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Re: How are import resolution and export resolution related.

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Joe Marler
Re: How are import resolution and export resolution related.
on Mar 20, 2018 at 11:52:33 pm

[vidda roshinns] "if i shoot in one codec render in a different codec and export to another seperate codec. If I shoot in MP4, render in pro res 422 and export back to pro res is the actual quality of the render the same as if i had shot in pro res to begin with?"

FCPX generally renders the timeline using ProRes 422, although you can modify the project to use several other ProRes variants. So no matter what the acquisition codec or the export codec, the render codec is usually ProRes.

MP4 is a container format that frequently contains 8-bit 4:2:0 H264 material. ProRes is a codec family which supports at least 10 bits per color channel and various chroma sampling resolutions.

In many cases you don't decide to shoot in MP4 or ProRes, your camera supports one or the other. Sometimes a camera that records MP4 internally can record ProRes to an external recorder. Some higher-end cameras can record either ProRes or MP4 internally. Some cameras which don't record ProRes can nonetheless optionally record in higher bitrates, bit depths, chroma sampling, or can use a lower-compression or "intra frame" codec. Using these is a tradeoff of data management, workflow complications and evaluation of whether it makes a perceptible difference to the final viewer.

The recording bit depth and chroma sampling is determined by the camera, not by the codec. IOW adding a ProRes HDMI recorder to a low-end camera will not magically provide true 12-bit 4:4:4 4k recording. The camera's sensor, imaging pipeline and output logic determines what it can export.

So in most cases a lower-to-midrange camera will record MP4 and there's nothing you can do about it. In the edit phase FCPX renders in ProRes and your only non-ProRes option is uncompressed 10-bit 4:2:2; IMO there's no reason to use that. So up the the point of export, there is often no codec decision to make. You normally shoot at the best resolution, codec and bit rate your camera produces, but in low-to-midrange cameras this is often only a choice of bitrate.

Your export codec will be determined by your delivery requirements. If you are uploading to Youtube or Vimeo, unless your program length is very short it may be impractical to export to ProRes. Using an H264 output codec will save a lot of space and upload time, yet produce pretty good quality if the encoding bit rate is high enough. Exporting to HEVC/H265 is also becoming and option, which can cut upload size roughly in half while retaining the same quality. However it requires hardware acceleration (and software which harnesses that) to provide decent encoding performance.

If you are handing off material to a downstream editor or color finisher, you'd generally export in ProRes not a "long GOP" codec like H264. This isn't because ProRes makes it better but it avoids possible quality loss of re-encoding to a compressed codec. Also I think the most common long GOP versions of H264 only support 8 bit 4:2:0. There are intra-frame versions of H264 which support higher bit depths and chroma samplings but they are less common. To achieve the quality of ProRes requires an H264 variant which is encoded at nearly the same bit rate, so "turnovers" are often done using ProRes.

In general the image quality is "baked in" at acquisition time and transcoding to another codec will not improve that. Thus there's no reason to transcode to ProRes optimized media before editing in FCPX -- from an image quality standpoint. From a performance standpoint it could be beneficial.

However It is theoretically possible to transcode 4k 8-bit 4:2:0 H264 to true 1080p 10-bit 4:4:4 on output. Achieving this requires (1) Your rendering codec must be ProRes 4:4:4, (2) You don't crop the image during editing, and (2) You export to ProRes 4:4:4. That is one case where choice of rendering and output codec could make a positive difference. Whether that difference is visible to the final viewer on typical playback devices after distribution and upload encoding is another matter. See white paper by Barry Green "The Benefits of Shooting in 4k": ftp://ftp.panasonic.com/provideo/agdvx200pj/4kbenefits_techbrief.pdf


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