In publish manufacturing, artists alternate digital information between purposes and departments all the time.
Renders, transcodes, conforms, deliverables…there’s a relentless move of information processing and reprocessing. The bigger the workforce, the extra complicated the info move, with totally different variations of the identical software program, totally different working techniques, totally different file storage, and totally different workflows. So it’s actually necessary to have a transparent understanding of the alternatives being made throughout these essential file exchanges.
My purpose with this text is to supply an understanding of the widespread pitfalls that you simply’re prone to expertise, and to supply a guidelines you should use to troubleshoot widespread points. That manner, you’ll be able to deal with being inventive and spend much less time fixing or researching issues that went fallacious.
Alpha channels may be notably difficult relying on which software they’re coming from. So let’s contemplate what an alpha channel is earlier than digging into the potential points they’ll trigger.
Any time a picture or video asset file—fairly than a undertaking file—has an embedded alpha channel, it sometimes accommodates three RGB channels, usually referred to as a Fill and a black and white alpha channel usually referred to as a Matte.
How this matte interacts with the fill is the place a few of the confusion begins, and it’s normally because of the two key strategies of rendering embedded alpha channels; straight and premultiplied.
| Picture codecs with Alpha | Video codecs with Alpha |
| DPX | Apple Animation |
| GIF | Apple ProRes 4444 |
| JPG2000 | AVID Meridien |
| Maya IFF | AVID DNxHD |
| OpenEXR | AVID DNxHR |
| PNG | AVIF |
| RLA | Cineform |
| RPF | Cineon |
| SGI | HEVC / H.265 |
| TGA | |
| TIFF | |
| WebP |
Straight alpha channel
A straight alpha channel signifies that solely the alpha channel (matte) accommodates transparency information and any RGB (fill) components are opaque. With a straight alpha channel, the fill can look unusual by itself, because it’s not being restricted by the alpha channel (as you’ll be able to see within the second instance beneath).
One of many methods to inform what sort of alpha channel is embedded in a file is to show off the alpha channel. If the file seems pixelated or has odd colours on the edges, then it most likely has an alpha channel that obscures these unusual edges or filters them by means of transparency.
Premultiplied alpha channel
A premultiplied alpha channel signifies that the RGB channels include transparency data as effectively as alpha channel. The RGB values are mixed with a specified matte shade (normally black) that defines the quantity of transparency. This matte shade is subsequently faraway from the RGB values by the software program that you simply open the file with.
In less complicated phrases, the transparency worth for a pixel is saved as a price between 0 and 1 (with 0 being fully clear and 1 being fully opaque). For premultiplied transparency, the software program merely multiplies the RGB values by the alpha worth to calculate the end result—therefore the time period premultiply. So an alpha worth of 1 leads to a totally opaque pixel, a price of 0 leads to a very clear pixel, and a price of 0.5 would offer a 50 % clear pixel.
With a premultiplied alpha channel, because the RGB channels include transparency data, you may not see a lot of a change for those who flip the alpha channel on and off.
Recordsdata with a premultiplied alpha channel will most likely look near how they need to, in comparison with a straight alpha channel which can look fallacious exterior of a correct compositing atmosphere.
In layer-based compositing packages like After Results and Photoshop, information with alpha channels are handled as one single unit in a single layer. The embedded alpha channel can work together with layers above or beneath it.
In packages like this, alpha channels are dealt with within the background with out a lot consumer management. This may work effectively for artists who’re extra comfy with textual content or graphic animations as there isn’t as a lot want for alpha administration or interpretation for these components. So it’s so simple as choosing an alpha channel interpretation once you render.
Nonetheless, working with alpha channels and reside motion footage or CGI is a unique story. Functions like Flame, Fusion and Nuke are all node-based compositing techniques. Alpha channels are dealt with a lot in a different way in some of these packages than layer-based compositing techniques.
The most important distinction between node-based and layer-based compositing techniques is that fills and mattes are handled like separate parts, not one self-contained file or layer.
For instance, in After Results a masks that cuts out part of a shot will mechanically have transparency behind it within the compositing atmosphere. That occurs in a single layer. In distinction, in Flame a masks is created in a node. This masks node must be piped right into a merge or comp node with a foreground and background ingredient to composite one thing.
It’s a two-stage course of. When rendering or exporting, the consumer must explicitly pipe the right masks into an alpha channel output to appropriately interpret it.
