How to export an alpha channel: ProRes 4444, EXR and PNG sequences

To export an alpha channel, pick a format that can actually store one: Apple ProRes 4444 or ProRes 4444 XQ in a QuickTime file, or an image sequence such as PNG or OpenEXR. H.264 MP4 exports from editing apps have no alpha, and neither do the ProRes 422 codecs. Then settle two things with whoever receives the file: whether the alpha is straight or premultiplied, and how many bits it has. Most “my matte looks wrong” problems come from one of those two.

Below: what alpha is, a format comparison, straight vs premultiplied, bit depth, how to export from After Effects, Premiere Pro and DaVinci Resolve, and how to hand off sequences so frame counts and timecode survive the trip.

What an alpha channel is, and when you want a matte-only file

An alpha channel is a fourth channel stored next to red, green and blue that says how opaque each pixel is. Adobe’s After Effects help describes it the way compositors read it: white is fully opaque, black is fully transparent, and grays are partially transparent. An image with that fourth channel is called RGBA.

You can hand a matte over in two ways:

• RGBA. The picture and its alpha in one file. It drops straight onto a timeline and composites over whatever is underneath. Use it when the receiver needs the cut-out element itself: a title, a keyed subject, a graphic.
• Matte only. A grayscale image where the matte is the picture, sometimes called a luma matte. The receiver applies it to their own copy of the plate, as a track matte in After Effects or through a Copy or Merge node in Nuke. This is often the better handoff for roto, because the plate pixels never leave the compositor’s own full-quality source.

Premiere Pro and Adobe Media Encoder have a Render Alpha Channel Only option for the second case. Adobe’s Premiere help says it renders only the alpha as grayscale and is useful for formats that can’t store transparency. If you’re new to where mattes come from, start with what rotoscoping is.

Which formats store alpha

Only some formats carry alpha, and they differ in bit depth, compression and size. Data rates below are Apple’s published targets for 3840×2160 at 24 fps, measured without alpha; Apple says an alpha channel “typically results in only a small increase” (ProRes white paper, April 2022).

Format	Alpha?	Bit depth	Compression	Size notes	Where it’s used
ProRes 4444 (.mov)	Yes	Up to 12-bit color, up to 16-bit alpha	Color visually lossless; alpha mathematically lossless	About 1,061 Mb/s target at UHD 24p	Motion graphics and comp exchange, editorial
ProRes 4444 XQ (.mov)	Yes	Up to 12-bit color, up to 16-bit alpha	As ProRes 4444, at a higher data rate	About 1,591 Mb/s target at UHD 24p	High-dynamic-range sources, heavy grading
ProRes 422 HQ, 422, LT, Proxy	No	10-bit, 4:2:2	Visually lossless (422 HQ) and lighter variants	About 707 Mb/s target for 422 HQ at UHD 24p	Plates, editorial, RGB deliveries
PNG sequence	Yes	8 or 16 bits per channel	Lossless	No fixed data rate; size depends on the image	Graphics, mattes, handoffs between apps
OpenEXR sequence	Yes (channel “A”)	16-bit half float or 32-bit float	Lossless (ZIP, PIZ, RLE…) or lossy (DWAA, DWAB, B44…)	Depends on channels and compression	Nuke, CG renders, VFX pipelines
H.264 / HEVC (.mp4)	No, as exported by editing apps	Typically 8 or 10-bit	Lossy	Small	Review copies, web delivery

Sources: Apple’s About Apple ProRes and white paper, the OpenEXR technical introduction and the PNG specification. Apple states plainly that ProRes 4444 and 4444 XQ are the only ProRes codecs that support alpha. Adobe also lists TIFF and TGA among the formats that can include an alpha channel.

One exception to the MP4 row: Apple added HEVC video with alpha in 2019 for compositing video in apps and in Safari. It’s a playback format, not something to send a compositor.

Straight vs premultiplied: why edges go dark or bright

Straight (unmatted) alpha stores color independently of transparency. Premultiplied (matted) alpha stores color that has already been multiplied by the alpha, usually against black. Adobe’s footage interpretation help puts the trade-off this way: straight channels keep more accurate color, while premultiplied channels work in a wider range of programs.

Neither is wrong. The problem is reading one as the other:

• Premultiplied read as straight. Adobe notes that semitransparent areas then keep some of the background color. With a black matte color, that shows up as a dark fringe around hair, motion blur and feathered edges.
• Straight read as premultiplied. The app assumes the edge color has already been darkened by the alpha and doesn’t darken it, so soft edges come out too bright, like a thin glow.

