How Do I Use Transparent WebM Stream Alerts in OBS?

For stream alerts in OBS, use a VP9 WebM with alpha, add it as a Media Source, leave Loop off, and turn on Restart playback when source becomes active so each trigger starts at frame one. Do not add a Chroma Key or Colour Key filter on a file that already has alpha. Browser Source is better when the alert is hosted by an alert widget URL. ExportAlpha writes WebM up to 60 seconds or ProRes 4444 up to 8 seconds—it is not an alert SaaS and not a live virtual camera.

Updated October 30, 2026

Spec box

Input	MP4 / MOV / WebM / M4V
Alert-friendly export	Transparent VP9 WebM (yuva420p), typically a few seconds
Max WebM length	60 seconds
Optional master	ProRes 4444 MOV, ≤ 8 seconds
Not offered	GIF · animated WebP · PNG sequence · HEVC-with-alpha · public API · real-time virtual camera
Sign-in	Google
Trial	Most new accounts: 5 free credits (1 credit ≈ 1 video second or 1 still)
Packs	Pack credits never expire

One alert clip ≤60s ≈ Quick $6.99

Export one alert clip

Download a short VP9 WebM with alpha, then play it once per trigger in OBS.

Try 5 free credits — no card One project ≈ Quick pack $6.99

Google sign-in required · Failed jobs refund credits

Alert types (follow / sub / raid)

Stream alerts are short one-shot animations that play when something happens: a follow, subscribe, cheer, raid, or custom tip event. Visually they sit above the game or camera for a second or two, then disappear. The file workflow is the same across event types—what changes is how you trigger the Media Source (hotkey, automation, or an alert provider’s Browser Source).

This page owns the transparent WebM + OBS Media Source path. Platform-specific Alert Box upload rules vary by product and version; treat those as separate upload checklists. If your alert is only a URL hosted by a widget, skip Media Source and use Browser Source instead.

Why green-screen alerts fail

Green-screen alert MP4s look fine in a vacuum and break on stream:

Spill on hair, glow, or particle edges turns into a green fringe after keying.
Colour collision with game HUDs, team colours, or chat overlays punches holes in the wrong places.
You must keep a Chroma Key filter tuned forever; every scene change can need a retune.
Everyday H.264 MP4 still has no alpha—the green is opaque pixel data until something keys it.

A VP9 WebM with a real alpha channel ships the holes in the file. OBS composites them without a keyer. If you still have a green-screen master, convert it properly first—see Green screen to transparent WebM.

Encode / export checklist (yuva420p)

Start from a clip that either already has alpha or has a background you can remove or key.
Deliver VP9 WebM with alpha (ExportAlpha default pixel format: yuva420p).
Keep alert length short; credits are about 1 per video second.
Do not “save space” by re-exporting to everyday H.264 MP4—that drops alpha and brings back the black or green box problem (MP4 transparency).
Optional editorial master: ProRes 4444 ≤8s on ExportAlpha, then convert to WebM for a lighter Media Source if needed.

Format matrix: Which formats support transparency?.

OBS Media Source settings

Sources → Add → Media Source.
Browse to your .webm alert file.
Loop: off — alerts should play once per trigger.
Enable Restart playback when source becomes active so every show starts at frame one.
Leave Chroma Key / Colour Key filters off when the file already has alpha.
Place the Media Source above gameplay and camera.
Optional: toggle Apply alpha in linear space if soft edges look wrong; compare both states (OBS overlay guide).
Trigger visibility with a hotkey, scene switcher, or your automation. Exact macros are outside this page’s product scope.

Media Source vs Browser Source

Situation	Prefer	Why
Local transparent .webm you exported	Media Source	Direct file playback; Loop and Restart controls
Alert hosted by a widget URL	Browser Source	The provider renders the alert page
Window capture of a video player	Neither	Players often paint black behind “transparent” previews

Pick Media Source for files you control. Pick Browser Source for hosted widgets.

Stinger vs alert

	Alert (this page)	Stinger
Role	Event toast over a scene	Scene transition wipe
OBS home	Media Source (or Browser Source widget)	Stinger transition slot
Loop	Off	Plays through the cut
Guide	This page	OBS stinger transparent WebM

Do not drop a stinger file into a Media Source and expect transition timing to match—and do not put a one-shot alert into the stinger slot unless you intentionally redesigned it as a wipe.

Jagged or green-fringed edges

Jagged / stair-step: often resolution mismatch, heavy compression, or scaling a tiny canvas up. Export closer to your canvas size; avoid stacking sharpen filters.
Green fringe: leftover chroma key on a file that already has alpha—remove the filter. Or the source was still a green MP4.
Dark or dirty soft edges: try Apply alpha in linear space on Media Source; confirm the file is WebM with alpha, not MP4.
Black box: wrong format or window capture—see Why transparent video turns black.

ExportAlpha limits

ExportAlpha is a file processor: upload a clip, download transparent WebM or short ProRes. It does not connect to Twitch events, does not replace an alert-widget host, and does not run as an OBS plugin. Most new accounts get 5 free credits after Google sign-in. Pack credits never expire.

One alert clip ≤60s ≈ Quick $6.99

When this path is wrong

You need a live face-tracked avatar during the alert (use tracking software; see also VTuber overlay for pre-rendered loops).
You need GIF stickers, animated WebP, PNG sequences, or HEVC-with-alpha—ExportAlpha does not export those.
Your alert is only an HTML widget—use Browser Source, not a WebM Media Source.

Frequently Asked Questions

Media Source vs Browser Source for alerts—which should I pick?

Local transparent WebM: Media Source. Hosted alert widget URL: Browser Source. Do not window-capture a player.

Do I need Chroma Key for a transparent WebM alert?

No. If the file already stores alpha, Chroma Key or Colour Key usually creates fringing. Leave those filters off.

Why do my alert edges look jagged or green-fringed?

Jagged edges often mean scaling or compression issues. Green fringe usually means a chroma filter is still on, or the file is still a green-screen MP4 without real alpha. Re-export WebM with alpha and remove extra key filters.

Should Loop be on for alerts?

No for one-shots. Leave Loop off and enable Restart playback when source becomes active.

Is there a free trial?

Most new accounts get 5 free credits after Google sign-in. 1 credit is about 1 video second or 1 still.

Next step

Upload at exportalpha.com (Google sign-in). Most new accounts get 5 free credits.
Download transparent VP9 WebM (default) or ProRes 4444 MOV (≤ 8 seconds).
In OBS: Media Source → Loop off → Restart when active → no Chroma Key.
Out of trial credits? One project ≈ Quick pack $6.99.

Export the alert file

One transparent WebM, then Media Source with Loop off.