To confuse issues additional, alpha channels may be rendered in many alternative methods in node-based compositing techniques. Because the fill and matte are handled individually, it’s doable to render out a fill that accommodates the entire shot and an alpha channel that solely cuts out a bit of it. On this case, the render could be used as a separate matte inside an software on an underlying piece of footage.
Another choice is that every particular person RGB channel might be rendered as its personal black and white matte layer. For sure pictures involving rotoscoping, every of these RGB channels might be used as separate mattes that might be mixed, subtracted or manipulated in additional refined methods than a easy single channel alpha channel.
Much more difficult are embedded channels inside an OpenEXR file which may include many layers. Normally OpenEXR information are used with CG work in order that the compositor has extra management over the CG components throughout the scene. Channels inside an OpenEXR may be piped to inputs on merge nodes for manipulating components individually.
So with node-based compositors, the consumer has extra management over the matte layers. However the matte layers and embedding course of must be managed extra explicitly than a layer-based compositor. And like several guide course of, it opens the door to consumer error, inflicting mattes to render incorrectly.
Comparatively, layer-based compositors have much less express management, however this comes with an ease of use and automation that may make them extra simple. Neither manner is fallacious. It’s simply necessary to grasp the distinctions.
So when rendering or deciphering alpha channels, it’s crucial that whomever renders or interprets the alpha channel can reply these questions…
- Is the alpha straight or premultiplied?
- If a node-based system was used to render the alpha channel, what does it include?
If the solutions aren’t clear, or the data isn’t handed alongside to others, you’ll find yourself with misinterpreted alpha channels. For instance, if a file is rendered premultiplied and interpreted in one other program as straight, there’s prone to be a darkish halo across the edges of the alpha channel. Or if a file from a node-based compositor containing RGB channels as mattes is interpreted as a standard RGB fill and matte in a layer-based program, it’ll be unusable.
After Results has choices to mechanically detect which kind of alpha channel is embedded in a file, however the communication is essential. If each the particular person rendering and the particular person importing are on the identical web page, that’s the best technique to keep away from this pitfall.
It may be tough to catch incorrect alpha interpretation in Premiere or DaVinci Resolve or Avid by eye. So when importing a file with an alpha channel, it’s necessary that the applying explicitly is aware of what sort of alpha channel is embedded within the file. With out that piece of knowledge, it’s anybody’s guess.
Exchanging timelines between purposes requires the usage of text-based lists that seek advice from file metadata, like XMLs, EDLs, AAFs or OMFs.
One of many largest pitfalls when utilizing these lists is to imagine that the results, media, file interpretation and layer construction will mechanically translate correctly between packages. That is not often the case even with easy tasks.
Earlier than discussing how one can keep away from this assumption, it’s necessary to grasp what data is contained inside these lists and the way that data is formatted and skim by purposes. As well as, file metadata itself is essential in the case of exchanging lists—information have to have correct metadata in order that packages could make sense of how one can use them.
“One of many largest pitfalls when utilizing these lists is to imagine that the results, media, file interpretation and layer construction will mechanically translate correctly between packages.”
These lists work along side varied file sorts. Each file accommodates some sort of metadata about itself embedded from the {hardware} that captured it, or the software program that created it.
File alternate lists like EDLs, XMLs, and AAFs, seek advice from this metadata, which may be particulars about timecode, file names, reel names, and rather more. With out correct metadata, information may be tough to translate to totally different packages. For instance, if the supply software doesn’t include obligatory data like embedded timecode or distinctive file names, exporting an XML or EDL to rebuild in one other software is usually a very time consuming guide course of, if it could possibly even be accomplished in any respect.
Relying on the supply program that exports the listing and this system that imports it, there are sure to be some pitfalls in rebuilding the timeline. It’s necessary to grasp the restrictions and strengths of those alternate codecs and in addition perceive the quirks of explicit packages. Not solely that, however the media being referred to by these lists will play a component in rebuilding the timeline within the vacation spot software.
EDLs
The EDL (Edit Resolution Listing) is among the many oldest and commonest file alternate codecs, and it’s a easy customary that the majority packages perceive. EDLs solely include one layer and don’t include any resizing data. Whereas these are vital limitations, it’s additionally simpler to troubleshoot an EDL, because the formatting may be very primary and clear.
EDLs are nonetheless utilized in many elements of the business partly as a result of they’re the least error susceptible, but additionally as a result of they’re simply editable. An EDL may be opened and modified in a easy textual content editor, so customized scripts may be created to switch parameters.