What each format expects:

• PNG is straight by definition. The spec says color values “are not affected by the alpha value”, which it calls unassociated alpha.
• OpenEXR is premultiplied by convention: the OpenEXR docs say all color channels are premultiplied by alpha, so a standard “over” works directly.
• ProRes 4444 can hold either. After Effects, for example, lets you write either one. That’s why ProRes 4444 files need a label.

On the receiving end, After Effects lets you change the alpha interpretation of footage after import. Nuke’s Read node has a premultiplied control that unpremultiplies color before the colorspace conversion and premultiplies it again after. Put “straight” or “premult” in the filename or the handoff note, and nobody has to guess.

Bit depth: why soft edges need more than 8 bits

An 8-bit alpha has 256 levels from transparent to opaque. A 16-bit alpha has 65,536. A hard-edged matte doesn’t care. A soft one does: a motion-blurred arm or a feathered edge across a UHD frame has to spread its falloff over those levels, and with only 256 of them the matte can step, or band. You may not see it in the matte itself. You’ll see it once someone chokes the matte, grades through it, or pushes exposure on the comp.

Practical rules:

• For RGBA or mattes with soft edges, use 16-bit PNG, ProRes 4444 (alpha up to 16-bit, lossless) or half-float EXR.
• Writing an 8-bit matte into a 16-bit file doesn’t add precision; it only avoids losing any more. Keep the whole chain at high bit depth, from render to export.
• In Premiere Pro, Render at Maximum Depth renders effects at the highest bit depth the format supports; Adobe says that’s in most cases 32-bit float processing, and that it helps reduce banding in smooth gradients.
• 8-bit is fine for review proxies and hard mattes (garbage mattes, holdouts).

How to export with alpha from After Effects

After Effects writes alpha from the Render Queue, through the output module. Adobe’s rendering help documents the settings that matter:

1. Add the comp to the Render Queue and open the Output Module.
2. Choose a format that stores alpha: QuickTime with Apple ProRes 4444 set in the format options, or a PNG or OpenEXR sequence.
3. Set Channels to RGB + Alpha. Adobe notes this implies a depth of “Millions of Colors+”; the plus sign means alpha.
4. Set Depth as high as the format allows.
5. Set Color to Straight (Unmatted) or Premultiplied (Matted), whichever the receiver expects, and write that down.
6. For sequences, set Starting #. Adobe’s example: set it to 38 and the first frame is named [file_name]_00038. Use Comp Frame Number takes the numbering from the work area instead.

Areas of the comp with no layers are transparent in the output. Check the alpha channel view before you render to confirm it’s what you think it is.

How to export with alpha from Premiere Pro

Premiere Pro exports transparency only where the sequence is actually transparent: areas with no clip underneath, or clips that already carry alpha. Conceptually:

1. Choose a format and codec that store alpha: QuickTime with ProRes 4444 or 4444 XQ, or a PNG sequence.
2. Make sure the export is set to include alpha. Where that option sits depends on the format and the Premiere version, so check the video settings for your codec before you render.
3. Turn on Render at Maximum Depth for soft edges.
4. For a matte-only file, use Render Alpha Channel Only.
5. Leave Set Start Timecode off to keep the source timecode, or turn it on to set a different start. Adobe’s Media Encoder reference describes both options.

Test with one short export: bring the file back in over a colored solid and look at the edges.

How to export with alpha from DaVinci Resolve

Resolve renders from the Deliver page, and the same principles apply. Choose a QuickTime ProRes 4444 or 4444 XQ render, or an image-sequence format that carries alpha, and turn on alpha export in the render’s video settings. Resolve can only write transparency that exists in the timeline, so make sure the alpha you built, in Fusion for example, actually reaches it. Option names and placement change between versions, so check the render settings chapter of the DaVinci Resolve 21 Reference Manual for yours. As in Premiere, do a short test render and check the edges over a solid.

