpness, DR, rolloff, aliasing, moire etc. is all the same.

Downscaling 4k DCI (17:9) to 1920x1080 (16:9) will either add black bars at the top and bottom to preserve your aspect ratio, or crop off the sides to force 16:9.

The question is, do you need to deliver in 16:9? If not, you could export at 2kDCI (2048x1080) which will preserve your 17:9 aspect ratio, without cropping anything or adding black bars to the file.

If you need to deliver in 16:9, I'd suggest also editing in 16:9, so you can keep an eye on what's being cropped. You could still shoot DCI, which would allow you to reframe slightly if you want more or less space on the left or right side of your frame. But if you edit in 17:9 and let your NLE crop the sides during the export, you'll have estimate what's going to be cropped for the 16:9 ratio.

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1 hour ago, Yannick Willox said:

To summarize: I shoot in 4k dci, because of the aspect ratio and also because my lenses actually get a little wider. But would it be better practice to shoot in normal 4k, to downscale to 1080p ? Performance and or quality wise ?

If you (and/or your client) like the aspect ratio and like the fact that you are using a wider portion of the image circle of your lenses, then, to me, those are the most important considerations.

 

So, you are probably best shooting at 4096x2160 (DCI 4K) and down-converting cleanly to 2048x1080 (DCI 2K) or less cleanly to 1920x1013.  Any extra rendering time for the odd height pixel in the "less clean" resolution would likely be minimal, but it would probably be a good idea to test it, just to make sure.

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Good suggestions, thanks.

I'd probably need to find time to test. I never thought of putting everything on a 2K DCI timeline ... I should test the performance hit as well, sometimes we have a 8-10 cam setup, so even a small difference adds up when doing the multicam edit.

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23 hours ago, Yannick Willox said:

To summarize: I shoot in 4k dci, because of the aspect ratio and also because my lenses actually get a little wider.

Only because in most implementations 4K/1080p 16:9 aren't using the full horizontal width of the sensor. So your 24mm lens in DCI is actually closer to true 24mm FOV than when shooting 16:9 modes...unless your Sony.

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I remember Andrew and a german site said DCI 4K was slighlty softer than UHD on the NX1 (and GH5) because the size of the sensor used on DCI 4K was a bit smaller than UHD. However, the rolling shutter must be a tiny bit better on the DCI 4K, again because of the size of the sensor used with DCI 4K.

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1 hour ago, seanzzxx said:

@Beritar the OP stated they're using a Pocket 4k, which does have a native DCI 4k sensor, so your remark is not really relevant to them (just to avoid any confusion ahead of time).

Yes, my mistake. Thank you for pointing that out, I never used the Pocket 4K. I thought it was the same process.

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On 3/20/2021 at 3:18 AM, Yannick Willox said:

Good suggestions, thanks.

I'd probably need to find time to test. I never thought of putting everything on a 2K DCI timeline ... I should test the performance hit as well, sometimes we have a 8-10 cam setup, so even a small difference adds up when doing the multicam edit.

Should be less of a performance hit if any as it is a direct downsample ... and I shoot everything that I can in 4K DCI and render 75% as 2K DCI ... love the look.

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