Quite a lot of the performance (and limitations) of EDLs come from their origin in previous tape-based techniques, the place information contained reel or tape names that corresponded to a bodily piece of media alongside timecode. With tape, every reel title contained a sequential timecode so the reel title was used to seek advice from the correct tape, whereas the timecode was used to seek out which a part of the tape contained the best video. With digital information, it’s totally different. Reel names are sometimes distinctive to every supply file so every digicam take is basically the digital tape.
EDLs use embedded reel names to appropriately determine which digicam file to make use of when rebuilding in one other software. So it’s necessary for information to include embedded reel names when utilizing EDLs to alternate between purposes.
XMLs and AAFs
XML (eXtensible Markup Language) and AAF (Superior Authoring Format) information are additionally text-based lists, however they include extra data than EDLs. To start with, XMLs and AAFs, a number of layers may be exported as an alternative of only one. XMLs and AAFs additionally include details about resizing which EDLs don’t. They’re additionally extra versatile when it comes to reel names and file names for rebuilding timelines.
Whereas XMLs and AAFs have extra performance, there’s a widespread false impression that it’s simpler to prep timelines with XMLs or AAFs than EDLs. Whereas this may be considerably true because of the truth that XMLs and AAFs include resizing data and a number of layers, it’s simply as necessary to correctly prep sequences when exporting XMLs and AAFs.
Recordsdata with out timecode, graphics, or audio layers received’t conform correctly with simply an XML or AAF. Whereas the vacation spot software may need higher instruments like Flame to evolve, it’s nonetheless necessary to speak with whoever is rebuilding the timeline to verify all the pieces rebuilds correctly on the opposite finish.
Prepping timelines and information
Earlier than exporting an EDL, XML or AAF, it’s necessary to be sure that the information within the timeline have the correct metadata for rebuilding in one other software. A very powerful metadata embedded in supply information are:
- Timecode
- Reel names / Tape ID
- File Names
If file names have been altering throughout editorial, reel names or tape IDs develop into much more necessary for conforming and rebuilding. Timecode can also be important for correctly rebuilding timelines in different apps. A straightforward technique to examine that information match is to import proxies and the digicam authentic information and evaluate the metadata. If all matches, then the XML/AAF/EDL shouldn’t have a problem discovering the best items of the best information.
Past the fundamental metadata, it’s additionally necessary to grasp:
- How nests or compound clips will translate between packages.
- If clips are multicam and must be flattened again to supply information earlier than exporting a listing.
- If information include CDL (shade resolution listing) data for VFX or shade plate pulls.
- If there are markers or different data that’s crucial to embed.
Normally a very good rule of thumb is to make sure that the prepped timeline accommodates information from the unique media information with primary results like timewarps and resizes however not nests, multicam clips or comps earlier than exporting.
It will make sure that the exported listing accommodates metadata that really matches the information, not components created throughout the supply software that received’t translate to a different software. Checking exported lists after exporting additionally helps make sure that nothing is missed or misplaced in translation when despatched off to a different artist.
Compressed file high quality is one other necessary pitfall to grasp when working in publish manufacturing.
Sure codecs and file containers can compress media to a a lot larger diploma like h.264 or h.265 in comparison with extra constrained formats like ProRes or DNxHD. When choosing render settings, it’s essential to grasp the intent of the render—whether or not that’s supply, hand off to a different artist, shopper approval, VFX plates, and so forth. Particularly once you’re dealing with codecs like h.264 or h.264, the place chroma subsampling, bit charges, and encoding profiles outline the standard and measurement of the information.
Normally, compressed codecs like these are greatest used to effectively document media to cheap {hardware} or effectively ship information for distribution. Compressed codecs aren’t as helpful for manipulating photographs in publish or exchanging information between artists. They’re called “lossy” for a reason.
When exchanging information between artists, it’s necessary to keep up supply bit-depth high quality particularly when exporting log-encoded photographs again to log. Log information particularly are designed to work at larger bit depth ranges in order that they are often reworked with sufficient data from a log-type gamma area to a display-type gamma area.
With out that data, banding and low shade constancy can have a really detrimental impact on imagery when shade grading. Digital information can depreciate in a short time with re-rendering, transcoding or exporting. So be sure that your intent is evident earlier than you resolve how one can export or transcode a file. For instance, an edit is locked with 10-bit supply media information in a log-type gamma and exported for shade grading. If the render settings are modified to a decrease bit depth like 8-bit, that exported timeline can be rather more tough to grade and result in banding and shade data falling aside in areas of gradation.