Timecode and frame counts

A matte is only useful if it lines up with its plate frame for frame. Check three things on every handoff:

• Frame count. The matte must have exactly as many frames as the plate range it belongs to, including any handles you agreed on. One missing frame and everything after it slips.
• Frame rate. 23.976 and 24 fps are not interchangeable. If the matte was rendered at the wrong rate, it drifts against the plate.
• Timecode. QuickTime movies can carry a start timecode; Adobe Media Encoder uses the source’s timecode unless you set a different start. OpenEXR has an optional timeCode attribute, but not every app writes it. PNG has no timecode field, so for PNG sequences the frame number in the filename is all the timing information there is.

In Nuke, the Read node’s frame controls (start at, offset) let you remap a sequence whose numbering doesn’t match the plate. Better not to need them.

Naming and handing off image sequences

A clean handoff saves a round of emails. A checklist that works:

1. One folder per pass, for example SH010_matte_v003/.
2. Name frames shot_pass_version.frame.ext with fixed zero padding, such as SH010_matte_v003.1001.exr. Every frame gets the same number of digits.
3. Agree the start frame with the receiver. Some facilities start every shot at a fixed number, such as 1001, to leave room for handles; others match the source frame numbers.
4. Check for gaps: the first frame, the last frame and the file count should agree.
5. Label the alpha and color: straight or premultiplied, and the colorspace, in the folder name or a short note.
6. Match the plate’s resolution, not a proxy size, unless both sides agreed on a proxy.
7. Version up, never overwrite. v004 next to v003 is safer than a replaced v003.

Where nolanlabs fits

For a subject, nolanlabs hands back a single-channel matte and an RGBA pass (source RGB plus the matte as straight alpha), for example as ProRes 4444, 16-bit PNG or half-float EXR sequences, matched to the source’s frame rate and frame count. One honest limit: the model stages work on 8-bit Rec.709 proxies, and the matte is conformed back to source resolution afterward. Access is by invitation, starting with a demo. More under what you get back, and on how AI masks compare today in Object Matte vs Magic Mask.

Whatever made the matte, the handoff rules are the same: a format that stores alpha, enough bits for soft edges, a clear label for straight or premultiplied, and frame numbers that match the plate. If you’re compositing a subject over a new background, background replacement without a green screen picks up from here.

Questions

Can an MP4 or H.264 file have an alpha channel?

Not in practice for post-production. The H.264 MP4s that After Effects, Premiere Pro and DaVinci Resolve export don't carry transparency. Apple's HEVC with alpha exists for playback in apps and Safari, but it isn't an interchange format for compositing. For mattes, use ProRes 4444, a PNG sequence or an OpenEXR sequence.

Should I send a matte as ProRes 4444 or as an EXR sequence?

Send whatever the receiving app and person prefer. ProRes 4444 is a single file that editors and motion designers open easily, with a mathematically lossless alpha of up to 16 bits, according to Apple. An OpenEXR sequence stores half-float or 32-bit float channels, one file per frame, and is the usual choice for Nuke and CG pipelines. Both hold soft edges well; 8-bit formats are where edges start to band.

Why does my matte have a dark or bright fringe after import?

Almost always because the alpha is interpreted the wrong way. Premultiplied footage read as straight keeps some of the background color in its soft edges, which usually shows as a dark fringe. Straight footage read as premultiplied tends to brighten or glow at the edges. Check how the file was written and set the interpretation to match.

Sources

1. About Apple ProRes, Apple Support · accessed 2026-09-24
2. Apple ProRes white paper (April 2022), Apple · accessed 2026-09-24
3. Technical Introduction to OpenEXR, OpenEXR documentation · accessed 2026-09-24
4. PNG Specification 1.2, Data Representation, libpng.org · accessed 2026-09-24
5. Alpha channels and masks, Adobe After Effects Help · accessed 2026-09-24
6. Importing and interpreting footage items, Adobe After Effects Help · accessed 2026-09-24
7. Basics of rendering and exporting, Adobe After Effects Help · accessed 2026-09-24
8. Basic video settings, Adobe Premiere Help · accessed 2026-09-24
9. Export settings reference, Adobe Media Encoder Help · accessed 2026-09-24
10. DaVinci Resolve 21 Reference Manual, Blackmagic Design (PDF) · accessed 2026-09-24
11. Read node (Nuke 17.1), Foundry Learn · accessed 2026-09-24
12. HEVC Video with Alpha (WWDC19), Apple Developer · accessed 2026-09-24

Written by the nolanlabs team. nolanlabs does AI shot work for post-production; product names mentioned belong to their owners. Tool details were accurate on the date shown above. Check the vendor’s documentation before relying on them.