video output on blackmagic card vs graphic card

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cris cros

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video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 8:34 am

hi,

what is the advantage of outputting the video signal from resolve on a blackmagic card (decklink,...) rather than on the graphic card of the computer ?

thanks

chris
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Hendrik Proosa

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 8:42 am

IO card bypasses some potential color messups from OS side and outputs standardized video signal.

How much OS intervenes with color is difficult topic, because depends a lot on OS and application itself. As far as I know, rendering opengl surfaces can bypass all color management from OS and thus can be valid way to produce accurate color through gpu itself. But implementations can vary, so most people prefer IO. There is a lot of FUD surrounding gpu output, and a lot of it is "I heard it form the internet" kind of fuss with little actual technical backing.
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Steve Fishwick

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 9:07 am

The mathematical calculations for accurate calibration rise exponentially when you use the computer's graphic card to grade with: there's the pc/mac colour profiles, then the graphic card's driver settings, and finally the monitor calibration, and in some instances you cannot be sure you are getting true 10 bit either. With a video i/o device the output signal is pure, 10/12bit and unadulterated, to the correct standard of your chosen format, leaving only the monitor to be calibrated. Since all those other pc variables can easily be messed with and changed, over time, using an US/Decklink, in a professional grading environment, avoids them completely and I do not know of any such facility that would use the graphic card output. I see many users here agonise about various monitors and cards for grading, yet a simple, cheap and effective Decklink or US device, seems to be outside their priorities.
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Hendrik Proosa

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 9:14 am

Steve Fishwick wrote:I do not know of any such facility that would use the graphic card output.

Compositors outnumber colorists at least 100:1 ratio and they all use gpu output to monitor. It isn't final color ofcourse but so isn't grading on rec709 monitor for cinema distribution to bring a blunt analogue. So do all graphics designers, and pretty much anyone else not doing video work.

There is just one metric for measuring whether some IO path is valid or not and it is the calibration results of monitor. If that is good, it just becomes a trust issue, either you don't trust your eyes, your calibration, your os not messing it up or calibration device itself. Bypassing those trust issues is easier with dedicated io device.
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Steve Fishwick

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 9:25 am

Hendrik Proosa wrote:Compositors outnumber colorists at least 100:1 ratio and they all use gpu output to monitor. It isn't final color ofcourse but so isn't grading on rec709 monitor for cinema distribution to bring a blunt analogue. So do all graphics designers, and pretty much anyone else not doing video work.


I know of many (if not most here) VFX/compositor facility houses who use proper video i/o devices, so to say they all don't, is not quite true. Nonetheless I take your point but it all ends up down the funnel that is finishing/grading and there is an absolute professional requirement for a calibrated true video output using such hardware. I imagine most DVR users here do their own finishing as well as compositing, so as I say at the price of the entry BMD video i/o hardware, which is a major step up from the graphics card output it's, for me, a no brainer.
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Hendrik Proosa

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 10:20 am

Can’t argue with the no-brainer part.

I’m simply not convinced with most of the random fuss surrounding the gpu output. I’m not an opengl expert but os randomly messing with render surfaces, especially in fullscreen mode doesn’t ring my bell.
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Jim Simon

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 1:20 pm

This is oversimplified, but it's a good place to start.

Decklink=Accurate
GPU=Not Accurate
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Hendrik Proosa

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 4:23 pm

You should first define ”accurate”, because without it I could say with great confidence that
Decklink=okay
Gpu=better
And it is up to you to think what my measure for bad-good-better actually is.
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carlomacchiavello

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Re: video output on blackmagic card vs graphic card

PostThu Jun 17, 2021 7:09 pm

If you work with video broadcast mean interlaced video, most of gpu output progressive video or mess up a interlaced video in a progressive signal.

If you work on pal land, most if gpu / Os do a lots of signal messing about 25p/50i signal, often with double convertion from 25 to 30 (digital) to 25 output, and more.
I lost the number of time that I show how many problems are not in animation or in video but only a playback fault.

Not all people need a deck link or a video card to output their video, but… that not mean not all people can avoid to do that.

If you work in a progressive 30p world, you not need color accurance or you not need a broadcast signal test, and you gpu is enough powerful to accelerate video reproduction and send out a clean signal, you not need a video card to output signal.


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