Bit depth particularly comes into play with log-encoded photographs as a result of larger bit-depth sources make it doable to govern the vary of delicate colours and variation s throughout the picture. So be sure that when exporting log information for grading from Premiere Professional that the “render at most depth” field is checked and 16-bpc is chosen when rendering to a format like ProRes444 or DNxHD. This ensures that every one the bit-depth of the supply imagery can be preserved for the colorist or additional manipulation of the log imagery down the road. (For extra element, try Jarle Leirpoll’s post on Premiere Pro’s Render Quality settings.)
Information charges come up principally when exporting or encoding compressed media for supply like h.264 or h.265 information. However they’ll additionally come into play when coping with compressed digicam sources. Sure cameras like GoPros, for instance, use a model of h.264 for recording video. So be sure you perceive how these file have been encoded and at what information fee. The decrease the info fee, the extra compressed the file.
Information ranges are available two primary flavors: video and full. In my expertise, except there’s a particular workflow and pipeline constructed round full vary information, video vary information are rather more widespread particularly with offline editorial sort workflows in Premiere, After Results, and Avid.
There isn’t actually a good way to embed details about the info vary of a given file except it’s correctly exported with bars. Sure packages export sure file sorts as full vary mechanically like ProRes444. Historically 444 codecs are full vary with RGB information, however newer codecs don’t at all times encode data that manner. And relying on the applying, the file could be interpreted as full or video vary.
More often than not, a file’s information vary is interpreted based mostly on the codec and file container. So if a file is encoded with data that doesn’t match the standard information vary for that file sort, the interpretation of that file might be fallacious.
For file codecs like DPX image sequences, most packages assume a full information vary. For file codecs like Quicktime, most packages assume video vary information. It may be robust to inform when taking a look at information which vary that file is. If there’s a query concerning the information vary of a file, encoding bars in the beginning of a program is a good way to have an concept concerning the information vary of the file.
Sure packages can swap the interpretation of information from full to video vary. If the bars look appropriate, then the applying is seeing the right information vary of the file. If the bars look lifted or too darkish on the scopes, then the information aren’t being interpreted appropriately.
When rendering media for conforming or from shade, it’s necessary to grasp file handles. Edits can oftentimes include time warps. File handles make sure that there’s sufficient media to rebuild timelines. When pulling media or VFX, it’s necessary to ship the correct quantity of handles to make sure a very good rebuild when information are again within the edit or conform. With quick time warps and over-cranked footage, further handles are wanted to correctly conform. It’s additionally necessary to make use of supply timecode when rendering out information that can be used for conforms.
Colour area and gamma
For VFX prep and renders, shade area and gamma can simply be interpreted incorrectly between apps. Recordsdata may be exported with linear gamma and supply digicam shade area and interpreted again with the identical gamma however a unique shade area. It’s at all times nice if information may be rendered the identical because the supply for gamma and shade area, however generally wires can get crossed.
Simply keep in mind that EXRs are used to alternate linear gamma, whereas DPX information are normally full-range and include log gamma. ProRes or DNX information needs to be rendered with a excessive bit-depth for VFX exchanges. In After Results notably, these log information can simply find yourself being rendered in 8-bit, which makes them ineffective for shade grading.
Body charges
Body charges may be difficult in how they’re interpreted between apps. File sequences specifically don’t at all times include metadata that tells apps how one can interpret them. It’s additionally doable to re-interpret body charges throughout the apps which may make alternate file lists (XML, EDL, AAF) difficult if the opposite app doesn’t have that very same interpretation.
Colour tags and shows
Colour tags could make it tough to correctly see how one thing is meant to be seen between apps. Apps all have totally different shade administration and customers may be on computer systems with a wide range of shows or working techniques.
So long as shade shifts aren’t baked into renders, it’s okay if issues look totally different between apps. Verify the file renders from totally different packages towards one another in a identified shade area and gamma in a single app.
A few of these issues are particular to sure forms of software program, some are part of how information are encoded as we speak, some have been a problem in publish for a few years. Expertise is the most effective instructor to acknowledge these widespread issues.
Typically there isn’t time throughout publish manufacturing to analysis or dig deeply into discovering an answer. However hopefully you’re now in a greater place to identify these widespread pitfalls earlier than they develop into an issue that may’t be mounted.
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